Newton's second law says the resultant force equals mass times acceleration: F = ma. A resultant force of 96 N on a 12 kg particle gives a = 96 ÷ 12 = 8 m/s².
Objects moving freely under gravity have acceleration g = 9.8 m/s² downwards. A ball thrown up at 19 m/s stops at the top, where v = 0. Using v = u + at with a = −9.8 gives the time to reach the top: t = 19 ÷ 9.8 = 1.94 s. Draw a force diagram for every problem.
Worked example
A ball is thrown vertically up at 24 m/s. Find its greatest height. Use g = 9.8.
- At the top v = 0: v² = u² + 2as gives 0 = 576 − 19.6s.
- s = 576 ÷ 19.6 = 29.4.
Answer:29.4 m
Key words
- resultant force
- the overall force after combining all forces
- g
- acceleration due to gravity, 9.8 m/s²
- force diagram
- a sketch showing every force
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