Question

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.

Which best describes Lenny’s error?

In Step 1, he should have found the square root of 400 J instead of squaring 8 m/s.
In Step 1, he should have multiplied 8 m/s by 2 instead of squaring it.
In Step 2, he should have divided 64 m2/s2 by 400 J instead of dividing 400 J by 64 m2/s2.
In Step 3, he should have multiplied 6.25 J per m2/s2 by 2 instead of dividing it by 2.

Answer 1

The fourth option.

Because KE = 1/2 m v^2, m = 2KE / v^2. Lenny correctly computed v^2 = 64 and 400/64 = 6.25, but he then divided by 2 instead of multiplying by 2. He should have done 2 × 6.25 = 12.5 kg (not 3.125 kg).