Senior 1 practice book · Unit 4 of 9

Newton's Laws of Motion

50 questions that complete the Senior 1 quiz for this unit: 26 core and 24 challenge. Easy and hard questions are mixed. Try each question, then tap “Show answer”.

Common misconceptions

A moving object needs a force to keep it moving.By Newton's first law an object keeps moving at constant velocity unless a resultant force acts. Things slow down on Earth because friction and air resistance act on them.
Action and reaction cancel, so nothing can ever move.The two forces of a third-law pair act on DIFFERENT objects, so they never cancel each other. Each object moves according to the resultant force on itself.
Heavier objects fall faster.Without air resistance every object falls with the same acceleration g. A bigger weight is matched by a bigger mass to accelerate, so a = W/m = g for all.
In a collision the bigger vehicle pushes harder on the smaller one.By the third law the forces are equal and opposite. The small car is damaged more and accelerates more because its mass is smaller, not because the force on it is bigger.

What this unit covers

Topics marked new are not tested much in the quiz, so this book gives them extra questions.

Newton's first law of motion
Inertia and mass; everyday effects of inertia (seat belts, loose loads)
Newton's second law: F = ma calculations
Resultant force with friction and resistance in F = ma
Newton's third law: action–reaction pairs
Applications of the third law (rockets, swimming, walking, boats)
Weight, mass and g on the Earth and other bodies
Air resistance and terminal velocity
Free fall: all bodies fall with the same acceleration g without air resistance
Investigating F = ma by experiment (trolleys, data, friction compensation)
Connected bodies, towing and lifts: tension when accelerating

Go to the questions

Questions (1–50)

Easier and harder questions are mixed, just like in a real exam. The 24 harder ones are marked ★ Challenge.

