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KS3 Science · Year 8Chapter 13 · Forces and motion
Year 8 · Chapter 13

Forces and motion

By the end of this chapter you will be able to

  • Sort forces into contact and non-contact forces, and name the unit of force
  • Find the resultant of forces acting along one line, and say whether they are balanced
  • Explain friction and air resistance, and say what they do to motion
  • State Newton’s first law, and apply it to simple cases
  • Calculate average speed, and read a distance-time graph

You should already know

  • Speed, distance and time (Year 7 Chapter 11)
  • Weight and mass (Year 7 Chapter 11)

Where this leads

  • Further motion and energy in later science work

National Curriculum for England, Key Stage 3 science, Year 8. Content: 8P6 contact and non-contact forces; resultant force; balanced and unbalanced forces; friction and air resistance, 8P7 Newton’s first law (basic): an object stays at rest or keeps moving at a constant velocity unless an unbalanced force acts, 8P8 speed as distance over time, average speed and motion in simple cases, with distance-time graphs.

Exercise P  Do you remember?

1
FWhat is the unit of mass in the SI system? Write the word.
2
FWhat is 150 divided by 5? Write the number.
3
FWhat is the word for the distance travelled divided by the time taken? Write the word.
4
FWhat is 12 plus 9 plus 4? Write the number.
5
FWhat is the name of the force that pulls objects towards the Earth? Write the word.
13.1

Contact and non-contact forces

Year 8
8P6: contact forces (friction, air resistance, normal reaction) and non-contact forces (gravity, magnetic and electrostatic forces)
Video lesson: Contact and non-contact forcesComing soon
Story. Sam pushes a trolley across the lab floor, and it slows down and stops. “Something is pushing back,” he says. His teacher says, “That is friction, a contact force. Gravity pulls the trolley down without touching it; that is a non-contact force.”

A force is a push or a pull. It is measured in newtons (N), and it is a vector: it has a size and a direction. Contact forces act when two objects touch. Non-contact forces act at a distance, without touching.

The table sorts six forces into contact and non-contact.

ForceTypeWhere it comes from
frictioncontacttwo surfaces rubbing
air resistancecontactair particles hitting a moving object
normal reactioncontacta surface pushing back on an object
weightnon-contactgravity pulling an object down
magnetic forcenon-contacta magnet pulling a magnetic material
electrostatic forcenon-contactcharged objects attracting or repelling

Key ideas

  • A force is a push or a pull, measured in newtons (N).
  • Contact forces need the objects to touch.
  • Non-contact forces act at a distance, such as gravity and magnetism.
  • Friction, air resistance and the normal reaction are contact forces.
Worked example 13.1.1
Is friction a contact force or a non-contact force? Write the words.

Method. Check whether the objects touch.

  1. Think. Friction acts between two surfaces that are touching.
    touching
  2. Name it. So friction is a contact force.
    contact force
  3. Write the answer.
    contact force

Check. Friction opposes motion. ✓

Worked example 13.1.2
Is gravity a contact force or a non-contact force? Write the words.

Method. Check whether the objects touch.

  1. Think. Gravity pulls a falling object without touching it.
    without touching
  2. Name it. So gravity is a non-contact force.
    non-contact force
  3. Write the answer.
    non-contact force

Check. Weight is caused by gravity. ✓

Worked example 13.1.3
What is the unit of force? Write the word.

Method. Recall the unit.

  1. Recall. Force is measured in newtons.
    newtons
  2. Name it. Its symbol is N.
    N
  3. Write the answer.
    newton (N)

Check. Named after Isaac Newton. ✓

Watch out

Watch out: a force is a push or a pull, not a speed. An object can have a large force acting and still be at rest, if the forces balance.

Exam tip

Exam tip. Say whether the objects touch: yes gives a contact force; no gives a non-contact force.

Try it (with your teacher, or a grown-up at home). Look at the force table. Say whether each force needs the objects to touch. Do not push or pull anything hard; work from the table.

