Physics · Unit 11

Forces and Newton's Laws

Why things move the way they do

Three laws that between them explain the motion of everything from a dropped ball to a spacecraft, and which say something genuinely counter-intuitive: a force is needed to change motion, not to maintain it.

The unit covers all three laws, free-body diagrams as the tool for applying them, and momentum and impulse.

This unit breaks down into 20 short steps and 119 questions, starting at difficulty 2 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 20
Questions: 119
Difficulty: 2-3

What this unit covers

Newton's Second Law
Newton's First Law
Newton's Third Law
Free-Body Diagrams
Momentum and Impulse

Where this fits

Needs Motion in Depth. The core of classical mechanics.

Where people slip

The third-law pair acts on two different objects, which is why they never cancel. The forces on the book and on the table are a pair; the forces on the book alone are not.

How the unit is structured

Forces and Newton's Laws runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 2 and climb to 3 as you progress.

Step 1 · easierStep 20 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 119 in the unit altogether.

Free-Body Diagrams

Choose all that applyLevel 2
1. Which of these are forces you might label on a free-body diagram? Select all that apply.
- Weightcorrect
- Normal reactioncorrect
- Frictioncorrect
- Temperature
Weight, the normal reaction, and friction are forces; temperature is not a force.
Fill the blankLevel 2
2. The single force that has the same effect as all the forces acting on an object is called the ____ force.
- resultantcorrect
- normal
- gravitational
- applied
The resultant, or net, force is the vector sum of all forces and determines how the object accelerates.
Guess the numberLevel 2
3. A box is pulled right with 10 N while friction pulls left with 4 N. What is the resultant force in newtons?
Answer: 6 N
The forces act along the same line in opposite directions, so the resultant is 10 N - 4 N = 6 N to the right.
Match the pairsLevel 2
4. For a car driving along a flat road, match each force to the direction its arrow points.
Answer: Weight = Straight down; Normal force = Straight up; Driving force = Forward; Friction and drag = Backward
Weight points down, the normal force up, the driving force forward, and friction and drag backward.
Multiple choiceLevel 2
5. What does a free-body diagram show?
- All the forces acting on a single objectcorrect
- The path the object travels
- The object's speed over time
- Only the object's weight
A free-body diagram shows all the forces acting on a single object, drawn as arrows from that object.
Build the sentenceLevel 3
6. Build the rule for resolving weight on an incline (theta measured from the horizontal).
Answer: weight component along the slope equals m g sine theta
The component of weight acting down the slope is m g sin(theta), while the component pressing into the surface is m g cos(theta).

Momentum and Impulse

Fill the blankLevel 2
7. The impulse given to an object is equal to its change in ____.
- momentumcorrect
- mass
- volume
- temperature
Impulse equals the change in momentum, which is why a longer contact time reduces the force in a collision.
Guess the numberLevel 2
8. A 4 kg ball rolls at 5 m/s. Using p = mv, what is its momentum in kg m/s?
Answer: 20 kg m/s
p = mv = 4 kg x 5 m/s = 20 kg m/s.
Multiple choiceLevel 2
9. Momentum is best described as...
- Mass times velocitycorrect
- Mass times acceleration
- Force times distance
- Mass divided by speed
Momentum equals an object's mass multiplied by its velocity, so heavier or faster objects have more momentum.
Put in orderLevel 2
10. Order these moving objects by momentum, from smallest to largest.
Answer: Rolling ball: 2 kg m/s -> Running dog: 30 kg m/s -> Cyclist: 100 kg m/s -> Moving car: 20000 kg m/s
Multiplying mass by velocity gives 2, 30, 100, and 20000 kg m/s, so the car has by far the most momentum.
Match the pairsLevel 3
11. Match each mechanics quantity to its correct SI unit.
Answer: Momentum = kg m/s; Impulse = N s; Rate of change of momentum = newton (N); Kinetic energy = joule (J)
Momentum is kg m/s, impulse is N s (equivalent to kg m/s), rate of change of momentum is force in newtons, and kinetic energy is measured in joules.
Sort into groupsLevel 3
12. Sort each collision as approximately elastic or inelastic.
Answer: Two billiard balls bouncing apart = Elastic; Gas molecules colliding = Elastic; A car crash where the cars crumple = Inelastic; Two lumps of clay sticking together = Inelastic
Elastic collisions conserve kinetic energy (billiard balls, gas molecules); inelastic ones lose it to deformation or sticking (crashes, clay).

