KS3 Science Study Pack: Forces

Key Knowledge

Forces are everywhere. Every time you open a door, ride a bike, kick a football, sit on a chair, jump, brake, swim, or pick up a bag, forces are involved. A force is a push, pull, twist, stretch, or squeeze. Forces can change an object's speed, direction, or shape.

Forces are measured in Newtons, written as N. A small force might be a push of 2 N on a pencil. A much larger force might be a 600 N weight acting on a person. Forces have both size and direction, so scientists often show them with arrows. A longer arrow represents a larger force, and the arrow points in the direction in which the force acts.

Objects often have several forces acting on them at the same time. To understand what happens, we need to think about the resultant force. The resultant force is the single overall force after all the forces have been combined. If the resultant force is zero, the forces are balanced. If the resultant force is not zero, the forces are unbalanced.

Balanced forces do not always mean an object is stationary. A stationary object with balanced forces stays stationary, but a moving object with balanced forces continues moving at a steady speed in a straight line. Unbalanced forces can make an object speed up, slow down, change direction, or change shape.

What Forces Are

A force is a push, pull, twist, stretch, or squeeze. Forces are caused by interactions between objects. Sometimes the objects touch. Sometimes they do not.

Forces can:

• make a stationary object start moving
• make a moving object speed up
• make a moving object slow down
• make an object change direction
• change the shape of an object
• keep an object still by balancing other forces

For example, when you kick a football, your foot applies a contact force to the ball. The ball speeds up and changes shape very briefly during the kick. Once the ball is moving through the air, gravity pulls it downwards and air resistance acts against its motion.

Forces are measured using a force meter, sometimes called a Newton meter. The unit of force is the Newton, named after Isaac Newton. The symbol is N.

Because forces have size and direction, it is not enough to say "there is a force of 20 N". A better scientific answer says where the force acts, what type of force it is, and which direction it acts in. For example:

• "The push force is 20 N to the right."
• "The weight of the object is 8 N downwards."
• "Friction is 5 N to the left, opposite the motion."

Contact and Non-Contact Forces

Forces can be divided into two main groups: contact forces and non-contact forces.

A contact force acts when objects touch. For example, friction acts when surfaces rub or try to slide past each other. Tension acts in a stretched rope or string. A normal contact force acts when a surface pushes back on an object.

A non-contact force acts without objects touching. Gravity, magnetic forces, and electrostatic forces are non-contact forces. These forces can act at a distance.

Force name	Type	What it acts on	Everyday example
Friction	Contact	Surfaces that slide or try to slide	Bike brakes slowing a wheel
Air resistance	Contact	Objects moving through air	A parachute slowing a parachutist
Water resistance	Contact	Objects moving through water	A swimmer being slowed by water
Normal contact force	Contact	An object resting on or pressing against a surface	A table pushing up on a book
Tension	Contact	A stretched rope, string, chain, or cable	A rope pulling a sledge
Upthrust	Contact	Objects in fluids such as water or air	Water pushing up on a boat
Applied push or pull	Contact	An object being pushed or pulled	A person pushing a trolley
Gravity	Non-contact	Objects with mass	Earth pulling a ball downwards
Magnetic force	Non-contact	Magnetic materials and magnets	A magnet attracting a steel paperclip
Electrostatic force	Non-contact	Charged objects	A rubbed balloon attracting hair

Contact and non-contact forces are both real forces. Non-contact forces can seem surprising because objects do not need to touch, but gravity, magnetism, and electrostatic attraction or repulsion can all produce measurable effects.

Gravity, Mass, and Weight

Gravity is a non-contact force of attraction between masses. Near Earth, gravity pulls objects towards the centre of the Earth. This force acts all the time, not only when objects are falling.

Mass and weight are not the same thing.

Mass is the amount of matter in an object. It is measured in kilograms, written as kg. Your mass would be the same on Earth and on the Moon because you are made of the same amount of matter.

Weight is the force of gravity acting on an object. It is measured in Newtons, written as N. Your weight would be smaller on the Moon because the Moon's gravity is weaker than Earth's gravity.