1True or false
An object in deep space, far from any planet or star, still has mass but almost no weight.
Show answer
Answer: True
Mass is the amount of matter and does not change; weight is the pull of gravity, which is almost zero far from any body.
Common mistakeLearners think 'weightless' means 'massless'; the object still has inertia and is hard to stop.
2True or false · ★ Challenge
Newton's first law says that a moving object will naturally slow down and stop when no force acts on it.
Show answer
Answer: False
The first law says the object keeps moving at constant velocity; it is friction that makes real objects stop.
Common mistakeThis is the old idea that motion needs a force; everyday friction hides the first law.
3True or false
A full jerrycan is harder to start moving than an empty one, even on a smooth floor, because it has more inertia.
Show answer
Answer: True
The full jerrycan has more mass, so more inertia; a larger force is needed to give it the same acceleration.
Common mistakeLearners think this is only because of friction or weight; it is still true on a smooth floor.
4True or false · ★ Challenge
The two forces in a Newton's third-law pair are always of the same type, for example both gravitational.
Show answer
Answer: True
The Earth pulls the mango gravitationally and the mango pulls the Earth gravitationally; contact forces pair with contact forces.
Common mistakeLearners pair a weight with a normal reaction; those act on the same object and are different types, so they are not a third-law pair.
5True or false
In F = ma, F is always the force of the engine, even when friction also acts.
Show answer
Answer: False
F in F = ma is the RESULTANT force: driving force minus friction and other resistances.
Common mistakeUsing the applied force alone gives an acceleration that is too big.
6True or false · ★ Challenge
In a trolley experiment, friction can be compensated by tilting the runway slightly until the trolley rolls down it at a constant speed.
Show answer
Answer: True
At constant speed the resultant is zero, so the slope's pull just balances friction; any extra force then gives the acceleration.
Common mistakeLearners tilt the runway until the trolley speeds up; that over-compensates and adds an extra force.
7True or false
Without air resistance, a falling body accelerates at about 10 m/s² whatever its mass.
Show answer
Answer: True
All bodies in free fall near the Earth have the same acceleration g ≈ 10 m/s².
Common mistakeThe everyday idea that heavy things fall faster comes from air resistance, not from gravity.
8Multiple choice · ★ Challenge
Headrests in cars mainly protect a passenger's neck when the car is:
1. ABraking hard to avoid an accident
2. BTurning sharply at a roundabout
3. CHit from behind by another vehicle
4. DMoving along at a high, steady speed
Show answer
Answer: C. Hit from behind by another vehicle
When hit from behind the car and body jerk forwards but the head, by inertia, stays behind; the headrest pushes the head forwards with the body.
Common mistakeBraking is tempting, but then the head moves forwards and the seat belt, not the headrest, protects the passenger.
9Fill in the blank
An object falling with only its weight acting on it is said to be in ______ fall.
Show answer
Answer: free
Free fall means gravity is the only force acting.
Common mistakeLearners write 'terminal', but terminal velocity needs air resistance, so it is not free fall.
10Multiple choice · ★ Challenge
A blown-up balloon is let go with its neck open and it flies round the room. Which explanation is correct?
1. AThe air outside pushes on the back of the balloon
2. BThe balloon flies because it is lighter than the air round it
3. CIt pushes air out, and the air pushes it the other way
4. DNo force acts; the balloon simply follows the escaping air
Show answer
Answer: C. It pushes air out, and the air pushes it the other way
The stretched balloon squeezes air backwards out of the neck; the air pushes the balloon forwards, like a rocket.
Common mistakeThinking the outside air pushes it is a common error; rockets work even in space where there is no air.
11Fill in the blank
The mass of an object is a measure of its ______: the bigger the mass, the harder it is to change its motion.
Show answer
Answer: inertia
A larger mass resists changes in motion more strongly.
Common mistakeWriting 'weight' confuses inertia with the pull of gravity; inertia exists even where there is no gravity.
12Multiple choice · ★ Challenge
A loaded lorry collides with a small car. Which statement about the forces during the collision is correct?
1. AThe lorry exerts a bigger force on the car
2. BEach exerts an equal-sized force on the other
3. CThe car exerts a bigger force on the lorry