Newton's First Law

Choose all that applyLevel 2
13. Which situations are explained by Newton's first law of inertia? Select all that apply.
- A passenger lurches forward when a car brakescorrect
- A spacecraft keeps drifting with its engines offcorrect
- A book stays still on a table until it is pushedcorrect
- A stretched spring stores elastic energy
A resting book, a coasting spacecraft, and a lurching passenger all show inertia; a stretched spring is about stored elastic energy instead.
Fill the blankLevel 2
14. Newton's first law of motion is also called the law of ____.
- inertiacorrect
- gravity
- momentum
- friction
Because it describes an object's resistance to changing its motion, the first law is often called the law of inertia.
Multiple choiceLevel 2
15. What does the word 'inertia' describe?
- An object's tendency to resist changes in its motioncorrect
- The pull of gravity on an object
- The energy stored in a moving object
- The friction between two surfaces
Inertia is a body's tendency to resist changes to its state of motion, which is the heart of Newton's first law.
Build the sentenceLevel 3
16. Arrange the words to state Newton's first law.
Answer: an object stays at constant velocity unless a resultant force acts
An object stays at constant velocity unless a resultant force acts on it.
Fact or fibLevel 3
17. A moving object will slow down and stop by itself even if no force acts on it.
Answer: False
False; without a net force such as friction, an object keeps moving at constant velocity, so something must act on it to slow it down.
Guess the numberLevel 3
18. A block of weight 50 N sits in equilibrium on a rough plane inclined at 30 degrees. What is the friction force acting up the slope?
Answer: 25 N
In equilibrium the friction balances the weight component along the slope, 50 times sin 30 = 25 N.

Newton's Second Law

Fill the blankLevel 2
19. For a fixed mass, the acceleration of an object is directly proportional to the ____ acting on it.
- net forcecorrect
- mass
- distance
- temperature
From F = ma, with mass held constant, acceleration rises in step with the net force.
Guess the numberLevel 2
20. A net force accelerates a 2 kg object at 3 m/s^2. Using F = ma, what is the force in newtons?
Answer: 6 N
F = ma = 2 kg x 3 m/s^2 = 6 N.
Match the pairsLevel 2
21. Match each physical quantity to its SI unit.
Answer: Force = newton (N); Mass = kilogram (kg); Acceleration = metre per second squared (m/s^2); Momentum = kilogram metre per second (kg m/s)
Force is measured in newtons, mass in kilograms, acceleration in m/s^2, and momentum in kg m/s.
Multiple choiceLevel 2
22. Newton's second law is written F = ma. What does this equation tell us?
- Net force equals mass times accelerationcorrect
- Force equals mass times velocity
- Force equals mass divided by acceleration
- Force equals weight times speed
The net force on an object equals its mass multiplied by the acceleration that force produces.
Choose all that applyLevel 3
23. When a falling object reaches terminal velocity, which statements are true?
- The drag force equals the object's weightcorrect
- The resultant force is zerocorrect
- The acceleration is zerocorrect
- The velocity keeps increasing
At terminal velocity the drag equals the weight, so the resultant force and acceleration are zero and the speed stays constant.
Fact or fibLevel 3
24. On a frictionless inclined plane, the acceleration of a sliding block does not depend on its mass.
Answer: True
Since a = g sin theta, the mass cancels and every block accelerates at the same rate.

Newton's Third Law

Build the sentenceLevel 2
25. Build Newton's third law of motion.
Answer: every action has an equal and opposite reaction
Newton's third law states that every action has an equal and opposite reaction.
Fact or fibLevel 2
26. For every action force there is an equal and opposite reaction force.
Answer: True
This is Newton's third law: forces always come in equal, oppositely directed pairs.
Fill the blankLevel 2
27. By Newton's third law, action and reaction forces are equal in size but opposite in ____.
- directioncorrect
- mass
- speed
- time
The paired forces have the same magnitude but point in opposite directions.
Choose all that applyLevel 3
28. Which statements about Newton's third-law force pairs are correct?
- They are equal in magnitudecorrect
- They point in opposite directionscorrect
- They act on two different bodiescorrect
- They act on the same body and cancel out
The paired forces are equal in size, opposite in direction, and act on two different bodies, so they never cancel on a single object.
Guess the numberLevel 3
29. A 60 kg astronaut pushes off a 120 kg free-floating module and accelerates at 2.0 m/s^2. By Newton's third law the module feels the same size force. Find the module's acceleration.
Answer: 1 m/s^2
The force on each is F = 60 x 2.0 = 120 N, so the module's acceleration is a = 120 / 120 = 1.0 m/s^2.
Match the pairsLevel 3
30. Match each action force to its Newton's third-law reaction.
Answer: Rocket pushes exhaust gas downward = Exhaust gas pushes rocket upward; Swimmer pushes water backward = Water pushes swimmer forward; Foot pushes the ground backward = Ground pushes the foot forward; Bat pushes the ball forward = Ball pushes the bat backward
Each reaction is equal in size and opposite in direction to the action, acting on the other body.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 16 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Forces and Newton's Laws

119 questions across 20 steps. Start with step one and crawl at your own pace.

Play this unit

Read about Forces and Newton's Laws

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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