Feature	Mass	Weight
Meaning	Amount of matter in an object	Force of gravity on an object
Unit	kilogram, kg	Newton, N
Type of quantity at KS3	A property of the object	A force
Changes on different planets?	No, not unless matter is added or removed	Yes, because gravity can be different
Measuring instrument	Balance	Force meter
Example	A bag has a mass of 3 kg	The bag has a weight of about 30 N on Earth

At KS3, it is most important to remember that mass is measured in kg and weight is measured in N. Weight is a force caused by gravity.

Book Resting on a Table

A book resting on a table has gravity pulling it down. This downward force is the book's weight. The table also pushes up on the book with a normal contact force. If the upward force is equal to the downward force, the forces are balanced and the book stays still.

       Normal contact force = 8 N
                 ^
                 |
             [ book ]
                 |
                 v
            Weight = 8 N

Resultant force = 0 N, so the book remains at rest.

Friction and Resistance Forces

Friction is a contact force that opposes sliding or attempted sliding between surfaces. It acts in the opposite direction to the motion, or the direction the object is trying to move.

Friction can be useful:

• Shoes grip the ground so you can walk without slipping.
• Bike brakes use friction to slow the wheels.
• Tyres use friction to grip the road.
• A pencil makes marks because of friction with the paper.
• Rock climbers rely on friction between hands, shoes, and rock.

Friction can also be unwanted:

• Moving machine parts can wear down.
• Energy can be transferred by heating, making parts hot.
• Cars use more fuel when friction and air resistance are large.
• Rough surfaces can make objects harder to move.

Friction can be increased by using rougher surfaces, adding grip patterns, pressing surfaces together more strongly, or using rubber. Friction can be reduced by using smoother surfaces, lubricants such as oil, wheels, rollers, or streamlined shapes when moving through fluids.

Air resistance is a contact force that opposes motion through air. It acts opposite to the direction of motion. Air resistance is larger when an object moves faster, has a larger surface area, or has a less streamlined shape.

Water resistance is a contact force that opposes motion through water. It is important for swimmers, boats, fish, submarines, and falling objects in water. Streamlined shapes reduce water resistance.

Force	Where the force acts	What it opposes	How it can be increased	How it can be reduced
Friction	Between touching solid surfaces	Sliding or attempted sliding	Rougher surfaces, more grip, pressing surfaces together	Lubricants, smoother surfaces, wheels
Air resistance	Between an object and air	Motion through air	Larger surface area, higher speed, less streamlined shape	Streamlining, smaller surface area
Water resistance	Between an object and water	Motion through water	Larger surface area, higher speed, less streamlined shape	Streamlining, smooth surfaces

Falling Object

When a ball falls, gravity pulls it downwards. As it moves through the air, air resistance acts upwards, opposite to the motion.

        Air resistance
              ^
              |
            [ ball ]
              |
              v
            Weight

At first, the ball may have a large downward resultant force because air resistance is small. As the ball gets faster, air resistance increases. If air resistance becomes equal to weight, the forces are balanced and the ball falls at a steady speed.

Parachutist

Before a parachute opens, the parachutist has weight downwards and air resistance upwards. As speed increases, air resistance increases. When the parachute opens, the surface area becomes much larger, so air resistance increases greatly.

          Large air resistance
                  ^
                  |
             \---------/
              \       /
               \_____/
                  |
                [person]
                  |
                  v
                Weight

The large upward air resistance gives an upward resultant force for a short time, so the parachutist slows down. Later, the forces may become balanced again, and the parachutist falls at a lower steady speed.

Drawing and Reading Force Diagrams

Force diagrams are simplified models. They do not show every detail of a real object. They help us think clearly about the forces acting on an object.

Rules for force diagrams:

• Use arrows to show forces.
• The arrow points in the direction the force acts.
• Longer arrows show larger forces.
• Shorter arrows show smaller forces.
• Arrows should start at or near the object.
• Label each arrow with the force name.
• Add values in Newtons if they are provided.
• Do not assume the arrow shows the direction of movement. It shows force direction.

