Newton's second law (F = ma)

Resultant force = mass × acceleration.

The common mistake

Using one force instead of the resultant (net) force when several act.

Instead: Find the net force first (subtract opposing forces), then F = ma.

Worked example with Guide me

A 5 kg box is pushed with 30 N against 10 N friction. Find its acceleration.

Sugar · Net force?
Student · 30 − 10 = 20 N.
Sugar · Rearrange: a = F/m.
Student · 20 ÷ 5 = 4.

Answer: 4 m/s²

Try 3 practice questions

1. Force to accelerate 2 kg at 3 m/s²?
Show answer
6 N
2. Mass if 50 N gives 10 m/s²?
Show answer
5 kg
3. Forces 12 N right and 12 N left — acceleration?
Show answer
0 (balanced)

Quick answers

What is the most common mistake with newton's second law (f = ma)?: Using one force instead of the resultant (net) force when several act.
How do I get newton's second law (f = ma) right?: Find the net force first (subtract opposing forces), then F = ma.
Can you show a worked example?: A 5 kg box is pushed with 30 N against 10 N friction. Find its acceleration. 4 m/s²

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