METRONMETHOD. REASON. LEARN.
Forces · 12 min

Newton’s laws

They explain why a trolley needs a push to accelerate and why an object can keep moving even when the resultant force is zero.

01Why this matters

They explain why a trolley needs a push to accelerate and why an object can keep moving even when the resultant force is zero.

02See it

12 N4 kg
A 12 N resultant accelerates a 4 kg trolley to the right at 3 m/s². Only the resultant force is drawn.
12 N resultant4 kg massa = F/m3 m/s²
Read left to right: 12 N resultant → 4 kg mass → a = F/m → 3 m/s². Each panel connects one part of the example.

03Understand it

Newton’s laws connect forces to changes in motion and describe how interacting objects exert forces on each other.

The first law says an object maintains constant velocity when the resultant external force is zero, in an inertial reference frame. The second law gives Fnet = ma for constant mass. The third law says interacting objects exert equal and opposite forces on each other; those forces act on different objects.

04Guided example

A 4 kg trolley experiences a resultant horizontal force of 12 N. Find its acceleration.

  1. Use the resultant force, after combining all horizontal forces: Fnet = 12 N.
  2. Rearrange Fnet = ma to a = Fnet/m.
  3. Substitute: a = 12/4 = 3 m/s², in the direction of the resultant force.

05Try it

Mass 5 kg, acceleration 2 m/s². Resultant force?
A little hint

Multiply mass by acceleration.

06Practise it

Resultant force 18 N, mass 6 kg. Acceleration?
A little hint

Divide resultant force by mass.

A 2 kg object has zero resultant force. Acceleration?
A little hint

Use a = Fnet/m.

07Use it in the real world

Pushing a loaded trolley with the same resultant force gives less acceleration because the mass is greater.

Go a little deeper

Newton’s laws work extremely well for ordinary speeds and macroscopic objects. Relativity and quantum mechanics extend our models outside those conditions.

08Quick check

Forces 15 N right and 5 N left act on 2 kg. Acceleration right?
A little hint

Find the resultant first.

09Summary

Newton’s laws connect forces to changes in motion and describe how interacting objects exert forces on each other. Substitute: a = 12/4 = 3 m/s², in the direction of the resultant force.

Before moving on, explain the worked example in your own words. Use the hint on any question you want to revisit.

10Key idea

Fnet = ma

Fnet: resultant force in N; m: mass in kg; a: acceleration in m/s².

a = Fnet/m; m = Fnet/a when a ≠ 0

Find this in the formula library →