Interaction of Forces

Force as a push or pull. Effects of force. Types: magnetic, gravitational, frictional, elastic spring. Weight due to gravity. Investigating friction and elastic spring force.

frictional forcegravitational forceelastic spring
Forces — types, definitions & effects
Gravitational force
A pull towards the centre of the Earth. Acts on all objects. Gives objects their weight. Weight = gravitational force acting on an object.
Frictional force
Acts between two surfaces in contact. Always opposes motion. Can be useful (grip) or unhelpful (slows machines). Produces heat and sound.
Elastic spring force
A push or pull exerted by a compressed or stretched spring/elastic. Returns to original shape when force is removed.
Magnetic force
A push or pull between magnets or between a magnet and a magnetic material. Acts without contact.
Effects of a force (any force can do these):
1.Move a stationary object
2.Speed up a moving object
3.Slow down a moving object
4.Stop a moving object
5.Change the direction of a moving object
6.Change the shape of an object
Syllabus note: The terms "air resistance" and "water resistance" are not required by the 2023 syllabus. However, the concept that friction can oppose motion through air or water IS tested. Questions 5–6 below use these terms for clarity — in the actual PSLE, the question would describe the scenario without using the specific terms.
Part A — Multiple choice (Booklet A style · 2 marks each)
Question 1 — MCQ (2 marks) Booklet A Effects of force

A goalkeeper catches a fast-moving football and brings it to a stop. Which effect of force best describes what happens when the goalkeeper catches the ball?

AThe force moves a stationary object.
BThe force changes the direction of a moving object.
CThe force stops a moving object.
DThe force speeds up a moving object.

The ball was moving — the goalkeeper's hands exert a force that brings it to rest. This is "stops a moving object." Option B would be correct if the ball changed direction but was not stopped — e.g. a deflection.

Question 2 — MCQ (2 marks) Booklet A Gravitational force · weight

Which statement about gravitational force is correct?

AGravitational force only acts on objects that are falling.
BThe weight of an object is the gravitational force acting on it.
CGravitational force is a push that acts away from the Earth's surface.
DOnly large objects experience gravitational force.

Gravity acts on all objects at all times, not just falling ones — a book sitting on a table has gravitational force acting on it (its weight). Gravity is always a pull towards the Earth's centre, never a push. Weight and gravitational force are the same thing.

Part B — Structured questions (Booklet B style)
Question 3 — Structured (4 marks) Booklet B Friction investigation · variables

A student investigates how the type of surface affects frictional force. She places a wooden block on four different surfaces and measures the force needed to slide it at a steady speed. The results are shown below.

Surface type Force needed to slide block (N) Sandpaper 9 Carpet 6 Wood 4 Glass 1

(a) State the conclusion that can be drawn from this experiment. (1 mark)

(b) State two variables that must be kept the same for this to be a fair test. (1 mark)

(c) The student claims that glass is the best surface for a skateboard ramp. Do you agree? Explain your answer, considering both advantages and disadvantages. (2 marks)

(a)The rougher the surface, the greater the frictional force. Smoother surfaces produce less friction.
(b)Any two of: same wooden block (same mass and size) / same speed of sliding / same direction of pulling / same area of block in contact with surface.
(c)Disagree. Although glass produces very little friction (an advantage for speed), it is dangerous for a skateboard ramp because the very low friction means a skateboarder would have almost no grip and could not control their speed or stop safely. A surface with moderate friction — like wood — would be safer and more practical.
Rougher surface → greater friction · Less friction ≠ always better
Part (c) is an evaluate-and-justify question — the examiner wants you to consider both sides. Saying only "yes, because less friction means faster speed" or only "no, because it's dangerous" earns 1 mark. Both the advantage AND disadvantage are needed for 2 marks.
Question 4 — Structured (4 marks) Booklet B Elastic spring force · predict & explain Distinction level

A student hangs different masses on a spring and measures how much the spring stretches. The table shows her results.

Mass hung (g) Extension of spring (cm) 100 2 200 4 300 6 400 ?

(a) Describe the relationship between the mass hung and the extension of the spring. (1 mark)

(b) Predict the extension when 400 g is hung. Explain your prediction. (1 mark)

(c) The spring is then compressed (pushed down) instead of stretched. What type of force does the spring exert when compressed? State the direction of this force. (1 mark)

(d) When the mass is removed, the spring returns to its original length. What property of the spring does this demonstrate? (1 mark)

(a)As the mass increases, the extension of the spring increases. The extension is directly proportional to the mass — doubling the mass doubles the extension.
(b)8 cm. The pattern shows every 100 g adds 2 cm of extension. 400 g → 4 × 2 = 8 cm.
(c)Elastic spring force — a push force acting upward (pushing back against the compression, away from the compressed end).
(d)This demonstrates the spring's elasticity — it can return to its original shape after the force is removed.
More mass → more extension · Spring force opposes deformation · Elasticity = returns to shape

For (a), the key phrase is "directly proportional" — every equal increase in mass produces an equal increase in extension. Spotting this pattern earns the mark; just saying "as mass increases, extension increases" is correct but incomplete.

Question 5 — Structured (5 marks) Booklet B Multiple forces · real-world · evaluate Distinction level

A skydiver jumps from an aeroplane. The diagram shows the forces acting on the skydiver at two moments — just after jumping (A) and after reaching a steady falling speed (B).

Moment A (just jumped) person Gravity (large) Air resist. (small) Moment B (steady speed) person Gravity Air resist. (equal to grav.) Gravity > Air resistance Gravity = Air resistance

(a) At moment A, gravity is greater than air resistance. What effect does this have on the skydiver's speed? (1 mark)

(b) As the skydiver falls faster, air resistance increases. Explain why, at moment B, the skydiver falls at a steady speed. (2 marks)

(c) The skydiver then opens a parachute. This greatly increases air resistance. Describe and explain what happens to the skydiver's speed immediately after the parachute opens. (2 marks)

(a)The skydiver speeds up (accelerates). When gravity is greater than air resistance, there is a net downward force, causing the skydiver to move faster.
(b)As the skydiver falls faster, air resistance increases until it equals gravitational force. When the two forces are equal and opposite, they balance each other out — there is no net force. With no net force, there is no change in speed, so the skydiver falls at a constant (steady) speed.
(c)The skydiver slows down. Opening the parachute greatly increases the surface area, which increases air resistance. Air resistance now exceeds gravitational force, so there is a net upward force. This net upward force slows the skydiver down until a new, slower steady speed is reached where the forces balance again.
Forces unequal → speed changes · Forces equal → steady speed
The key principle: a steady (constant) speed means the forces are balanced — NOT that there are no forces acting. This is one of the most tested and most misunderstood ideas in forces. "No forces means no movement" and "steady movement means no forces" are both wrong.