Newton's second law

Objective. Apply F = ma to systems including weight and friction.

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Learn: 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.

1 $Resultant force: 20 - 4 = 16 N.$
2 $a = F \div m = 16 \div 4.$

Answer: 4 m/s²

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Common mistake. Using mass in newtons or weight in kilograms.

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