Experiment E1935 Mechanical Energy

Figure 1. A girl throws a softball. Discussion The Model Marking the Video Graphical Analysis

Discussion

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?

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The Model

Free Body Diagram

Figure 2 shows three free body diagrams for this experiment, where the direction of DD_x and DD_yare 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 DD_y. 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 DD_y. 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?

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Marking the Video

1. Play the video and observe the motion.
2. Mark a point on the object until the video does not step any more.
3. What are your observations? Is the object moving in the positive x, negative x, positive y, or negative y direction(s)? What is happening to the object's height during the motion? What is happening to the object's speed during the motion?
4. Use the Circles - Vectors menu item to observe first the velocity and acceleration and then the momentum and net force. What are your observations?
5. Based on your observations, make rough sketches (no numbers, just the general shape) of the kinetic energy, potential energy and total energy versus time graphs. You should verify if your sketches are correct when you view these graphs.

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Graphical Analysis

1. Press the GRAPHS Button and the Data Analysis Choices will appear. Pick Option 1, then click the Next button.
2. Direction of Motion? Pick the correct direction(s) of motion based on your observations.
3. When selecting either y or both x and y for the direction of motion, you will see another window. This window asks if y is horizontal or vertical. If y is real-world vertical, then select vertical. The computer will then plot the gravitational potential energy on the energy plot.
4. From the plots options, select plots that will allow you to check the model. Click the Next button.
5. Examine the graphs. Are they the same shape as your sketches?
6. Does this experiment support the model?

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