Figure 1. A girl throws a softball. |
Figure 1 shows a girl throwing a softball into the air. If you watch the video closely, you will see that the throwing motion is included in this experiment. What will happen to the object's total mechanical energy, kinetic energy and potential energy during the throwing motion? What will happen to these energies after the softball leaves the girl's hand?
Free Body Diagram
Figure 2 shows three free body diagrams for this experiment, where the direction of DDx and DDyare show for each case. We will discuss each case separately below.
The Throw
While the girl is throwing the ball, there will be two forces. The normal is at an angle and has both horizontal and vertical components, while the weight's direction is straight down. What kind of work, positive or negative, will each force do on the ball in the vertical direction? Do either of the forces do work in the horizontal direction? What will happen to the total, kinetic, and potential energies during the throw?
Moving Up
After the ball leaves the girl's hand and while it is traveling upward, the weight will be the only significant force acting on the ball. In this region, the weight is in the opposite direction of DDy. What kind of work, positive or negative, will the weight do on the ball in the vertical direction? Will the weight do any work in the horizontal direction? What will happen to the total, kinetic, and potential energies while the ball is traveling upward?
Moving Down
While the ball is traveling downward, the weight is still the only significant force acting on the ball. However, the weight is now in the same direction as DDy. What kind of work, positive or negative, will the weight do on the ball in the vertical direction? Will the weight do any work in the horizontal direction? What will happen to the total, kinetic, and potential energies while the ball is traveling downward?