a scooter weighs 120kg. Brakes are applied so that wheels stop rolling and start skidding. Find the force of friction if the coefficient of friction is 0.4

Assuming the scooter is moving horizontally on a flat surface, then by Newton's second law, the vertical forces (the normal force and the scooter's weight) cancel out:

n - w = 0

n = w

n = (120 kg) g = 1176 N

(where g = 9.8 m/s²)

Then the friction force has a magnitude of

f = µ n

where µ is the coefficient of kinetic friction, so that

f = 0.4 (1176 N) = 470.4 N ≈ 470 N

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