Forces — Gravity, Friction, and Elastic Spring Force✓ Updated 2026
Forces explained for PSLE Science. Gravity, friction, elastic spring force, weight vs mass — with Singapore examples, balanced and unbalanced forces, and exam tips for P5/P6.
What Is a Force?
A force is a push or pull that acts on an object. Forces can make things start moving, stop moving, speed up, slow down, change direction, or change shape. Forces are measured in Newtons (N) using a device called a spring balance (or force meter).
The three forces tested in PSLE are: gravitational force (pulls everything with mass towards the centre of Earth), frictional force (opposes motion between surfaces), and elastic spring force (a stretched or compressed spring tries to return to its original shape).
Forces in Singapore's Daily Life
When you ride a bicycle along East Coast Park: you push the pedals (applied force), the tyres grip the path through friction, and gravity pulls you and the bike downward. When you stop pedalling on a flat road, friction gradually slows you to a stop. Cycling downhill, gravity accelerates you — unless friction and air resistance are large enough to balance it out.
The NAPFA standing broad jump at school involves all three forces. When you jump, your legs push against the ground (reaction force by Newton's third law). While airborne, gravity pulls you down, determining how long you stay in the air. The moment you land, friction between your shoes and the mat prevents you from sliding forward.
Escalators in MRT stations use motors to apply force against both gravity (when going up) and friction. The handrail uses friction between the belt and your hand to keep pace with you safely.
A bungee jumper at adventure parks: gravity accelerates them downward; the elastic cord stretches, building up elastic spring force that eventually decelerates and then reverses their motion.
Effects of Forces on Objects
- Start an object moving (from rest)
- Stop a moving object
- Speed up a moving object
- Slow down a moving object
- Change the direction of motion
- Change the shape of an object (e.g. stretching, compressing, bending)
Forces can be contact forces (require touching: friction, push, pull, spring force) or non-contact forces (act at a distance: gravitational force, magnetic force).
Weight vs Mass — A Critical Distinction
Gravitational force is a non-contact force that pulls all objects with mass towards each other. On Earth, it pulls everything towards the centre of the Earth.
- Mass: the amount of matter in an object. Measured in kilograms (kg). Does not change with location — your mass on the Moon is the same as on Earth.
- Weight: the force of gravity acting on an object. Measured in Newtons (N). Changes with location — you weigh about 6 times less on the Moon because the Moon's gravity is weaker.
- Formula: Weight (N) = Mass (kg) × Gravitational field strength (10 N/kg on Earth)
A 5 kg bag of rice has a weight of 50 N on Earth. On the Moon (gravitational field strength ≈ 1.6 N/kg), the same bag would weigh only 8 N — but its mass is still 5 kg.
Friction — Both Helpful and Harmful
Friction is a contact force that opposes motion when two surfaces are in contact. The amount of friction depends on the roughness of the surfaces and the force pressing them together.
| Friction is useful when... | Friction is harmful when... |
|---|---|
| Walking — grip prevents slipping | Moving parts in engines — wastes energy as heat |
| Braking — slows cars safely | Tyres and brake pads wearing down |
| Writing — pencil deposits graphite on paper | Drawers and hinges getting stiff |
| Striking a match — generates heat to ignite | Slowing objects that need to move freely (conveyor belts) |
Ways to reduce friction: use lubricants (oil, grease), use wheels or rollers, smooth the surfaces, use ball bearings. Ways to increase friction: use rougher surfaces, increase weight/pressure between surfaces, use grip-enhancing materials (rubber soles).
Springs — Stretch, Compress, Restore
When a spring is stretched or compressed, it exerts a force trying to return to its original shape. This is the elastic spring force (or restoring force).
- The more you stretch or compress a spring, the greater the elastic force it exerts
- Spring balances (used to measure weight/force) work on this principle — the more force applied, the more the spring stretches, and the reading increases
- Beyond the elastic limit: if stretched too far, the spring permanently deforms and does not return to its original shape
- Elastic bands, trampolines, and bungee cords all use elastic restoring forces
When Forces Are Balanced vs Unbalanced
- Balanced forces: Equal forces acting in opposite directions. Result: NO change in motion. The object stays still OR continues moving at constant speed in the same direction. Example: a book sitting on a table — gravity pulls down, table pushes up equally.
- Unbalanced forces: Forces are not equal. Result: change in motion — the object accelerates in the direction of the larger force. Example: when you push a trolley harder than friction resists, it accelerates forward.
Why Is Friction Both a Friend and Enemy in Singapore?
In a tropical city like Singapore, friction management is everywhere. The non-slip tiles on wet market floors and MRT station platforms are deliberately rough to maximise friction and prevent dangerous slipping in wet conditions. Conversely, Singapore's MRT train wheels on smooth steel rails have very low friction, which is why trains are so energy-efficient — a small motor can propel a heavy train because almost no energy is wasted to friction between wheel and rail.
Singapore's LTA spends significant resources on road surface texture: smooth highways reduce tyre noise and rolling resistance for fuel efficiency, while pedestrian crossings have rougher surfaces to maximise friction for safe stopping.
Common Mistakes
Key Points at a Glance
- Force = push or pull, measured in Newtons using a spring balance
- Weight (N) = Mass (kg) × 10. Mass stays constant; weight changes with gravity
- Friction: opposes motion, acts at surfaces, increases with roughness and weight pressing down
- Elastic spring force: restoring force when spring is stretched/compressed
- Balanced forces → no change in motion (still or constant speed)
- Unbalanced forces → object accelerates in direction of larger force
⚡ The 3 PSLE Forces — Visual Summary
All three forces you must know for PSLE, with type, direction and key rule.
⚖️ Weight vs Mass — Earth vs Moon
Mass NEVER changes wherever you go. Weight changes because it depends on gravity. This is one of the most common PSLE traps.
⚖️ Balanced vs Unbalanced Forces
🌀 Spring Extension — Proportional Relationship
The extension of a spring is directly proportional to the load applied. Double the load = double the extension. Key for PSLE calculation questions.
Ready to test yourself? 🧠
You've read the full guide on Forces — Gravity, Friction, and Elastic Spring Force — now lock it in with a quiz and flashcards.
Related PSLE Topics
These topics are closely linked in the PSLE syllabus.
Help a classmate — share this free resource