Longer arrow = larger force
Shorter arrow = smaller force
Arrow direction = direction of force

Box with Push and Friction

Friction = 20 N  <---- [ box ] ---->  Push = 50 N

Resultant force = 30 N to the right

The push is larger than friction, so the forces are unbalanced. The resultant force is 30 N to the right. If the box is free to move, it will speed up to the right.

Tug-of-War

Team A pulls 300 N  <---- [ rope ] ---->  Team B pulls 260 N

Resultant force = 40 N to the left

The forces act in opposite directions. Team A's pull is larger, so the resultant force is 40 N to the left.

Resultant Forces

The resultant force is the single overall force after combining all the forces acting on an object. It has a size and a direction, unless it is zero.

When forces act in the same direction, add them.

When forces act in opposite directions, subtract the smaller force from the larger force. The direction of the resultant force is the direction of the larger force.

If equal forces act in opposite directions, they cancel out. The resultant force is 0 N.

Worked Example 1: Forces in the Same Direction

A student pushes a trolley with 35 N. Another student pushes in the same direction with 20 N.

Step 1: Identify the directions.

Both forces act in the same direction.

Step 2: Add the forces.

35 N + 20 N = 55 N

Step 3: State the answer with direction.

The resultant force is 55 N in the direction of the pushes.

Worked Example 2: Forces in Opposite Directions

A box is pushed right with 60 N. Friction acts left with 25 N.

Step 1: Identify the larger force.

The larger force is 60 N to the right.

Step 2: Subtract the smaller force.

60 N - 25 N = 35 N

Step 3: State the answer with direction.

The resultant force is 35 N to the right.

If the box is free to move, it will speed up to the right because the resultant force acts to the right.

Worked Example 3: Balanced Forces

A book has a weight of 8 N downwards. The table pushes up with a normal contact force of 8 N.

Step 1: Identify the forces.

Weight = 8 N downwards. Normal contact force = 8 N upwards.

Step 2: Compare the forces.

The forces are equal in size and opposite in direction.

Step 3: Find the resultant force.

8 N - 8 N = 0 N

The resultant force is 0 N. The forces are balanced, so the book remains at rest.

Worked Example 4: Cyclist at Steady Speed

A cyclist pedals with a forward force of 120 N. Air resistance and friction total 120 N backwards.

The forward and backward forces are equal, so the resultant force is 0 N. The forces are balanced. If the cyclist is already moving, the cyclist continues at a steady speed in a straight line.

Worked Example 5: Car Braking

A car is moving forwards. The driving force becomes smaller because the driver stops accelerating and applies the brakes. Friction from the brakes and tyres, plus air resistance, acts backwards.

If the backward forces are larger than the forward driving force, the resultant force is backwards. A backward resultant force on a forward-moving car makes the car slow down.

Balanced and Unbalanced Forces

Balanced forces are equal in size and opposite in direction. Their resultant force is 0 N.

Unbalanced forces do not cancel out. Their resultant force is not 0 N.

Feature	Balanced forces	Unbalanced forces
Resultant force	0 N	More than 0 N
Effect on a stationary object	Remains stationary	Starts moving in the direction of the resultant force
Effect on a moving object	Continues at steady speed in a straight line	Speeds up, slows down, or changes direction
Example	A book resting on a table	A trolley pushed harder forwards than friction acts backwards

A common mistake is to think balanced forces always mean an object is not moving. This is not correct. Balanced forces mean there is no change in motion. The object may be stationary, or it may already be moving at steady speed in a straight line.

Forces and Motion

Forces explain changes in motion. The key question is: what is the resultant force?

If the resultant force acts in the same direction as the motion, the object speeds up.

If the resultant force acts opposite to the motion, the object slows down.

If the resultant force acts sideways, the object changes direction.

If the resultant force is zero, the object stays stationary or continues moving at steady speed in a straight line.

Football Being Kicked

During the kick, the football experiences an applied contact force from the foot. This force changes the ball's speed, direction, and shape very briefly. After the ball leaves the foot, gravity pulls it downwards and air resistance acts opposite to its motion.

