Question

1. what happens in a perfectly elastic collision when two objects of the same mass are both moving before the collision?

the velocity for both objects would increase
they would exchange velocities.
the velocity in both objects would stay the same
the velocity of both objects would be zero.

Explanation:

In a perfectly elastic collision, both momentum and kinetic energy are conserved. For two objects of equal mass ($m_1=m_2=m$) with initial velocities $v_{1i}$ and $v_{2i}$, solving the conservation of momentum ($m v_{1i} + m v_{2i} = m v_{1f} + m v_{2f}$) and conservation of kinetic energy ($\frac{1}{2}m v_{1i}^2 + \frac{1}{2}m v_{2i}^2 = \frac{1}{2}m v_{1f}^2 + \frac{1}{2}m v_{2f}^2$) equations shows that $v_{1f}=v_{2i}$ and $v_{2f}=v_{1i}$, meaning the objects swap velocities.

Answer:

B. They would exchange velocities.