Exercise 13A  Contact and non-contact forces

1
FWhat is the unit of force? Write the symbol.
2
FWhat is the name of a quantity that has both a size and a direction? Write the word.
3
FIs air resistance a contact force? Write yes or no.
4
FIs the magnetic force a contact force? Write yes or no.
5
FIs the electrostatic force a non-contact force? Write yes or no.
6
FName a non-contact force that pulls objects towards the Earth. Write the word.
7
FName the contact force that a surface exerts to hold up an object resting on it. Write the words.
8
FIs friction a force that acts between two surfaces that touch? Write yes or no.
9
FName one force from the table that acts only at a distance and does not touch. Write the word.
10
PSAli says that gravity is a contact force because the Earth touches the object. Is Ali right? Write yes or no.
11
PSWorking scientifically: a student says that a magnet pulling a paperclip is a contact force. Is the student right? Write yes or no.
12
RExplain why friction is called a contact force. Write one sentence.
13
RChallenge: explain why a magnet can lift a paperclip without touching the other magnet pole that is far away. Write one sentence.
13.2

Resultant force and balanced forces

Year 8
8P6: resultant force for forces along one line; balanced and unbalanced forces
Video lesson: Resultant force and balanced forcesComing soon
Story. Ruby pushes a trolley to the right with 30 N, and a friend pulls it back to the left with 20 N. “Which way does it go?” she asks. Her teacher says, “Add the forces that act along the same line, taking one direction as positive. That gives the resultant force.”

The resultant force is the single force that has the same effect as all the forces acting on an object. For forces along one line, add them with the direction taken as positive. If the resultant is zero, the forces are balanced. If the resultant is not zero, the forces are unbalanced, and the object accelerates in the direction of the resultant.

trolley30 N20 Nresultant = 30 N - 20 N = 10 N to the right
A trolley with a 30 N push to the right and a 20 N pull to the left.
at restbalanced: equal and opposite forces, no change in motion
A block at rest with two equal and opposite forces: balanced.

The table gives the resultant force for three pairs of forces on an object. Right is positive.

Forces on an objectResultant force (N)
push 30 N to the right, pull 20 N to the left10
push 15 N to the right, friction 15 N to the left0
pull 50 N to the right, friction 20 N to the left30

Key ideas

  • Resultant force: the single force with the same effect as all the forces.
  • Forces along one line: add them, with one direction positive.
  • Zero resultant means balanced forces.
  • A non-zero resultant means unbalanced forces, and the object accelerates.
Worked example 13.2.1
A push of 30 N to the right and a pull of 20 N to the left act on a trolley. What is the resultant force? Write the number of newtons and its direction.

Method. Take right as positive and add.

  1. Set up. Right is positive: +30 N and -20 N.
    +30 and -20
  2. Add. The resultant is 30 minus 20 = 10 N.
    30 - 20 = 10
  3. Direction. The resultant is 10 N to the right.
    10 N to the right
  4. Write the answer.
    10 N to the right

Check. 10 N to the right. ✓

Worked example 13.2.2
Two forces of 15 N each act on a block, in opposite directions. Are the forces balanced? Write yes or no.

Method. Find the resultant.

  1. Calculate. The forces are equal and opposite: 15 minus 15.
    15 - 15 = 0
  2. Decide. A resultant of zero means the forces are balanced; the answer is yes.
    yes
  3. Write the answer.
    yes

Check. Balanced forces give no change in motion. ✓

Worked example 13.2.3
A resultant force of 30 N acts on an object. Are the forces balanced? Write yes or no.

Method. Check the resultant.

  1. Check. A resultant of 30 N is not zero.
    not zero
  2. Decide. So the forces are unbalanced; the answer is no.
    no
  3. Write the answer.
    no

Check. Unbalanced forces change the motion. ✓

Watch out

Watch out: a resultant of zero does not mean there are no forces. It means the forces are balanced, and there is no change in motion.

Exam tip

Exam tip. Choose a positive direction, write each force with its sign, then add. Give the direction with the answer.

Try it (with your teacher, or a grown-up at home). Pull a book across a table with a grown-up. Note the forces you feel, and say which direction the resultant is. Do not push or pull anything hard or fast.

