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Unit 3 · Force, mass and acceleration
Motion and forces · Year 10 Science · Science understanding
Topic hub: Overview · Previous: Inertia and first law · Next: Interaction pairs (third law)
This unit is the quantitative core: net force, mass and acceleration in SI units.
Try it first
Change mass and net force to the right, then Push. How do acceleration, speed, and position respond? The formulas below are the compact way to predict what you are already exploring.
Interactive: push the block (Fnet = m a)
The force on the block is always a push to the right (net force stays at 0 N or higher on the slider). Pick a preset or set mass and net force, then Push. The block and rail are drawn from above, like a bird's-eye view of the table; a simple arrow marks the push at the contact edge. Friction is turned off — only your chosen net force acts horizontally.
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1. What you should be able to do
- Use and its rearrangements , with correct units.
- Choose a positive direction in 1D and keep signs consistent.
- Explain in words why a large mass small acceleration can happen even when you push hard (hint: other forces on the object, and third-law pairs — Unit 4).
2. Second law —
Net force means the vector sum of all external forces on one chosen object.
For constant mass at Year 10 level:
SI units: newtons (N) for force, kilograms (kg) for mass, metres per second squared (m/s²) for acceleration.
Rearrangements:
3. One dimension — signs
Pick one axis (e.g. east = +). Forces in the + direction are positive; forces in the − direction are negative. Add them (with signs) to get . The acceleration you calculate points the same way as .
4. Worked examples
Example 1
A 12 kg cart has a net horizontal force of 36 N east. Find acceleration.
Example 2 — pushing a heavy object
You push a heavy cupboard; it barely moves. Your forward push is one of several forces on the cupboard. Static friction from the floor can balance your push so on the cupboard is ≈ 0 and ≈ 0.
Your push does have a third-law partner (the cupboard pushes you back) — but that partner acts on you, not on the cupboard, so it does not belong in for the cupboard.
5. Your turn
Q1. A 25 kg box accelerates at 2.0 m/s². Find the net force.
Sample answer
in the direction of acceleration.
Q2. A 400 N net force acts on an object of mass 80 kg. Find .
Sample answer
.
Q3. Why is it wrong to say “ means if I push harder, the object must move faster right away”?
Sample answer
in the law is net force on that object. Other forces (friction, components of weight, etc.) can cancel much of your push, so can stay small or zero until slip — and mass also matters: .