4. DOnly the lorry exerts a force, being heavier
Show answer
Answer: B. Each exerts an equal-sized force on the other
The forces are a third-law pair, so they are equal in size and opposite in direction; the car is damaged more because its mass is smaller.
Common mistakeChoosing 'the lorry exerts a bigger force' confuses the force with its effect; the car's acceleration is bigger, not the force.
13Fill in the blank
When a lift starts to move downwards (speeding up), the tension in its cable is ______ than the weight of the lift.
Show answer
Answer: less
A downward acceleration needs a downward resultant, so the weight must be bigger than the tension.
Common mistakeLearners write 'greater' because the lift is moving; what matters is the direction of the acceleration.
14Multiple choice · ★ Challenge
A skydiver falling at a steady speed opens her parachute. What happens immediately afterwards?
1. AShe speeds up, as the parachute adds to her weight
2. BShe slows, as air resistance now exceeds her weight
3. CShe stops at once and then floats down very gently
4. DShe keeps the same speed, as the forces stay balanced
Show answer
Answer: B. She slows, as air resistance now exceeds her weight
The open parachute greatly increases air resistance, so there is a resultant upward force and she decelerates.
Common mistakeLearners think she moves upwards; she still falls, but more slowly, until the forces balance at a lower steady speed.
15Fill in the blank
The steady top speed reached by a falling object when the air resistance equals its weight is called the ______ velocity.
Show answer
Answer: terminal
At terminal velocity the resultant force is zero, so the object falls at a constant speed.
Common mistakeWriting 'final' or 'maximum' is close, but the physics term is terminal velocity.
16Short answer · ★ Challenge
Name the reaction force for each action: (a) your foot pushes backwards on the ground; (b) the Earth pulls a mango downwards; (c) a hammer pushes a nail into wood.
Show answer
Model answer: (a) The ground pushes your foot forwards. (b) The mango pulls the Earth upwards. (c) The nail pushes back on the hammer (upwards/backwards) with an equal force.
Common mistakeFor (b) learners often give 'the branch holds the mango up'; the reaction must be the same type of force between the same two bodies.
17Multiple choice
When you walk forwards, which force actually pushes you forwards?
1. AYour foot pushing backwards on the ground
2. BYour own weight acting downwards
3. CThe air pushing on your back
4. DThe ground pushing forwards on your foot
Show answer
Answer: D. The ground pushing forwards on your foot
Your foot pushes the ground backwards; the ground's equal and opposite push forwards on your foot moves you.
Common mistakeChoosing 'your foot pushing backwards' names a force on the ground, not on you.
18Multiple choice
When a minibus starts suddenly, standing passengers fall backwards because:
1. AThe bus pushes them backwards with a force
2. BBy inertia, their bodies tend to stay at rest
3. CThe air inside the bus pushes them towards the back
4. DGravity becomes stronger when the bus speeds up
Show answer
Answer: B. By inertia, their bodies tend to stay at rest
By inertia the passengers' bodies resist the change from rest, so the bus moves forwards under them.
Common mistakeLearners imagine a backward force on the passengers; in fact no backward push acts, the bus simply moves forwards under them.
19Multiple choice · ★ Challenge
A lunar landing craft weighs 1440 N while it stands on the Moon (g = 1.6 N/kg). What is its mass?
1. A2304 kg
2. B144 kg
3. C90 kg
4. D900 kg
Show answer
Answer: D. 900 kg
m = W/g = 1440 ÷ 1.6 = 900 kg.
Common mistake144 kg uses Earth's g (1440 ÷ 10); on the Moon you must divide by the Moon's g.
20Multiple choice
A magnet pulls an iron nail with a force of 0.3 N. What force does the nail exert on the magnet?
1. ANo force, as nails are not magnets
2. B0.3 N, pulling towards the nail
3. CLess than 0.3 N towards the nail
4. DMore than 0.3 N towards the nail
Show answer
Answer: B. 0.3 N, pulling towards the nail
By Newton's third law the nail pulls the magnet with an equal and opposite force, 0.3 N.
Common mistake'No force' is tempting because the nail is not a magnet, but every force is one of an equal pair.
21Short answer · ★ Challenge
Describe, using forces, the three stages of a skydiver's fall before she opens her parachute.
Show answer
Model answer: 1. Just after jumping, her speed is low, air resistance is small and her weight is much bigger, so she accelerates downwards. 2. As she speeds up, air resistance increases, the resultant force falls and her acceleration gets smaller. 3. When air resistance equals her weight, the resultant is zero and she falls at a steady terminal velocity.