Exercise 13B  Resultant force and balanced forces

1
FWhat do we call the single force with the same effect as all the forces on an object? Write the words.
2
FWhen the resultant force is zero, are the forces balanced or unbalanced? Write the word.
3
FWhat is the resultant of 20 N to the right and 20 N to the left? Write the number of newtons.
4
FWhat is the resultant of 40 N to the right and 15 N to the right? Write the number of newtons.
5
FWhat is the resultant of 50 N to the right and 20 N to the left? Write the number of newtons.
6
FWhat is the direction of the resultant of 50 N to the right and 20 N to the left? Write the word: left or right.
7
FIs a resultant of 10 N unbalanced? Write yes or no.
8
FUsing the table, what is the resultant when the trolley is pushed 15 N right and has 15 N friction left? Write the number of newtons.
9
FUsing the table, what is the resultant of 50 N right and 20 N friction left? Write the number of newtons.
10
PSAli says that two forces of 10 N each, in the same direction, have a resultant of 0 N. Is Ali right? Write yes or no.
11
PSWorking scientifically: a pupil says that an object at rest has no forces on it. Is the pupil right? Write yes or no.
12
RExplain why a trolley at rest on a table can have forces acting on it but still not move. Write one sentence.
13
RChallenge: forces of 25 N right, 10 N left and 5 N left act on a block. What is the resultant, and in which direction? Write the number of newtons and the direction.
13.3

Friction and air resistance

Year 8
8P6: friction opposes motion between surfaces; air resistance opposes motion through a fluid
Video lesson: Friction and air resistanceComing soon
Story. Omar rolls a ball along a smooth floor, and it goes a long way. He rolls it along a carpet, and it stops quickly. “Why the difference?” he asks. His teacher says, “The carpet makes more friction. Friction always acts in the opposite direction to the motion.”

Friction is the force that opposes the motion of two surfaces sliding over each other. It acts in the direction opposite to the motion, and it transfers energy to the thermal store. Rougher surfaces give more friction.

Air resistance is the friction-like force that opposes an object moving through air. It increases as the speed increases. When the air resistance balances the weight, the object reaches a steady speed called its terminal velocity.

motionfriction opposes motion
A block sliding to the right, with friction acting to the left and opposing the motion.

Key ideas

  • Friction opposes motion between two surfaces.
  • Friction acts opposite to the direction of motion.
  • Air resistance opposes motion through air and increases with speed.
  • When air resistance balances weight, the object reaches terminal velocity.
Worked example 13.3.1
In which direction does friction act on a sliding block that is moving to the right? Write the word: left or right.

Method. Think about opposing motion.

  1. Think. Friction opposes the motion, so it acts against the direction of motion.
    opposes
  2. Decide. So friction acts to the left.
    left
  3. Write the answer.
    left

Check. Friction always opposes the motion. ✓

Worked example 13.3.2
Does air resistance increase or decrease as an object moves faster? Write the word.

Method. Recall the effect of speed.

  1. Recall. Air resistance increases as the speed of the object increases.
    increases
  2. Decide. So it increases.
    increase
  3. Write the answer.
    increase

Check. A faster object pushes more air. ✓

Worked example 13.3.3
What is the name for the steady speed reached when air resistance balances weight? Write the words.

Method. Recall the name.

  1. Recall. When the forces balance, the object reaches a steady speed.
    steady speed
  2. Name it. This steady speed is the terminal velocity.
    terminal velocity
  3. Write the answer.
    terminal velocity

Check. Balanced forces give a constant speed. ✓

Watch out

Watch out: friction does not always slow an object down. Friction is what allows a car tyre to grip the road and accelerate.

Exam tip

Exam tip. Say friction and air resistance both oppose the motion, and that air resistance increases with speed.

Try it (with your teacher, or a grown-up at home). Roll a small ball on a smooth table, and then on a rough cloth, with a grown-up. Note which goes further. Explain the difference in one sentence using friction.

