Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Exam tip: learn all three laws word-perfect
The three laws
| Law | What it says | In simple words |
|---|---|---|
| 1st | If the resultant force on an object is zero, a stationary object stays stationary and a moving object keeps moving at the same speed in the same direction. | Objects keep doing what they’re already doing. |
| 2nd | The acceleration of an object is directly proportional to the resultant force on it and inversely proportional to its mass. | F = m a. More force, more acceleration. More mass, less acceleration. |
| 3rd | Whenever two objects interact, the forces they exert on each other are equal and opposite. | Every push has an equal push back. |
Second law rearranged: a = F ÷ m | m = F ÷ a | (H) Inertial mass = F ÷ a – how hard it is to change an object’s velocity
How to answer
- Calculations: use FIFA – Formula, Insert, Fix, Answer (with the unit).
- Explain questions: name the forces, say which law applies, then say what happens to the motion.
- Third law pairs: always name BOTH objects and give the direction of each force.
Foundation: aim for Q1–8. Higher: aim for Q4–10.
Questions
Q1 (F) A book rests on a shelf. What is the resultant force on the book, and which of Newton’s laws explains why it stays still?
Show solution
Resultant force = 0 N. The weight and the normal contact force are balanced.
Newton’s first law: with zero resultant force, a stationary object stays stationary.
Q2 (F) You push on a wall with a force of 40 N. State the size and direction of the force the wall exerts on you.
Show solution
40 N, in the opposite direction – back towards you (Newton’s third law).
Q3 (F) A resultant force of 20 N acts on a 4.0 kg trolley. Calculate its acceleration.
Show solution
F: F = m a
I: 20 = 4.0 × a
F: a = 20 ÷ 4.0
A: 5.0 m/s²
Q4 (F) A bus brakes suddenly and the standing passengers lurch forwards. Use Newton’s first law to explain why.
Show solution
The braking force acts on the bus, not the passengers, so the bus slows down. No resultant force has acted on the passengers yet, so they keep moving forwards at the same speed (their inertia).
Q5 (F) Use Newton’s third law to explain how a swimmer moves forwards through the water.
Show solution
The swimmer pushes the water backwards. The water pushes the swimmer forwards with an equal and opposite force.
Q6 (F/H) A car travels at a constant 30 m/s along a straight road. The driving force is 2000 N. What is the total resistive force? Explain your answer.
Show solution
2000 N. The car moves at a constant velocity, so the resultant force is zero (Newton’s first law). The resistive force must balance the driving force.
Q7 (F/H) Two skaters stand still on ice and push each other apart. Skater A has a mass of 50 kg and skater B has a mass of 75 kg. Each feels a force of 150 N. Calculate each skater’s acceleration and say who moves away faster.
Show solution
F: F = m a, so a = F ÷ m
I: A: a = 150 ÷ 50 B: a = 150 ÷ 75
A: A = 3.0 m/s², B = 2.0 m/s²
Skater A moves away faster. The forces are equal (third law), but A has less mass (second law).
Q8 (F/H) A student says a rocket can’t accelerate in space because there’s no air to push against. Explain why the student is wrong.
Show solution
The rocket pushes its exhaust gases backwards. The gases push the rocket forwards with an equal and opposite force (third law), so no air is needed.
Q9 (H) A resultant force of 12 N gives an object an acceleration of 0.80 m/s². Calculate its inertial mass.
Show solution
F: inertial mass = F ÷ a
I: m = 12 ÷ 0.80
F: already the subject
A: 15 kg
Q10 (H) An apple of mass 0.10 kg falls from a tree. Calculate the force the Earth exerts on the apple, state the force the apple exerts on the Earth, and explain why the Earth doesn’t noticeably move. (g = 9.8 N/kg)
Show solution
F: W = m g
I: W = 0.10 × 9.8
A: 0.98 N downwards on the apple
The apple pulls the Earth upwards with 0.98 N (third law pair).
The Earth’s mass is enormous, so a = F ÷ m is tiny – the Earth’s movement is far too small to notice.
⚠️ Where students lose marks
- “No force means it stops.” Zero resultant force means constant velocity – the object could still be moving.
- Using one force in F = ma instead of the RESULTANT force.
- Saying third law forces cancel out. They act on different objects, so they can’t.
Next: F = ma questions · Resultant forces · Skate Park Physics lesson · ← Forces topic
