A 900-kg compact car moving at 60 miles/hr has approximately 320,000 Joules of kinetic energy. Estimate its new kinetic energy if it is moving at 30 miles/hr.

40 000 J

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80 000 J

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50 000 J

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20 000 J

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Solution

The correct option is B
80 000 J

The Kinetic Energy is directly related to the square of the speed.

If the speed is reduced by a factor of 2 (as in from 60 mi/hr to 30 mi/hr) then the KE will be reduced by a factor of 4.

Thus, the new Kinetic Energy is $\frac{(320000 J)}{4}$ = 80 000 J.

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