Kinematics and forces
Lesson 10 of 10
Newton's second law
Objective. Apply F = ma to systems including weight and friction.
The key idea
Resolve forces in the direction of motion. The resultant force equals mass times acceleration; weight is mg acting downwards.
Worked example
A 4 kg box is pulled with 20 N against 4 N friction. Find the acceleration.
- 1Resultant force: 20 − 4 = 16 N.
- 2a = F ÷ m = 16 ÷ 4.
Answer: 4 m/s²
Practice
Each question comes with a picture and a listen button. Try it first, then reveal the answer.
Common mistake. Using mass in newtons or weight in kilograms.
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