Why does the scale read lower in a lift going down?

The support force must be below the weight to give a downward net force.

Why work out tension from one block only?

The string pulls only the trailing block, so its mass times the shared acceleration gives the tension.

Why give the braking force as an average?

With a steady deceleration the force is steady too, so the average is the force throughout.

S4 Physics: Forces and Newton Laws Practice Paper 3

2026-11-01

Tutor Prep Guide · Secondary Physics

Paper three warms up with 15 N on 5 kg, W = mg and the third law, then works net force against friction, six seconds from rest, a gram conversion and a lift going down, before a two-block tension and a braking force, ending with the cancellation mix-up.

Quick answer　3 m/s^2, 30 N, 30 N opposite, 3 m/s^2, 2.5 m/s^2 with 15 m/s and 45 m, 5 m/s^2, 560 N, 2 m/s^2 with 16 N, 2.5 m/s^2 with 2 500 N, and two different objects.

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 15 N net force pushes a 5 kg box. Find the acceleration.
FoundationWhat is the weight of a 3 kg school bag on the Earth surface (g = 10 m/s^2)?
FoundationI push a wall with 30 N. How large a force does the wall exert on me?
FoundationA 25 N push acts on a 7 kg box while friction is 4 N. Find the acceleration.
IntermediateA 5 N net force acts for 6 s on a 2 kg object starting from rest. Find the acceleration, the speed after 6 s and the displacement.
IntermediateA 6 N net force acts on a 1 200 g trolley. Find the acceleration.
IntermediateA 70 kg person stands on a scale in a lift accelerating downwards at 2 m/s^2 (g = 10). What does the scale read?
ChallengeBlocks of 8 kg and 2 kg joined by a light string rest on a smooth table. A 20 N horizontal force pulls the 2 kg block. Find the acceleration and the tension.
ChallengeA 1 000 kg car travels at 20 m/s and stops in 8 s. Find the deceleration and the average braking force.
Error redoA student says action and reaction cancel out, so pushing a wall puts no force on them. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	3 m/s^2 (15 / 5)	From F = ma, acceleration is the net force divided by the mass.
2	30 N	Weight is W = mg, with force measured in newtons.
3	30 N in the opposite direction	Newton third law: action and reaction are equal in size and opposite in direction.
4	3 m/s^2 (net 21 N / 7 kg)	Take the net force 25 – 4 = 21 N first, then divide by the mass.
5	2.5 m/s^2, 15 m/s and 45 m	a is 2.5, v is at = 15 and s is 1/2 a t^2 = 45.
6	5 m/s^2 (6 / 1.2)	Convert grams to kilograms first: 1 200 g is 1.2 kg.
7	560 N	Accelerating downwards, the support force is less than the weight: 70 x (10 – 2).
8	2 m/s^2 and tension 16 N	For the whole system a is 20 / 10 = 2; the string pulls only the 8 kg block, so T is 8 x 2 = 16 N.
9	2.5 m/s^2 and 2 500 N	The deceleration is 20 / 8 = 2.5, so the force is 1 000 x 2.5.
10	The two forces act on two different objects, so they never cancel the forces on one object	To cancel, the forces must act on the same object; here one acts on the wall and one on the person.

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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