Physics Review Question

Unit: Energy
Year: 1996 Question#: 22
Question: A cart of mass (m) traveling at speed (v) has kinetic energy KE. If the mass of the cart is doubled and its speed is halved, the kinetic energy of the cart will be:

(1 ) half as great
(2 ) twice as great
(3 ) one-fourth as great
(4 ) four times as great

What is this question really asking?

Explanation by: Dustin C.

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Answer 1
half as great

CORRECT
Remembering KE = 0.5 mv2
First we are doubling the mass which equals 2x the kinetic energy. Then the speed is halved which results in one-quarter the kinetic energy. When combined the doubling times the quarter equals a half - which is why it is half as great. return to top























Answer 2
twice as great

INCORRECT

This is incorrect because when the mass is doubled, the kinetic energy is doubled. But then you have to account for the halving of the speed. Remember KE = 0.5 mv2 return to top



























Answer 3
one-fourth as great

INCORRECT

You seem to have correctly accounted for the hlving of the speed but have forgotten about the doubling of the mass. Remember KE = 0.5 mv2 return to top


























Answer 4
four times as great

INCORRECT

This is incorrect because when the mass is 2x, the kinetic=2x. When the velocity is halved it equals 1/4 the kinetic. Combining 2x times a quarter equals half not four times as great. Remember KE = 0.5 mv2 return to top


























What's this question really asking?


This question is really asking how to find the kinetic energy of the cart described when, while traveling, both the mass and speed are changed. return to top

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