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Motion and forces
Motion and forces · Year 10 Science · Science understanding
This lesson builds skill with Newton’s laws and quantitative problems using in SI units. Official checklist wording: Motion and forces (curriculum). Quiz: Motion and forces.
1. What you should be able to do
- State Newton’s first, second and third laws and recognise them in scenarios.
- Calculate acceleration from net force and mass, or net force from mass and acceleration.
- Identify third-law pairs (two forces on different objects, same type, opposite direction, equal magnitude).
2. First law — inertia
Newton’s first law: an object maintains its velocity (including rest, ) unless a non-zero net external force acts.
- Inertia is the tendency to resist change in motion; mass is a measure of inertia.
- Friction and air resistance are real forces that often make objects appear to “naturally” slow down—the net force is not zero.
3. Second law —
Net force (vector sum of all external forces on one object) equals mass × acceleration (for constant mass, Year 10 level):
SI units: newtons (N) for force, kilograms (kg) for mass, metres per second squared (m/s²) for acceleration.
Rearrangements:
Always draw or imagine which direction is positive before substituting signs for 1D problems.
Worked example 1
A 12 kg cart has a net horizontal force of 36 N east. Find acceleration.
Worked example 2 — why “pushing a truck” still moves you
You push a heavy truck; it barely moves. Your forward force on the truck is paired with the truck’s backward force on you (third law). The truck’s tiny acceleration is still consistent with for the truck: static friction from the ground on the truck can balance much of your push until slip.
4. Third law — interaction pairs
If body A exerts a force on body B, then B exerts a force on A that is equal in magnitude, opposite in direction, and of the same type (e.g. both gravitational, both contact).
Common mistake: do not add third-law partners when finding on one object—they act on different bodies.
Worked example — book on a table
- Earth pulls book down: weight .
- Book pulls Earth up: Newton’s third-law partner to weight (often skipped in for the book because we analyse the book).
- Table pushes book up: normal force .
- Book pushes table down: third-law partner to .
If the book is at rest, on the book, so balances (for a horizontal table with no other vertical forces).
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. A student says: “The book is at rest, so no forces act on it.” Correct them in one or two sentences.
Sample answer
Velocity can be zero while forces balance. The book typically has weight downward and normal force upward from the table; net force is zero, so acceleration is zero (first law).