Lenny's steps to determine the mass of an object using its kinetic energy and velocity include an error in the method he used to find the mass. The correct formula for kinetic energy (KE) is:
\[ KE = \frac{1}{2}mv^2 \]
where
- \( KE \) is the kinetic energy,
- \( m \) is the mass, and
- \( v \) is the velocity.
In this case, Lenny's calculations are as follows:
- He correctly found \( v^2 \) (which is \( (8 , \text{m/s})^2 = 64 , \text{m}^2/\text{s}^2 \)).
- He then incorrectly divided the kinetic energy by this value.
The correct approach from the kinetic energy formula is to rearrange to find the mass:
\[ m = \frac{2KE}{v^2} \]
This means that instead of dividing \( 400 , \text{J} \) by \( 64 , \text{m}^2/\text{s}^2 \), he should have multiplied \( 400 , \text{J} \) by 2 and then divided by \( 64 , \text{m}^2/\text{s}^2 \).
Thus, the best description of Lenny’s error is:
In Step 2, he should have divided \( 64 , \text{m}^2/\text{s}^2 \) by \( 400 , \text{J} \) instead of dividing \( 400 , \text{J} \) by \( 64 , \text{m}^2/\text{s}^2 \).
However, it should be clarified that he did not just need to switch the division; he should have been using the kinetic energy formula to calculate mass directly.