Cyclist

When a cyclist starts moving, the forward driving force from pedalling is larger than the backward forces of friction and air resistance. The resultant force is forwards, so the cyclist speeds up.

At steady speed, the forward force equals the backward forces. The forces are balanced, so the cyclist continues at steady speed in a straight line.

When the cyclist brakes, the backward forces are larger than the forward driving force. The resultant force is backwards, so the cyclist slows down.

When the cyclist turns, there must be a sideways resultant force. Friction between the tyres and the road helps change the cyclist's direction.

Sports and Safety

Trainers increase friction between shoes and the ground, helping athletes grip and change direction. Bike tyres and car tyres also need friction to grip the road.

Seat belts apply a force to slow a passenger during a sudden stop. Helmets and crumple zones help by increasing the time taken to stop. A longer stopping time means the stopping force can be smaller, which can reduce injury. This is a simple KS3 idea: safety equipment does not remove forces, but it can make forces act in a safer way.

Space and Orbit

Gravity can act at a distance. Satellites orbit planets because gravity keeps changing their direction. At KS3, you do not need to calculate orbits, but you should know that gravity is still acting even when an object is in space.

Key Vocabulary

Keyword	Meaning
Force	A push, pull, twist, stretch, or squeeze that can change motion or shape
Newton	The unit of force, written as N
Gravity	A non-contact force of attraction between masses
Friction	A contact force that opposes sliding or attempted sliding
Resultant force	The single overall force after combining forces
Balanced forces	Forces that cancel out because they are equal and opposite, giving 0 N resultant force
Unbalanced forces	Forces that do not cancel out, giving a non-zero resultant force
Air resistance	A contact force that opposes motion through air
Water resistance	A contact force that opposes motion through water
Normal contact force	A force from a surface pushing on an object touching it
Tension	A pulling force in a stretched rope, string, cable, or chain
Upthrust	An upward force from a fluid such as water or air
Mass	The amount of matter in an object, measured in kg
Weight	The force of gravity on an object, measured in N
Contact force	A force that acts when objects touch
Non-contact force	A force that acts without objects touching
Independent variable	The variable deliberately changed in an investigation
Dependent variable	The variable measured in an investigation
Control variable	A variable kept the same to make a test fair
Reliability	How trustworthy results are, often improved by repeats
Repeatability	Getting similar results when the same method is repeated
Accuracy	How close a measurement is to the true value
Precision	How close repeated measurements are to each other

Working Scientifically: Investigating Forces

Investigation Question

How does surface type affect the force needed to move a block?

Scientific Idea

Different surfaces produce different amounts of friction. Rougher surfaces usually produce more friction, so a larger force is needed to pull the same block at a steady speed.

Prediction

The block will need the greatest pulling force on sandpaper because sandpaper is the roughest surface. The block will need the smallest pulling force on smooth plastic because the surface is smoother.

Variables

Variable type	Variable	How to measure or control it
Independent variable	Surface type	Change the surface: smooth plastic, wood, carpet, sandpaper
Dependent variable	Force needed to pull the block	Measure with a force meter in N
Control variable	Same block	Use the same block each time
Control variable	Same added mass	Keep the same mass on the block
Control variable	Same pulling speed	Pull steadily at a slow, constant speed
Control variable	Same force meter	Use the same force meter for all readings
Control variable	Same distance pulled	Pull the block over the same distance each time

Safe Method

1. Place the block on the first surface.
2. Attach a force meter to the block.
3. Pull the force meter steadily so the block moves at a slow, constant speed.
4. Record the force shown on the force meter in Newtons.
5. Repeat three times for the same surface.
6. Repeat the method for each surface.
7. Calculate a mean for each surface.
8. Compare the mean forces to decide which surface produced the most friction.

Safety points:

• Keep the floor clear of bags and equipment.
• Pull the force meter steadily, without sudden jerks.
• Do not release a stretched force meter suddenly.
• Keep added masses secure so they do not fall.