Exercise 13C  Friction and air resistance

1
FWhat is the force that opposes the sliding of two surfaces over each other? Write the word.
2
FWhat is the force that opposes the motion of an object moving through air? Write the words.
3
FIn which direction does friction act, compared with the motion? Write the word: same or opposite.
4
FDoes air resistance increase or decrease as the speed increases? Write the word.
5
FWhat is the steady speed reached when air resistance balances weight? Write the words.
6
FDoes a rough surface give more friction than a smooth surface? Write yes or no.
7
FDoes friction transfer energy to the thermal store? Write yes or no.
8
FIs friction always bad for motion? Write yes or no.
9
FA block slides to the right. Which way does friction act on it: left or right? Write the word.
10
PSAli says that a parachutist reaches terminal velocity when the air resistance is greater than the weight. Is Ali right? Write yes or no.
11
PSWorking scientifically: a pupil compares a ball on a carpet and a smooth floor. Name one variable to keep the same. Accept any one: the ball, the starting push, or the starting point.
12
RExplain why a parachute increases the air resistance on a falling person. Write one sentence.
13
RChallenge: explain why a falling object stops accelerating once it reaches terminal velocity. Write one sentence.
13.4

Newton’s first law

Year 8
8P7: Newton’s first law (basic): an object stays at rest, or keeps moving at a constant velocity, unless an unbalanced force acts on it
Video lesson: Newton’s first lawComing soon
Story. Sam is on a bus that brakes suddenly, and he lurches forward. “Why does my body keep going?” he asks. His teacher says, “Your body keeps moving at the same velocity until a force acts on it. That is Newton’s first law, and it is why seat belts are needed.”

Newton’s first law says that an object stays at rest, or keeps moving at a constant velocity, unless an unbalanced force acts on it. This tendency to keep the same motion is called inertia.

Velocity is speed in a given direction. A constant velocity means both the speed and the direction stay the same. An unbalanced force changes the velocity.

Key ideas

  • An object at rest stays at rest unless an unbalanced force acts.
  • A moving object keeps its velocity unless an unbalanced force acts.
  • Inertia is the tendency to keep the same motion.
  • An unbalanced force changes the velocity.
Worked example 13.4.1
A book lies still on a table. Is the resultant force on it zero? Write yes or no.

Method. Apply the first law.

  1. Think. The book is at rest, so by the first law the forces on it are balanced.
    balanced
  2. Decide. So the resultant is zero; the answer is yes.
    yes
  3. Write the answer.
    yes

Check. Weight and the table push are balanced. ✓

Worked example 13.4.2
A trolley moves at a constant velocity on a smooth surface with no forces. Does its velocity change? Write yes or no.

Method. Apply the first law.

  1. Think. With no unbalanced force, the first law says the velocity stays the same.
    stays the same
  2. Decide. So the velocity does not change; the answer is no.
    no
  3. Write the answer.
    no

Check. No unbalanced force means no change. ✓

Worked example 13.4.3
What is the name of the tendency of an object to keep its motion? Write the word.

Method. Recall the word.

  1. Recall. The tendency of an object to keep its motion is inertia.
    inertia
  2. Name it. So the word is inertia.
    inertia
  3. Write the answer.
    inertia

Check. Inertia is related to mass. ✓

Watch out

Watch out: a moving object does not need a force to keep moving. It needs an unbalanced force to change its velocity.

Exam tip

Exam tip. State the law: an object stays at rest or at constant velocity unless an unbalanced force acts on it.

Try it (with your teacher, or a grown-up at home). Put a book on a smooth sheet of paper on a table. Gently pull the paper sideways and watch what happens to the book. Explain in one sentence using the first law. Do not pull anything hard or fast.

