Clamp the 18 inch aluminum bar to a sturdy table with the end hanging over the edge. One steel marble should be placed on either side of the bar. Measure the mass of the two marbles with one of the balances in the lab.
Sriking the bar with a hammer will cause the ball which is resting on the same side as the hammer to drop straight down but the ball on the other side will receive the kinetic energy from the hammer and will be propelled forward as it drops.
The position of the second ball when it drops can be used to solve for the force with which the hammer struck the bar because using what we know of vectors and the acceleration due to gravity, we can find the initial velocity of the ball.