Results Table

Surface	Reading 1 (N)	Reading 2 (N)	Reading 3 (N)	Mean force (N)
Smooth plastic	1.2	1.3	1.1	1.2
Wood	2.0	2.1	2.2	2.1
Carpet	3.4	3.6	3.5	3.5
Sandpaper	4.8	7.9	5.0	4.9 if anomaly ignored

The sandpaper result of 7.9 N is likely to be anomalous because it is much higher than the other sandpaper readings. The student may have jerked the force meter or the block may have caught on the surface. A good scientific response is to repeat the sandpaper test. If the new result is close to 4.8 N and 5.0 N, the anomalous 7.9 N should not be used in the mean.

Mean for sandpaper without the anomaly:

4.8 N + 5.0 N = 9.8 N

9.8 N / 2 = 4.9 N

Conclusion

The rougher surfaces needed a larger pulling force. Smooth plastic had the lowest mean force, 1.2 N. Sandpaper had the highest mean force, about 4.9 N when the anomalous result was ignored. This supports the prediction that rougher surfaces produce more friction.

Evaluation

The method could be improved by:

• repeating each surface more times
• using a data logger or force sensor to reduce reading errors
• pulling with a more constant speed
• checking the force meter starts at zero
• replacing anomalous results with repeat readings
• using surfaces of the same length and keeping them flat

The results are more reliable if repeated readings are close together. They are more repeatable if another student can follow the same method and get similar results.

Data and Graph Skills

Air Resistance and Speed

The table shows how air resistance changes as the speed of a model car increases in a wind tunnel.

Speed (m/s)	Air resistance (N)
0	0
2	1
4	4
6	9
8	16
10	25

Simple bar chart:

Air resistance (N)
25 |                         █
20 |                         █
15 |                    █    █
10 |               █    █    █
 5 |          █    █    █    █
 0 | █   █    █    █    █    █
     0   2    4    6    8   10   Speed (m/s)

Pattern: as speed increases, air resistance increases. The increase is not equal each time. At higher speeds, air resistance rises more quickly.

Explanation: a faster object collides with more air particles each second, so the force of air resistance is greater. Shape and surface area also affect air resistance.

Limitation: the graph uses simplified data from a model. Real cars may have different shapes, surfaces, and wheel friction, so the exact values may be different.

Diagram Interpretation

Study this force diagram.

             Air resistance = 40 N
                      <----
                  [ cyclist ] ---->
             Driving force = 90 N

Questions:

1. Which force is larger?
2. What is the resultant force?
3. Which direction is the resultant force?
4. What is likely to happen to the cyclist's speed?

Model answers:

1. The driving force is larger because 90 N is greater than 40 N.
2. 90 N - 40 N = 50 N.
3. The resultant force is 50 N to the right.
4. The cyclist will speed up to the right if the cyclist is free to move that way.

Now study this diagram.

           Water resistance = 70 N
                     <----
                  [ boat ] ---->
               Engine force = 70 N

The forces are equal and opposite, so the resultant force is 0 N. If the boat is already moving, it continues at steady speed in a straight line.

Common Misconceptions

Misconception	Correct scientific idea
Moving objects always need a forward force to keep moving.	If forces are balanced, an object can continue moving at steady speed in a straight line.
Balanced forces mean an object must be stationary.	Balanced forces can mean stationary or moving at steady speed in a straight line.
A larger object always has a larger force acting on it in every situation.	Force depends on the interaction, not only object size.
Gravity only acts when objects are falling.	Gravity acts on objects all the time near Earth, including when they rest on a surface.
Weight and mass are the same thing.	Mass is the amount of matter in kg. Weight is a force in N caused by gravity.
Friction always stops objects immediately.	Friction opposes motion, but the effect depends on the size of the forces.
Friction is always bad.	Friction is useful for walking, writing, gripping, braking, and many sports.
The arrow in a force diagram shows where the object is moving.	A force arrow shows the direction of a force, not necessarily the direction of motion.
The biggest individual force is always the resultant force.	The resultant force is the overall force after combining all forces.
If the resultant force is zero, there are no forces acting.	Forces may still act, but they cancel because they are equal and opposite.
Air resistance only acts upwards.	Air resistance acts opposite to motion through air.
Non-contact forces are not real because objects do not touch.	Gravity, magnetic forces, and electrostatic forces can act at a distance.

