Physics Review Question

Unit: Work and Energy
Year: 1992 Question#: 17
Question: A 20.-kilogram object strikes the ground with 1,960 joules of kinetic energy after falling freely from rest. How far above the ground was the object when it was released?

(1 ) 10. m
(2 ) 14 m
(3 ) 98 m
(4 ) 200 m

What is this question really asking?




Explanation by: Jessica S

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Answer 1
10 m

CORRECT

Because of the law of conservation of energy, there is no net gain or loss of energy in this system. Therefore, all the energy at the end of the fall (1960 joules) must also have been there at the beginning. The only difference is that the energy at the beginning was GPE and the energy at the end is KE. Since we know that KEbefore = GPEafter = 1960 joules = mgh, we can solve for h because w know m (20 kg) and g (9.8 m/s2). Substituting in these values, we get 1960 joules - 20. Kg x 9.8 m/s2 x h. Dividing through by 20. Kg x 9.8 m/s2, we get h = 10 m. return to top























Answer 2
14 m

INCORRECT

I don't know how you got this. Did you guess randomly? Try thinking through this problem again; use the law of conservation of energy. return to top



























Answer 3
98 m

INCORRECT

I don't know how you got this. Maybe you divided 1960 joules by 20 kg because these are the numbers given in the problem. This is incorrect. This would yield 98Nm/kg; this is not a height. Try this problem again and use the law conservation of energy. return to top


























Answer 4
200 m

INCORRECT

This is incorrect. I don't know how you got this. Perhaps you divided 1960 joules by the gravitatinoal constant, 9.8 m/s2. This is incorrect, and would yield 200 Ns2; this is not a height. Try this problem again, and use the law of conservation of energy. return to top


























What's this question really asking?

This question is testing your knowledge of the law of conservation of energy. return to top

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