Forces and Motion: The Complete PSLE Guide

Forces are pushes or pulls that act on objects. In Singapore Primary Science, understanding forces is essential because the topic appears across P4 and P5, and is heavily tested at PSLE. You need to be able to identify types of forces, describe their effects on objects, and explain everyday situations involving forces — including why objects move, stop, change direction, or change shape.

What Is a Force?

A force is a push or a pull. Forces can cause objects to start moving, stop moving, speed up, slow down, change direction, or change shape. Forces are measured in Newtons (N) using an instrument called a force meter (or spring balance).

A key concept for PSLE: forces always act between two objects. You cannot have a force without something exerting it and something receiving it.

Types of Forces: Contact vs Non-Contact

The most important classification for PSLE is whether a force requires physical contact between two objects.

Gravitational Force

Gravity is a non-contact force that pulls objects towards each other. On Earth, gravity pulls everything downward towards the Earth's centre. This is why objects fall when dropped. The gravitational force acting on an object is called its weight.

Weight is different from mass. Mass is the amount of matter in an object and does not change wherever you are. Weight is the gravitational force on that mass and changes depending on location — an astronaut on the Moon weighs less because the Moon's gravity is weaker, but their mass stays the same.

Friction

Friction is a contact force that acts between two surfaces in contact. It always opposes motion — meaning it acts in the direction opposite to movement. Friction can be useful or unhelpful depending on the situation.

When Friction Is Useful

• Shoe soles grip the floor so we don't slip
• Bicycle brakes use friction to slow down and stop
• Tyres grip the road for safe driving
• We can hold objects because friction acts between our hands and the surface

When Friction Is Not Useful

• Machine parts wearing down over time
• Heat produced in engines due to friction between moving parts
• Soles of shoes wearing thin after prolonged use

Factors That Affect Friction

• Rougher surfaces produce more friction than smooth surfaces
• Heavier objects experience more friction than lighter objects on the same surface

Ways to Reduce Friction

• Use lubricants (oil, grease) to separate surfaces
• Use wheels or rollers instead of sliding
• Polish or smooth the surfaces
• Use ball bearings in machines

Spring Force

When a spring (or elastic material) is stretched or compressed, it exerts a force to return to its original shape. This is the spring force. A force meter uses a spring — when a weight pulls down on the spring, it stretches, and the scale shows how large the force is.

Effects of Forces

For PSLE, you must be able to state clearly what forces can do to objects. There are four key effects:

• Start an object moving — a push on a stationary ball makes it roll
• Stop a moving object — friction from brakes stops a bicycle
• Change the direction of a moving object — a goalkeeper deflects a ball sideways
• Change the shape of an object — squeezing clay changes its shape; compressing a spring shortens it

Balanced and Unbalanced Forces

When two or more forces act on an object, their combined effect determines whether the object moves or stays still.

Balanced forces are equal in size and opposite in direction. An object with balanced forces acting on it stays still (if stationary) or continues moving at the same speed in the same direction. For example, a book resting on a table has gravity pulling it down and the table exerting an equal upward force — the forces are balanced.

Unbalanced forces are forces where one direction is stronger. The object will move in the direction of the stronger force, or accelerate. For example, if you push a box with 10 N but friction opposes it with only 6 N, the net force is 4 N in your direction, so the box moves.

Model Exam Answers

Question: A book is lying still on a table. What can you say about the forces acting on the book?

Model Answer: The forces acting on the book are balanced. Gravity acts downward on the book, and the table exerts an equal upward force (normal force) on the book. Since the forces are equal and opposite, the book remains stationary.

Question: Explain why a parachutist eventually falls at a constant speed after jumping from a plane.

Model Answer: When the parachutist first jumps, gravity pulls them downward faster than air resistance acts upward, so they accelerate. As their speed increases, air resistance also increases. Eventually, air resistance equals the gravitational force. At this point, the forces are balanced and the parachutist falls at a constant speed.