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## A car moves at a constant velocity of 30 m/s and has 3.6 × 105 J of kinetic energy. The driver applies the brakes and the car stops in 95 m.

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## Answers ( )

The force needed to the stop the car is

-3.79 N.Explanation:The force required to stop the car should have equal magnitude as the force required to move the car but in opposite direction. This is in accordance with the

Newton’s third law of motion. Since, in the present problem, we know the kinetic energy and velocity of the moving car, we can determine themass of the car from these two parameters.So, here v = 30 m/s and k.E. = 3.6 × 10⁵ J, then mass will be

Now, we know that the

work doneby the brake tostop the carwill be equal to theproduct of force to stop the carwith thedistance travelled by the car on applying the brake.Here it is said that thecar travels 95 mafter the brake has been applied. So with thehelp of work energy theorem,Work done = Final kinetic energy – Initial kinetic energyWork done = Force × DisplacementSo, Force × Displacement = Final kinetic energy – Initial Kinetic energy.Thus, the force needed to the stop the car is

-3.79 N.