Key Concepts — Forces & Motion
| Type of Force | Contact or Non-contact? | Description | Singapore Example |
|---|---|---|---|
| Friction | Contact | Opposes motion between two surfaces in contact | Brakes on a bicycle slowing it down |
| Gravitational force | Non-contact | Pulls all objects toward the Earth (or any large mass) | A durian falling from a tree |
| Magnetic force | Non-contact | Attracts magnetic materials; magnets attract or repel each other | Magnets holding notes on a fridge |
| Elastic spring force | Contact | Stored in a stretched or compressed spring/elastic; returns object to original shape | Rubber band in a paper plane launcher |
| Situation | Forces | Effect on object |
|---|---|---|
| Balanced forces | Equal forces in opposite directions | Object stays still OR moves at constant speed in a straight line |
| Unbalanced forces | Unequal forces; one direction dominates | Object speeds up, slows down, changes direction, or changes shape |
When forces are unbalanced, the object changes its motion in the direction of the larger force.
Multiple-Choice Questions (1 mark each)
Q1Which of the following is a non-contact force?
Q2A book rests on a table and does not move. Which statement correctly describes the forces acting on the book?
Q3Amirah slides down a playground slide. Friction acts on her as she slides. What is the direction of friction on Amirah?
Q4A cyclist is riding on a flat road at a constant speed in a straight line. Which of the following must be true?
Q5Which of the following is an example of friction being useful?
Q6A ball is kicked along a flat, rough surface. After a while, the ball slows down and stops. Why does the ball stop?
Q7Two teams in a tug-of-war pull on a rope. Team A pulls with 400 N to the left, and Team B pulls with 550 N to the right. What happens to the rope?
Q8Which of the following would reduce friction between two surfaces?
Q9A magnet attracts an iron nail without touching it. What type of force is this?
Q10An astronaut on the Moon weighs less than on Earth. This is because ___.
Open-Ended Questions
Write your answers in the boxes below. Click Show Model Answer to check your response.
Q11Raj pushes a wooden crate across the floor of his flat with a force of 35 N. The friction acting on the crate is 20 N. [3 marks]
(a) Are the forces on the crate balanced or unbalanced? Explain your answer.
(b) What will happen to the crate? Give a reason.
(c) If Raj reduces his pushing force to exactly 20 N, what will happen to the crate?
✅ Model Answer
(a) The forces are unbalanced. Raj's pushing force (35 N) is greater than friction (20 N), so the forces in opposite directions are not equal.
(b) The crate will accelerate (speed up) in the direction of Raj's push. This is because the net force is 35 − 20 = 15 N in the direction of the push, causing it to move faster.
(c) The crate will move at a constant speed (or remain stationary if it was not already moving). The forces are now balanced — the pushing force equals friction — so there is no net force to change its motion.
Q12Suggest two ways that the sole of a school shoe is designed to increase friction. For each, explain why it increases friction. [4 marks]
✅ Model Answer
Way 1: The sole has a rough, ridged (treaded) surface. Rough surfaces have more contact points, which increases friction between the shoe and the ground, preventing slipping.
Way 2: The sole is made of rubber. Rubber is a grippy material that has high friction with most floor surfaces, providing better grip than smooth plastic would.
Q13A parachutist jumps from a plane. At first, she accelerates downward. After some time, she reaches a constant speed (called terminal velocity). Explain in terms of forces why she eventually moves at a constant speed. [3 marks]
✅ Model Answer
When she first jumps, gravity (her weight) is greater than air resistance, so the forces are unbalanced and she accelerates downward. As her speed increases, air resistance (acting upward) also increases. Eventually, the air resistance equals the gravitational force (her weight). The forces are now balanced, so there is no net force and she moves at a constant speed — terminal velocity.
Q14Wei Ling stretches a rubber band and uses it to launch a paper ball. Name the type of force stored in the stretched rubber band and explain how it causes the paper ball to move. [2 marks]
✅ Model Answer
The stretched rubber band stores elastic spring force (elastic potential energy). When Wei Ling releases the rubber band, this stored force is transferred to the paper ball as a push, causing the ball to accelerate and move forward in the direction of the force.
Q15A student says: "If I drop a feather and a stone from the same height at the same time, the stone hits the ground first because it is heavier and gravity pulls it harder." Is this reasoning correct? Explain. [3 marks]
✅ Model Answer
The student is partially correct but for the wrong reason. In normal conditions (with air), the stone does hit the ground first — but this is because the feather experiences much greater air resistance relative to its weight, which slows it down significantly. Gravity does pull the heavier stone with a greater force, but it also has more mass to accelerate. If both were dropped in a vacuum (no air), they would hit the ground at exactly the same time because air resistance would be absent.
⚠️ Common PSLE Exam Mistakes
Mistake 1: Saying a stationary object has "no forces." A book on a table has TWO balanced forces: gravity pulling it down and the table pushing it up. Always identify all forces.
Mistake 2: Confusing "balanced forces" with "no movement." An object moving at constant speed in a straight line also has balanced forces — it doesn't have to be stationary.
Mistake 3: Saying friction always acts downward. Friction always acts opposite to the direction of motion — if an object moves right, friction acts left.
Mistake 4: Saying the Moon has no gravity. The Moon has gravity — it is just weaker than Earth's. This is why the astronaut weighs less, not nothing.
📋 Key Facts to Memorise
- A force is a push or pull that can change speed, direction or shape
- Contact forces require touching: friction, elastic spring force
- Non-contact forces act at a distance: gravity, magnetic force
- Balanced forces → no change in motion (object stays still or moves at constant speed)
- Unbalanced forces → object speeds up, slows down or changes direction
- Friction always opposes motion — it acts in the direction opposite to movement
- Friction can be reduced with lubricants (oil, grease) and smooth surfaces
- Weight is the gravitational force on an object; mass stays the same everywhere
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