Lenny wrote the steps he used to find the mass of an object with 400 J of energy moving at a velocity of 8 m/s.1. Find the square of 8 m/s, which is 64 m2/s2. 2. Divide kinetic energy, 400 J, by 64 m2/s2, which is 6.25 J per m2/s2.3. Divide 6.25 J per m2/s2 by 2 to get a mass of 3.125 kg.
Answer
A
In Step 1, he should have found the square root of 400 J instead of squaring 8 m/s.
B
In Step 1, he should have multiplied 8 m/s by 2 instead of squaring it.
C
In Step 2, he should have divided 64 m2/s2 by 400 J instead of dividing 400 J by 64 m2/s2.
D
In Step 3, he should have multiplied 6.25 J per m2/s2 by 2 instead of dividing it by 2.
Explanation
Kinetic energy is given by KE = ½mv², so solving for mass gives m = 2KE ÷ v². Lenny correctly found 8² = 64 and 400 ÷ 64 = 6.25, but he should multiply 6.25 by 2 to get 12.5 kg. Therefore, the correction in choice D is needed.
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