Question

a car is moving with the velocity of108km/h & it takes 4sec to stop it .after the brakes are applied . calculate the force exerted by brakes from the motor car . if the mass along its passenger is 1000 kg

Answer 1

force  = mass x acceleration 
F = ma
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u = 108 x 5/18 = 30m/s
v = 0
t=4 s
m = 1000kg
a = v - u/t

=0- 30 /4
= -7.5 m/s²
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F =ma

=1000 x (-7.5)

-7500N
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-ve sign indicates that the force is in opposite direction

Answer 2

108 kmph = 30 m/s

a = (v - u) / t
= (0 - 30) m/s / (4 s)
= -7.5 m/s²

F = ma
= 1000 kg × (-7.5 m/s²)
= -7500 N
Here -ve sign indicates that Force is acting opposite to the direction of motion of car.

Magnitude of Force exerted by brakes is 7500 N