Real-World Examples

Book on a Table

The book's weight acts downwards. The table's normal contact force acts upwards. If both forces are 8 N, the resultant force is 0 N. The book stays still.

Tug-of-War

Two teams pull in opposite directions. If Team A pulls with 300 N and Team B pulls with 260 N, the resultant force is 40 N towards Team A. If both teams pull with equal force, the rope may remain still because the forces are balanced.

Cyclist

A cyclist starting from rest needs a forward resultant force. At steady speed, the forward driving force is balanced by friction and air resistance. When braking, the backward forces are larger than the forward force, so the cyclist slows down.

Swimmer

A swimmer pushes water backwards. The water pushes the swimmer forwards. Water resistance acts backwards, opposing the swimmer's motion. Streamlined body position helps reduce water resistance.

Car at Steady Speed

A car can move at steady speed even though forces act on it. The engine provides a forward driving force. Friction and air resistance act backwards. At steady speed in a straight line, these forces are balanced.

Resultant Force Calculation Practice

For each question, give the size and direction of the resultant force.

Situation	Working	Answer
Push 30 N right, friction 10 N left	30 N - 10 N	20 N right
Pull 15 N left, pull 15 N right	15 N - 15 N	0 N
Push 40 N right, extra push 25 N right	40 N + 25 N	65 N right
Team A 500 N left, Team B 450 N right	500 N - 450 N	50 N left
Engine 200 N forwards, resistance 120 N backwards	200 N - 120 N	80 N forwards
Weight 12 N down, upthrust 8 N up	12 N - 8 N	4 N down

Remember: if the resultant force is not zero, include a direction.

Motion Explanation Task

For each scenario, decide whether the forces are balanced or unbalanced, then explain the motion.

Scenario	Balanced or unbalanced?	Explanation
A bus accelerates away from a stop.	Unbalanced	The forward driving force is larger than the backward resistance forces, so the bus speeds up.
A skateboard slows down on a rough path.	Unbalanced	Friction acts backwards and gives a resultant force opposite the motion, so the skateboard slows down.
A boat moves at steady speed in a straight line.	Balanced	The engine force forwards equals water resistance backwards, so the resultant force is 0 N.
A ball sits still on the floor.	Balanced	Weight downwards is balanced by the normal contact force upwards.
A cyclist turns a corner.	Unbalanced sideways	A sideways resultant force changes the cyclist's direction.

Exam-Style Questions

Multiple-Choice Questions

1. What is the unit of force? A. kilogram
   B. metre
   C. Newton
   D. second

2. Which force is non-contact? A. friction
   B. tension
   C. gravity
   D. water resistance

3. A box has a push of 50 N right and friction of 20 N left. What is the resultant force? A. 70 N right
   B. 30 N right
   C. 30 N left
   D. 0 N

4. Which statement about balanced forces is correct? A. Balanced forces mean no forces are acting.
   B. Balanced forces always make an object stop instantly.
   C. Balanced forces give a resultant force of 0 N.
   D. Balanced forces only happen in space.

5. Air resistance acts: A. always upwards
   B. opposite to motion through air
   C. only when an object is stationary
   D. in the same direction as gravity

6. Which pair is correct? A. mass in N, weight in kg
   B. mass in kg, weight in N
   C. mass in metres, weight in seconds
   D. mass and weight both in N

Short-Answer Questions

1. Define a force.
2. Give two examples of contact forces.
3. Give two examples of non-contact forces.
4. Explain why weight is a force.
5. Why does a parachute increase air resistance?
6. What does a longer arrow mean in a force diagram?
7. A rope is pulled with 80 N left and 80 N right. What is the resultant force?
8. Explain why a moving car at steady speed can have balanced forces.

Fill-the-Blank Questions

Use these words: Newtons, gravity, friction, resultant, balanced, direction, weight, mass.

1. Force is measured in ________.
2. The force of ________ pulls objects towards the centre of the Earth.
3. ________ is the amount of matter in an object.
4. ________ is the force of gravity on an object.
5. ________ opposes sliding between surfaces.
6. The ________ force is the overall force after combining forces.
7. Forces have size and ________.
8. Forces are ________ when the resultant force is 0 N.