Exercise 13D  Newton’s first law

1
FWhat is the name of the law that says an object stays at rest or at constant velocity unless acted on by an unbalanced force? Write the words.
2
FWhat is the tendency of an object to keep its motion called? Write the word.
3
FWhat must act on a moving object for its velocity to change? Write the words.
4
FA stationary object has balanced forces on it. Does it stay at rest? Write yes or no.
5
FDoes a moving object with balanced forces keep moving at the same speed? Write yes or no.
6
FDoes a braking bus change its velocity? Write yes or no.
7
FIs a seat belt useful in a sudden stop, because of inertia? Write yes or no.
8
FAn object at rest has no forces acting on it. Is this consistent with the first law? Write yes or no.
9
PSRuby says that a moving object needs a force to keep moving at constant velocity. Is Ruby right? Write yes or no.
10
PSWorking scientifically: a trolley is pushed and then left alone on a smooth floor. It slows down. What is the most likely reason? Write the word: friction.
11
RExplain why passengers lurch forward when a bus brakes. Write one sentence.
12
RChallenge: explain why a heavier object needs a bigger force to get it moving from rest than a lighter one. Write one sentence.
13.5

Speed, distance and time

Year 8
8P8: speed as distance over time; average speed for a journey with stops; distance-time graphs
Video lesson: Speed, distance and timeComing soon
Story. Omar walks to a bus stop, waits, and then rides the bus. “What is my average speed for the whole journey?” he asks. His teacher says, “Add up all the distance, and all the time, including the wait. Divide the total distance by the total time.”

Speed is the distance travelled divided by the time taken: speed = distance / time. It is measured in metres per second (m/s) or kilometres per hour (km/h). The average speed of a journey is the total distance divided by the total time, including any stops.

On a distance-time graph, a steeper line means a higher speed, and a flat horizontal line means the object is stopped.

The table gives the stages of Omar’s journey. The stop takes 10 seconds and covers no distance.

StageDistance (m)Time (s)
walk to the bus stop10020
wait at the stop010
ride on the bus6010
Distance-time graph of the journey (example data)01020304004080120160Time (s)Distance (m)
Distance-time graph: a rise to 100 m, a flat section for the stop, then a rise to 160 m.

Key ideas

  • Speed = distance / time.
  • Average speed = total distance / total time, including stops.
  • A steeper distance-time line means a higher speed.
  • A flat line on a distance-time graph means the object is stopped.
Worked example 13.5.1
What is the total distance of Omar’s journey? Write the number of metres.

Method. Add the distances of each stage.

  1. Add. The distances are 100 m, 0 m and 60 m.
    100 + 0 + 60
  2. Calculate. The total distance is 160 m.
    160
  3. Write the answer.
    160 m

Check. The stop covers no distance. ✓

Worked example 13.5.2
What is the total time of the journey, including the stop? Write the number of seconds.

Method. Add the times of each stage.

  1. Add. The times are 20 s, 10 s and 10 s.
    20 + 10 + 10
  2. Calculate. The total time is 40 s.
    40
  3. Write the answer.
    40 s

Check. The wait counts in the time. ✓

Worked example 13.5.3
What is the average speed of the whole journey? Write the number of metres per second.

Method. Divide the total distance by the total time.

  1. Divide. Average speed = 160 m divided by 40 s.
    160 / 40
  2. Calculate. The average speed is 4 m/s.
    160 / 40 = 4
  3. Write the answer.
    4 m/s

Check. 40 x 4 = 160. ✓

Watch out

Watch out: average speed is total distance divided by total time. It is not the average of the speeds of the stages, and it includes the time stopped.

Exam tip

Exam tip. Write the formula, then substitute the totals. Give the unit with the answer.

Try it (with your teacher, or a grown-up at home). Walk across a room, and time yourself with a watch or timer, while a grown-up measures the distance. Work out your speed in metres per second. Do not run near furniture or others.

