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Q.

A car weighing 1000 kg and travelling at 30 m/s stops at a distance of 50 m decelerating uniformly. What is the force exerted on it by the brakes?


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a

+15000 N

b

-9000 N

c

-15000 N

d

+10200 N 

answer is B.

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Detailed Solution

The force exerted by brakes is -9000 N.
Given data:
Mass of car, m=1000 kg
Initial speed of the car, u=30 m/s
The car stopped after the break then the final speed of the car, v=0 m/s
Displacement during the apply break, s=50 m
Step 1:
Find the acceleration of the car.
As we know, from the equation of motion.
v2=u2+2as
Where v is the final speed, u is the initial speed, s is the displacement and a is the acceleration.
02=302+2××50
100a=-900
a=-900100
a=-9 m/s2    …..(i)
Thus, Acceleration, = -9 m/s2
Step 2:
Find the force exerted by the brakes.
Formula used:
F=ma
Where F is the force, m is the mass of the object and a is the acceleration.
Now,
F=1000×(-9)           [From equation (i)]
F=1000×(-9)    F=-9000 N                 …… (ii)
The negative sign shows that it is a retarding force.
 
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