Calculation Questions

1. A trolley is pushed with 35 N to the right and 20 N to the right. Calculate the resultant force.
2. A box is pushed with 60 N to the right. Friction is 25 N to the left. Calculate the resultant force.
3. A swimmer produces a forward force of 90 N. Water resistance is 90 N backwards. What is the resultant force?
4. A parachutist has weight 700 N downwards and air resistance 500 N upwards. Calculate the resultant force.
5. A toy car has an engine force of 12 N forwards and friction of 4 N backwards. Calculate the resultant force.
6. Two students pull a rope. One pulls 140 N left and the other pulls 110 N right. Calculate the resultant force.

Diagram Drawing Questions

1. Draw a force diagram for a book resting on a table. Label weight and normal contact force.
2. Draw a force diagram for a box being pushed to the right while friction acts to the left.
3. Draw a force diagram for a falling ball with weight and air resistance.
4. Draw a force diagram for a tug-of-war where the left team pulls harder than the right team.

Data Interpretation Questions

Use the friction investigation results table.

1. Which surface had the lowest mean force?
2. Which surface had the greatest friction?
3. Which result is anomalous?
4. Suggest one reason for the anomalous result.
5. Explain why repeats are useful.
6. Calculate the mean for wood.

Use the air resistance table.

1. What happens to air resistance as speed increases?
2. Quote two values from the table to support your answer.
3. Explain why faster objects have greater air resistance.
4. Give one limitation of the data.

Practical Planning Question

A student wants to test the question: "How does surface type affect the force needed to move a block?"

Write a method for the investigation. Include:

• independent variable
• dependent variable
• at least three control variables
• how to measure the force
• how to improve reliability
• one safety point

Longer 6-8 Mark Question

Explain how the forces on a cyclist change when the cyclist starts moving, travels at steady speed, and then brakes. Use the terms resultant force, balanced forces, unbalanced forces, friction, and air resistance.

Model Answers

Multiple-Choice Answers

1. C. The unit of force is the Newton, written as N.
2. C. Gravity is a non-contact force because it acts without objects touching.
3. B. 50 N - 20 N = 30 N to the right.
4. C. Balanced forces give a resultant force of 0 N.
5. B. Air resistance acts opposite to motion through air.
6. B. Mass is measured in kg and weight is measured in N.

Short-Answer Model Answers

1. A force is a push, pull, twist, stretch, or squeeze that can change an object's speed, direction, or shape.
2. Friction and tension are contact forces. Other correct examples include air resistance, water resistance, normal contact force, upthrust, and applied push or pull.
3. Gravity and magnetic force are non-contact forces. Electrostatic force is another correct example.
4. Weight is a force because it is the pull of gravity on an object. It is measured in Newtons.
5. A parachute has a large surface area, so it pushes against more air and increases air resistance.
6. A longer arrow represents a larger force.
7. The resultant force is 0 N because the forces are equal and opposite.
8. A moving car at steady speed has balanced forces because the forward driving force equals the backward forces from friction and air resistance. The resultant force is 0 N, so the car continues at steady speed in a straight line.

Fill-the-Blank Answers

1. Newtons
2. gravity
3. mass
4. weight
5. friction
6. resultant
7. direction
8. balanced

Calculation Model Answers

1. The forces act in the same direction, so add them: 35 N + 20 N = 55 N to the right.
2. The forces act in opposite directions, so subtract: 60 N - 25 N = 35 N to the right.
3. 90 N - 90 N = 0 N. The forces are balanced.
4. 700 N - 500 N = 200 N downwards.
5. 12 N - 4 N = 8 N forwards.
6. 140 N - 110 N = 30 N left.

Data Interpretation Model Answers

The smooth plastic surface had the lowest mean force, 1.2 N. The sandpaper surface had the greatest friction because it needed the largest pulling force, about 4.9 N when the anomalous result was ignored. The anomalous result is 7.9 N for sandpaper because it is much higher than the other sandpaper readings. This may have happened because the block caught on the surface or the student pulled with a sudden jerk. Repeats are useful because they make the results more reliable and help identify anomalies.