Exercise 13E  Speed, distance and time

1
FWhat is the formula for speed? Write the words.
2
FWhat is the unit of speed in the SI system? Write the unit.
3
FA cyclist travels 80 m in 20 s. What is the speed? Write the number of metres per second.
4
FA bus travels 300 m in 30 s. What is its speed? Write the number of metres per second.
5
FWhat is the total distance in Omar’s journey? Write the number of metres.
6
FWhat is the total time of Omar’s journey, including the stop? Write the number of seconds.
7
FOn a distance-time graph, what does a flat horizontal line show? Write the words.
8
FA steeper line on a distance-time graph means a higher speed. Is this true? Write yes or no.
9
FUsing the graph, what distance is covered from 0 to 20 seconds? Write the number of metres.
10
PSAli says the average speed of Omar’s journey is the same as the speed on the bus. Is Ali right? Write yes or no.
11
PSWorking scientifically: a runner covers 90 m in 15 s at a steady speed. What is the speed, in m/s? Write the number.
12
RExplain why the average speed of a journey can be lower than the speed on the fastest stage. Write one sentence.
13
RChallenge: a car travels 120 m in 10 s, then stops for 5 s, then travels 60 m in 5 s. What is its average speed, in m/s? Write the number.

Chapter summary

SkillMethod
ForcesPush or pull, in newtons. Contact (friction, air resistance, normal reaction) or non-contact (gravity, magnetism, electrostatic).
ResultantAdd forces along one line with signs; zero means balanced; non-zero means unbalanced.
Friction and air resistanceOppose motion; air resistance grows with speed; terminal velocity when it balances weight.
First lawRest or constant velocity unless an unbalanced force acts; inertia.
SpeedSpeed = distance / time; average speed = total distance / total time; steeper graph means faster.

Can you…?

  • I can sort forces into contact and non-contact forces, and name the unit of force
  • I can find the resultant of forces along one line, and say whether they are balanced
  • I can explain friction and air resistance, and what they do to motion
  • I can state Newton’s first law and apply it to simple cases
  • I can calculate average speed, and read a distance-time graph

Key terms

force
a push or a pull, measured in newtons
contact force
a force that acts only when the objects touch
non-contact force
a force that acts at a distance, without touching
resultant force
the single force with the same effect as all the forces acting
balanced forces
forces whose resultant is zero, so there is no change in motion
friction
a force that opposes the sliding of surfaces
terminal velocity
the steady speed reached when air resistance balances weight
inertia
the tendency of an object to keep its state of rest or motion
average speed
total distance divided by total time, including stops

Exercise 13Z  Check-up

1
What is the unit of force? Write the symbol.
2
Is gravity a contact or non-contact force? Write the words.
3
What is the resultant of 40 N to the right and 15 N to the left? Write the number of newtons.
4
When the resultant force is zero, are the forces balanced? Write yes or no.
5
What is the force that opposes motion through air called? Write the words.
6
What is the steady speed when air resistance balances weight? Write the words.
7
What is the name of the law that describes an object staying at rest or moving at constant velocity? Write the words.
8
A cyclist travels 80 m in 20 s. What is the speed, in m/s? Write the number.
9
What is the average speed of a journey of 200 m in 50 s? Write the number of metres per second.
10
Explain why a passenger moves forward when a car brakes suddenly. Write one sentence.

Total 15 marks. Allow about 15 minutes.

Exercise 13X  Practice test questions

1
FWhat is the resultant of a 30 N push to the right and a 30 N friction force to the left? Write the number of newtons.[1 mark]
2
FName a non-contact force. Write the word.[1 mark]
3
FName the force that opposes the motion of two surfaces sliding over each other. Write the word.[1 mark]
4
RExplain what Newton’s first law says about a moving object with no unbalanced force. Write one sentence.[2 marks]
5
PSA trolley is pulled 50 N to the right and friction is 20 N to the left. What is the resultant force? Write the number of newtons.[2 marks]
6
FA runner covers 100 m in 20 s. What is the speed, in m/s? Write the number.[1 mark]
7
FOn a distance-time graph, what does a steeper line show? Write the words.[1 mark]
8
RExplain why a parachute reaches a lower terminal velocity than a falling stone. Write one sentence.[2 marks]
9
PSA journey covers 60 m in 10 s, then stops for 10 s. What is the average speed for the whole journey, in m/s? Write the number.[2 marks]
10
PSAli says a stationary trolley with forces of 10 N right and 10 N left has an unbalanced force. Is Ali right? Write yes or no.[2 marks]
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