Common mistakeLearners say she stops speeding up because gravity gets weaker; gravity stays the same, it is the air resistance that grows.
22Multiple choice
A water hose jerks backwards when a strong jet of water shoots out of it. Why?
1. AThe water it pushes forwards pushes it back
2. BThe water pulls the hose forwards along the jet
3. CAir pushes the hose backwards against the jet
4. DGravity acts more strongly on a hose full of water
Show answer
Answer: A. The water it pushes forwards pushes it back
This is a third-law pair: the forward push on the water comes with a backward push on the hose.
Common mistakeBlaming the air is the usual mistake; the backward force comes from the water itself.
23Multiple choice · ★ Challenge
A 2000 kg tractor pulls two trailers of 1000 kg each and accelerates at 0.5 m/s² (ignore friction). What is the force in the coupling between the tractor and the first trailer?
1. A500 N
2. B1000 N
3. C2000 N
4. D1500 N
Show answer
Answer: B. 1000 N
This coupling accelerates both trailers: F = (1000 + 1000) × 0.5 = 1000 N.
Common mistake500 N accelerates only one trailer; the first coupling must pull both trailers behind it.
24Multiple choice
As a raindrop falls faster and faster, what happens to the air resistance on it?
1. AIt decreases
2. BIt stays the same
3. CIt increases
4. DIt becomes zero
Show answer
Answer: C. It increases
Air resistance grows with speed, until it balances the weight of the drop.
Common mistakeLearners think air resistance is fixed, but it depends on how fast the object moves through the air.
25Short answer · ★ Challenge
Before Newton, people believed a force is needed to keep things moving. Use a moto on a road to explain why it seems so, and what Newton's first law really says.
Show answer
Model answer: A moto slows down when the engine stops because friction and air resistance act backwards on it, so it seems a force is needed to keep it moving. Newton's first law says an object keeps moving at constant velocity unless a resultant force acts; the engine is only needed to balance the resistive forces.
Common mistakeLearners say the engine force 'keeps' the moto moving; at steady speed it only cancels friction, the resultant is zero.
26Multiple choice
Why might a person stepping from a small canoe onto the shore fall into the water?
1. AThe water pulls the person downwards as they step
2. BThe canoe's weight suddenly becomes larger than before
3. CThe shore pushes the person back towards the canoe
4. DPushing the canoe back makes the canoe move away
Show answer
Answer: D. Pushing the canoe back makes the canoe move away
The person pushes the canoe back; the canoe pushes the person forwards, but the light canoe moves away, so the step falls short.
Common mistakeLearners blame the water, but the effect comes from the action–reaction pair between person and canoe.
27Multiple choice · ★ Challenge
A 50 kg girl stands on bathroom scales in a lift accelerating upwards at 2 m/s² (g = 10 N/kg). What do the scales read?
1. A600 N
2. B500 N
3. C400 N
4. D100 N
Show answer
Answer: A. 600 N
Resultant up = ma = 50 × 2 = 100 N, so the push of the scales = weight + 100 = 500 + 100 = 600 N.
Common mistake400 N subtracts the extra force; for an upward acceleration the scales must push MORE than the weight.
28Multiple choice
A flat sheet of paper falls more slowly than the same sheet crumpled into a ball. Why?
1. AIt meets more air resistance, at the same weight
2. BThe flat sheet weighs less than the crumpled ball
3. CGravity pulls less strongly on flat objects
4. DThe crumpled ball has a larger mass
Show answer
Answer: A. It meets more air resistance, at the same weight
Crumpling does not change the mass or weight; it reduces the area pushing against the air.
Common mistakeThinking the crumpled ball is heavier is the usual error; it is the same paper.
29Multiple choice · ★ Challenge
The engine of a 900 kg car gives a driving force of 2700 N and the car accelerates at 2 m/s². What is the total resistive force on it?
1. A900 N
2. B1800 N
3. C2700 N
4. D450 N
Show answer
Answer: A. 900 N
Resultant = ma = 900 × 2 = 1800 N, so resistance = 2700 − 1800 = 900 N.
Common mistake1800 N is the resultant force, not the resistance; subtract it from the driving force.
30Multiple choice
A coin and a feather are released together inside a tube from which all the air has been pumped out. What happens?
1. AThey both reach the bottom together
2. BThe coin reaches the bottom first
3. CThe feather reaches the bottom first
4. DThe feather floats and does not fall at all
Show answer
Answer: A. They both reach the bottom together
With no air resistance both fall with the same acceleration g, so they land together.