Mean for wood:

2.0 N + 2.1 N + 2.2 N = 6.3 N

6.3 N / 3 = 2.1 N

For the air resistance data, air resistance increases as speed increases. For example, at 2 m/s the air resistance is 1 N, but at 10 m/s it is 25 N. Faster objects have greater air resistance because they collide with more air particles each second. One limitation is that the data is simplified and may not match every real vehicle or object shape.

Practical Planning Model Answer

The independent variable is the surface type. The dependent variable is the force needed to pull the block, measured in Newtons using a force meter. Control variables include the same block, same added mass, same pulling distance, same force meter, and the same pulling speed.

Place the block on smooth plastic and attach a force meter. Pull the block steadily at a slow, constant speed and record the force in N. Repeat three times. Repeat the same method for wood, carpet, and sandpaper. Calculate a mean force for each surface and compare the results. To improve reliability, repeat readings and replace anomalous results with extra repeats. A safety point is to pull the force meter steadily and not release it suddenly.

Longer Question Model Answer

When the cyclist starts moving, the forward driving force from pedalling is larger than the backward forces from friction and air resistance. The forces are unbalanced, so there is a forward resultant force. This makes the cyclist speed up.

When the cyclist travels at steady speed in a straight line, the forward driving force is equal to the backward forces from friction and air resistance. The forces are balanced and the resultant force is 0 N. This does not mean the cyclist must stop. It means the cyclist continues moving at steady speed in a straight line.

When the cyclist brakes, the forward driving force becomes smaller and the backward forces become larger. Friction from the brakes and tyres acts backwards, and air resistance also acts backwards. The forces are unbalanced and the resultant force is backwards, opposite to the cyclist's motion. This makes the cyclist slow down.

A strong answer uses the terms balanced forces, unbalanced forces, resultant force, friction, and air resistance correctly, and links each force situation to the cyclist's motion.

Revision Checklist

Use this checklist before a quiz or test.

• I can define a force as a push, pull, twist, stretch, or squeeze.
• I know forces are measured in Newtons, written as N.
• I can explain that forces have size and direction.
• I can classify forces as contact or non-contact.
• I can give examples of friction, air resistance, water resistance, normal contact force, tension, upthrust, gravity, magnetic force, and electrostatic force.
• I can explain the difference between mass and weight.
• I know mass is measured in kg and weight is measured in N.
• I can explain gravity as a non-contact force.
• I can explain friction and give useful and unwanted examples.
• I can describe how air resistance and water resistance oppose motion.
• I can draw force arrows with correct direction, labels, and relative sizes.
• I can calculate resultant forces in one straight line.
• I can state the direction of a resultant force.
• I can explain balanced forces and unbalanced forces.
• I know balanced forces can apply to stationary objects or objects moving at steady speed in a straight line.
• I can explain how forces make objects speed up, slow down, change direction, or change shape.
• I can interpret force diagrams.
• I can identify independent, dependent, and control variables in a friction investigation.
• I can calculate a mean from repeat readings.
• I can recognise an anomalous result and suggest repeating it.
• I can describe a trend in a table or graph and use values as evidence.
• I can evaluate a practical method and suggest improvements.

Quick Retrieval Quiz

1. What symbol is used for Newtons?
2. Is gravity a contact or non-contact force?
3. What force opposes motion through air?
4. What force opposes motion through water?
5. What is the resultant force when two equal opposite forces act?
6. What does a force arrow show?
7. What happens to a stationary object if forces are balanced?
8. What happens to a moving object if forces are balanced?
9. Why is friction useful when walking?
10. Why does a parachute slow a person down?

Answers:

1. N
2. Non-contact
3. Air resistance
4. Water resistance
5. 0 N
6. The direction of a force, and relative size if arrows are drawn to scale
7. It remains stationary
8. It continues at steady speed in a straight line
9. It provides grip between shoes and the ground
10. It increases surface area and therefore increases air resistance