Why subtract friction before using F = m a?

Friction opposes the push, and only the net force drives the acceleration.

Why is the normal force larger than body weight in a rising lift?

A net upward force is needed for the upward acceleration.

Why do two blocks on one string share an acceleration?

The string cannot stretch, so both move together with the same change in speed.

S4 Physics: Forces and Newton Laws Practice Paper 1

2026-11-01

Tutor Prep Guide · Secondary Physics

Paper one starts with a 20 N push on 5 kg, the weight of 6 kg and inertia, then friction, a 30 N force over 4 s, a 500 g conversion and a lift, before a two-block string and a sliding stop, ending with the mass versus weight slip.

Quick answer　4 m/s^2, 60 N, rest or steady motion, 3 m/s^2, 12 m/s with 24 m, 6 m/s^2, 720 N, 2 m/s^2 with 4 N, 8 N, 20 N.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	25 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Physics: Forces and Newton Laws – Tutor Lesson Plan
Level	S4

02　Questions (10 items)

FoundationA 5 kg mass feels a net force of 20 N. Find its acceleration.
FoundationFind the weight of a 6 kg object on the ground with g = 10 m/s^2.
FoundationWhat happens to an object whose net force is zero?
FoundationA 20 N push acts on a 4 kg box against 8 N of friction. Find the acceleration.
IntermediateA 10 kg body starts from rest under a 30 N net force for 4 s. Find its final speed and displacement.
IntermediateA 500 g object feels a net force of 3 N. Find its acceleration.
IntermediateA 60 kg person stands in a lift accelerating upward at 2 m/s^2. Find the normal force from the floor.
ChallengeA 3 kg and a 2 kg block joined by a light string are pulled by 10 N on a smooth table. Find the acceleration and the tension.
ChallengeA 4 kg body sliding at 10 m/s stops uniformly in 5 s. Find the net force needed.
Error redoA student writes that a 2 kg book weighs 2 N. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	4 m/s^2 (F = m a, 20 / 5)	Newton second law links force, mass and acceleration.
2	60 N (W = m g = 6 x 10)	Weight is the pull of gravity and is measured in newtons.
3	It stays at rest or moves at a steady velocity in a straight line	That is Newton first law, also called the law of inertia.
4	3 m/s^2 ((20 – 8) / 4)	Find the net force first, then divide by the mass.
5	12 m/s and 24 m (a = 3 m/s^2)	Use F = m a for the acceleration, then the uniform acceleration formulas.
6	6 m/s^2 (0.5 kg)	Convert the mass to kilograms first, or the answer inflates a thousand times.
7	720 N (60 x (10 + 2))	Going up, the normal force must exceed the weight to give a net upward force.
8	Acceleration 2 m/s^2 and tension 4 N	Treat the pair as one for the acceleration, then look at the 2 kg block for the tension.
9	8 N backwards (a = 2 m/s^2)	Get the deceleration from the change in speed, then multiply by the mass.
10	Mass and weight differ; the weight is 2 x 10 = 20 N	The kilogram measures mass, while the newton measures force.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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