Common mistakeChoosing the coin comes from everyday experience in air; it is air resistance, not weight, that slows the feather.
31Short answer · ★ Challenge
A 3 kg ball is taken from the Earth to the Moon. Is it easier, harder or the same to (a) lift it and (b) start it rolling along a smooth level floor? Explain.
Show answer
Model answer: (a) Easier: its weight on the Moon (3 × 1.6 = 4.8 N) is much less than on Earth (30 N). (b) The same: starting it rolling depends on its mass (inertia), which is still 3 kg.
Common mistakeLearners say everything is easier on the Moon; only lifting is, because inertia does not depend on gravity.
32Fill in the blank
Pushing a 2 kg toy cart with a resultant force of just 1 N makes its speed grow by ______ m/s every second.
Show answer
Answer: 0.5
a = F/m = 1 ÷ 2 = 0.5 m/s².
Common mistakeAnswering 2 multiplies instead of divides; acceleration = force ÷ mass.
33Multiple choice · ★ Challenge
A constant force pulls trolleys of different mass. Results: m = 0.5, 1.0, 2.0 kg; a = 4.0, 2.0, 1.0 m/s². What is the size of the force?
1. A4 N
2. B2 N
3. C0.5 N
4. D8 N
Show answer
Answer: B. 2 N
F = ma = 0.5 × 4.0 = 1.0 × 2.0 = 2.0 × 1.0 = 2 N for every run.
Common mistake8 N comes from multiplying the largest mass by the largest acceleration, which belong to different runs.
34Short answer
Use the idea of inertia to explain why loads on a pickup truck must be tied down.
Show answer
Model answer: If the truck brakes suddenly, the untied load keeps moving forwards (inertia) and can slide into the cab or fall off; if the truck turns, the load keeps going straight and falls off the side. Ropes provide the force needed to change the load's motion together with the truck.
Common mistakeLearners say the load is 'thrown' forwards by a force; in fact it just continues moving while the truck slows.
35Multiple choice · ★ Challenge
A stone is dropped from rest off a bridge (ignore air resistance, g = 10 m/s²). How fast is it moving after 2 s?
1. A5 m/s
2. B10 m/s
3. C20 m/s
4. D40 m/s
Show answer
Answer: C. 20 m/s
v = u + gt = 0 + 10 × 2 = 20 m/s.
Common mistake10 m/s is the speed after 1 s only; the stone gains 10 m/s in every second.
36Short answer
Explain how a bird rises into the air by flapping its wings, using Newton's third law.
Show answer
Model answer: On each down-stroke the wings push air downwards (action). The air pushes the wings, and so the bird, upwards with an equal force (reaction). If this upward force is greater than the bird's weight, the bird rises.
Common mistakeLearners say the wings 'hold on to' the air; it is the push on the air and the air's equal push back that lifts the bird.
37Short answer · ★ Challenge
Students pull a trolley with 1, 2 and then 3 identical elastic cords, each stretched by the same length. Explain why this gives forces of F, 2F and 3F, and how they could measure the acceleration each time.
Show answer
Model answer: Identical cords stretched by the same amount each pull with the same force F, so two cords together give 2F and three give 3F. The acceleration can be found from a ticker tape attached to the trolley (change in speed between strips ÷ time) or from light gates.
Common mistakeLearners stretch the cords by different amounts; then the forces are not equal and the test is not fair.
38Multiple choice
In an experiment to show that acceleration is proportional to force, which quantity must be kept constant?
1. AThe pulling force used
2. BThe acceleration produced
3. CThe time taken by each run
4. DThe mass of the trolley
Show answer
Answer: D. The mass of the trolley
Only the force is changed and the acceleration measured, so the mass must stay the same for a fair test.
Common mistakeKeeping the force constant would be right for testing the effect of MASS, not of force.
39Multiple choice
A 60 g tennis ball is hit and accelerates at 500 m/s² while the racket touches it. What force acts on it?
1. A30.0 N
2. B30 000 N
3. C8.3 N
4. D0.00012 N
Show answer
Answer: A. 30.0 N
m = 60 g = 0.06 kg, so F = ma = 0.06 × 500 = 30 N.
Common mistake30 000 N comes from using 60 instead of 0.06 kg; F = ma needs the mass in kilograms.
40Multiple choice · ★ Challenge
The same resultant force acts on two trolleys. Trolley X accelerates at 6 m/s² and trolley Y at 2 m/s². How does the mass of Y compare with the mass of X?
1. AY has one third the mass of X
2. BY has the same mass as X
3. CY has 3 times the mass of X
4. DY has 4 times the mass of X
Show answer
Answer: C. Y has 3 times the mass of X
m = F/a: for the same F, a mass three times bigger gives one third of the acceleration (6 ÷ 2 = 3).
Common mistake'One third' reverses the relationship: the trolley with the SMALLER acceleration has the bigger mass.
