The best prediction for the second part of Mark's experiment is:
The wagon will have more kinetic energy than when it was empty.
This is because kinetic energy (KE) is calculated using the formula \( KE = \frac{1}{2}mv^2 \), where \( m \) is mass and \( v \) is velocity. When the bag of sand is added, the mass of the wagon increases, leading to a higher kinetic energy, assuming the velocity remains the same.