# A force of 2.5 kn acts on the car when braking. Determine the mass of the car if braking lasted 20s, and the initial speed was 36 km / h

January 18, 2021 | education

| 1) Let’s calculate the acceleration of the car.

V = V0 + a * t, where V (final speed) = 0 m / s, V0 (speed before deceleration) = 36 km / h or 10 m / s, t (deceleration time) = 20 s.

0 = V0 + a * t and a = -V0 / t (the movement is equally slow, a <0).

Calculation: a = -10 / 20 = -0.5 m / s2.

2) Let’s calculate the mass of the car (Newton’s second law):

F = m * a and m = F / a, where F (force acting when braking the vehicle) = 2.5 kN or 2500 N.

Calculation: m = 2500 / 0.5 = 5000 kg or 5 tons.

Answer: The vehicle weighs 5 tons.

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