Physics Review Question

Unit: mechanics
Year: 1990 Question#: 21
Question: If the speed of an object is doubled, its kinetic energy will be

(1 ) halved
(2 ) doubled
(3 ) quartered
(4 ) quadrupled

What is this question really asking?


Explanation by: Lindsay K

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Answer 1
1) halved

INCORRECT

Try using the formula KE = (1/2)(m)[(v)(v)]. KE stands for kinetic energy, m stands for mass, and v stands for the speed. Plug in any number for m any number for v, then see what KE equals. Then try it a second time, using the same number for m, but double v. You'll see that KE does not become halved when you double v. return to top























Answer 2
2) doubled

INCORRECT

Just plug in any number for m in the formula and any number for v and see what you get. Then try it again keeping m the same, but double v, and this time see what you get. You'll see KE is not doubled when v is doubled. See, the expalnation for answer one again if you are unsure of the formula to use. return to top



























Answer 3
3) quartered

INCORRECT

Use the formula twice and see what you get (see explanation one if you are still unsure how to use formula). The KE will not be quartered when you double v. Also for this one, it is important to not get mixed up between QUARTERED and QUADRUPLED. return to top


























Answer 4
4) quadrupled

CORRECT

Use the formula for finding kinetice energy, KE = (1/2)(m)[(v)(v)]. Just like in the previous explanations, put in any number for m, and any number for v and calculate to see what you get. Then try it again, using the same number for m, but this time doubling v. You'll see that when v is doubled the KE is quadrupled (4 times as big).
Example: KE = (1/2)(m)[(v)(v)] m=3m v=6m/s
KE = (1/2)(3kg)[(6m/s)(6m/s)] KE = 54J

Try a second time with v doubled: KE = (1/2)(m)[(v)(v)] m=3kg v=12m/s KE = (1/2)(3kg)[(12m/s)(12m/s)] KE = 216J

And 54J times 4 equals 216 J, so KE is quadrupled when speed is doubled.

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