Grade 11 · Physics

Apply Newton’s second law

Newton’s second law relates resultant force, mass and acceleration: Fnet = ma for an object of constant mass. In Grade 11 Physical Sciences, the important quantity is the vector sum of the forces, not just one applied force. Choose a positive direction and include opposing forces with negative signs before calculating acceleration. Give the answer with units and direction.

Original explanation prepared with AI assistance.

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What you will practise

Distinguish applied and net force, then calculate acceleration.

The method

Draw a free-body diagram, resolve forces along the chosen axis and apply Fnet = ma.

Worked example

A 5 kg trolley has a 20 N force to the right and 5 N friction to the left. Find its acceleration.

Choose right as positive.
The horizontal resultant is 20 − 5 = 15 N.
Use 15 = 5a to obtain a = 3 m/s².
The acceleration is 3 m/s² to the right. Vertical forces balance in this horizontal example.

Common mistake

Using 20 N as the net force ignores friction.

Try it yourself

A 4 kg object has forces of 18 N right and 6 N left. Find its acceleration.

Show the explained solution

Fnet = 18 − 6 = 12 N right. a = 12/4 = 3 m/s² right.

Curriculum and sources

Use the official CAPS subject document and your teacher’s annual plan to confirm the sequence and scope for your class. The link below is an official reference, not evidence of DBE approval of this explanation.

DBE: CAPS for the FET phase

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