41Short answer · ★ Challenge
A 2 kg stone and a 6 kg stone are dropped together. The 6 kg stone has three times the weight. Use F = ma to explain why both have the same acceleration (g = 10 N/kg, no air resistance).
Show answer
Model answer: 2 kg stone: W = 20 N, a = 20 ÷ 2 = 10 m/s². 6 kg stone: W = 60 N, a = 60 ÷ 6 = 10 m/s². The bigger force acts on a bigger mass, so a = W/m = g is the same for both.
Common mistakeLearners stop at 'it has three times the force, so it falls faster', forgetting it also has three times the mass to accelerate.
42Multiple choice
A 2 kg box on a rough floor is pushed with 10 N. Friction on it is 4 N. What is its acceleration?
1. A5 m/s²
2. B7 m/s²
3. C2 m/s²
4. D3 m/s²
Show answer
Answer: D. 3 m/s²
Resultant = 10 − 4 = 6 N; a = 6 ÷ 2 = 3 m/s².
Common mistake5 m/s² uses the push alone (10 ÷ 2) and ignores friction; F = ma needs the resultant force.
43Short answer · ★ Challenge
A 1500 kg car tows an 800 kg broken-down car with a rope and accelerates at 0.4 m/s² (ignore friction). Find the tension in the rope and the driving force needed.
Show answer
Model answer: Rope tension accelerates the towed car: T = 800 × 0.4 = 320 N. The driving force accelerates both cars: F = (1500 + 800) × 0.4 = 920 N.
Common mistakeLearners use only the front car's mass for the driving force; the engine must accelerate both cars.
44Multiple choice
A 400 kg boat on Lake Kivu speeds up from 2 m/s to 5 m/s in 6 s. What resultant force acts on it?
1. A333 N
2. B66.7 N
3. C200 N
4. D1200 N
Show answer
Answer: C. 200 N
a = (5 − 2) ÷ 6 = 0.5 m/s²; F = ma = 400 × 0.5 = 200 N.
Common mistake333 N uses only the final speed (5 ÷ 6 × 400); the acceleration needs the CHANGE in speed.
45Short answer · ★ Challenge
A cyclist and bicycle (80 kg in total) pedal with a driving force of 200 N while friction and air resistance total 40 N. Find the acceleration. As the speed rises, air resistance increases: explain what happens to the acceleration.
Show answer
Model answer: Resultant = 200 − 40 = 160 N; a = 160 ÷ 80 = 2 m/s². As air resistance grows, the resultant force falls, so the acceleration gets smaller; when resistance reaches 200 N the resultant is zero and she moves at a steady top speed.
Common mistakeLearners think acceleration stays at 2 m/s² as long as she pedals; it falls as resistance grows.
46Multiple choice
A lift is speeding up as it moves upwards. How does the tension in its cable compare with the lift's weight?
1. AThe tension is equal to the weight
2. BThe tension is less than the weight
3. CThe tension has dropped to zero
4. DThe tension is greater than the weight
Show answer
Answer: D. The tension is greater than the weight
To accelerate upwards there must be an upward resultant force, so tension > weight.
Common mistake'Equal' is true only when the lift moves at a steady speed.
47Multiple choice
A marble rolling on a long, smooth, level floor keeps going for a long way before it stops. Which law does this best illustrate?
1. ANewton's second law
2. BNewton's third law
3. CHooke's law
4. DNewton's first law
Show answer
Answer: D. Newton's first law
With very little friction, the marble keeps moving at almost constant velocity, as the first law says.
Common mistakeThe second law is tempting because forces are involved, but here the point is that motion continues when (almost) no force acts.
48Short answer · ★ Challenge
A moto taxi (total mass 250 kg) moves at a steady speed with a driving force of 900 N. (a) What is the total resistive force? (b) The driving force is raised to 1400 N with the same resistance. Find the acceleration.
Show answer
Model answer: (a) Steady speed means balanced forces, so resistance = 900 N. (b) Resultant = 1400 − 900 = 500 N; a = 500 ÷ 250 = 2 m/s².
Common mistakeLearners use 1400 ÷ 250 = 5.6 m/s², forgetting that the 900 N resistance still acts.
49Short answer
A loaded wheelbarrow of mass 30 kg is pushed with a resultant force of 45 N. Find its acceleration and its speed after 4 s if it starts from rest.
Show answer
Model answer: a = F/m = 45 ÷ 30 = 1.5 m/s². v = u + at = 0 + 1.5 × 4 = 6 m/s.
Common mistakeLearners stop at the acceleration and call it the speed; a = 1.5 m/s² is the gain in speed each second.
50Short answer · ★ Challenge
A 1.0 kg trolley is pulled by forces of 1, 2 and 3 N. The accelerations measured are 0.6, 1.6 and 2.6 m/s². Explain why each acceleration is less than F/m, and find the friction force.
Show answer
Model answer: Friction acts against the pull, so the resultant force is less than the pull. Resultant = ma = 0.6, 1.6 and 2.6 N, so friction = 1 − 0.6 = 2 − 1.6 = 3 − 2.6 = 0.4 N each time.
Common mistakeLearners blame measuring errors; a constant gap of 0.4 N in every run points to a steady friction force.