Energy of a Tossed Ball

Experiment 16 Energy of a Tossed Ball

When a juggler tosses a bean ball straight upward, the ball slows down until it reaches the top of its path and then speeds up on its way back down. In terms of energy, when the ball is released it has kinetic energy, KE. As it rises during its free-fall phase it slows down, loses kinetic energy, and gains gravitational potential energy, PE. As it starts down, still in free fall, the stored gravitational potential energy is converted back into kinetic energy as the object falls. If there is no work done by frictional forces, the total energy (mechanical energy) will remain constant. In this experiment, we will see if this works out for the toss of a ball.

Motion Detector

In this experiment, we will study these energy changes using a Motion Detector. OBJECTIVES  Measure the change in the kinetic and potential energies as a ball moves in free fall.  See how the total energy (mechanical energy) of the ball changes during free fall. MATERIALS computer bball Vernier computer interface Logger Pro Motion Detector PRELIMINARY QUESTIONS For each question, consider the free-fall portion of the motion of a ball tossed straight upward, starting just as the ball is released to just before it is caught. Assume that there is very little air resistance. 1. What form or form(s) of energy does the ball have while momentarily at rest at the top of the path?

2. What form or form(s) of energy does the ball have while in motion at the bottom of the path?

3. Sketch a graph of kinetic energy vs. time for the ball.

4. Sketch a graph of potential energy vs. time for the ball.

Physics with Computers 16 - 1 Experiment 16 PROCEDURE 1. Log on to the computer. While you are waiting for it to get going, measure and record the mass of the ball in the data table.

2. Connect the Motion Detector to the DIG/SONIC 1 channel of the interface. Place the Motion Detector on the floor. 3. Double click “Logger Pro”. Click on “open”, go to “Physics with Vernier” Click “16 Energy of a Tossed Ball”. 4. Get the mass of the ball and enter it into Logger Pre. There should be a box in the corner where you can input your mass. (If not, let me know and I can show you an alternate way to get the mass adjusted.) 5. Hold the ball about 0.5 m directly above the Motion Detector. In this step, you will use both hands to toss the ball straight upward above the Motion Detector and let it fall back toward the Motion Detector. Be sure to pull your hands away from the ball after it starts moving so they are not picked up by the Motion Detector. Throw the ball so it reaches maximum height no more than 1.5 m above the Motion Detector. Have your partner click to begin data collection. Toss the ball straight up after you hear the Motion Detector begin to click. Catch the ball about 0.5 m above the motion detector. You need to make sure that the position vs. time graph corresponding to the free-fall motion is a nice, pretty parabola, without spikes or flat regions, before you continue. This step may require some practice. If necessary, repeat the toss, until you get a good graph. When you have good data on the screen, proceed to the Analysis section to fill in the data table.

DATA TABLE

Mass of the ball (kg)

Position Time Height Velocity PE KE TE (s) (m) (m/s) (J) (J) (J)

After release

Top of path

Before catch

16 - 2 Physics with Computers Energy of a Tossed Ball

ANALYSIS 1. Click on the Examine button, , and move the mouse across the position or velocity graphs of the motion of the ball to answer these questions. a. Identify the portion of each graph where the ball had just left your hands and was in free fall. Measure and record in the data table the height and velocity of the ball at this time.

b. Identify the point on each graph where the ball was at the top of its path. Measure and record in the data table the time, height, and velocity of the ball at this point. c. Find a time where the ball was moving downward, but a short time before it was caught. Measure and record in the data table the height and velocity of the ball at that time.

d. For each of the three points in your data table, find the Potential Energy (PE), Kinetic Energy (KE), and Total Energy (TE) of each point on the page 3 data table in Logger Pro. 2. What does it mean to say that “energy is conserved”?

How well does this part of the experiment show conservation of energy? Explain.

3. Go to the page 2 by clicking on the Next Page button, . You should have graphs of KE, PE, and TE.

4. Compare your energy graphs predictions (from the Preliminary Questions) to the real data for the ball toss. Explain any discrepancies.

5. Print the energy graph. Label the section of the graph where the ball was in free fall. Color code or in some way indicate which line is PE, KE, and TE. Does the total energy of a ball in free fall remain constant? How can you tell? (Discuss the shape of the graph.)

6. Is the total energy of the ball constant when NOT in free fall? How can you tell? (Discuss the shape of the graph.)

Physics with Computers 16 - 3 Experiment 16 Physics Energy of a Tossed Ball Lab Preliminary Questions: For each question, consider the free-fall portion of the motion of a ball tossed straight upward, starting just as the ball is released to just before it is caught. Assume that there is very little air resistance. 1. (1 point) What form or form(s) of energy does the ball have while momentarily at rest at the top of the path?

2. (1 point) What form or form(s) of energy does the ball have while in motion at the bottom of the path?

3. (3 points) Sketch a graph of kinetic energy vs. time for the ball. Don’t forget to label your axes.

4. (3 points) Sketch a graph of potential energy vs. time for the ball. Don’t forget to label your axes.

Data Table: (10 points)

Mass of the ball (kg)

Position Time Height Velocity PE KE TE (s) (m) (m/s) (J) (J) (J)

After release

Top of path

Before catch

Analysis: 1. (see instructions online) 2. (1 point) When we say that “energy is conserved” we mean that the numerical value of energy is ______. (2 points) How well does this part of the experiment show conservation of energy? Explain.

3. (see instructions online)

16 - 4 Physics with Computers Energy of a Tossed Ball 4. Compare your energy graph predictions (from the Preliminary Questions) to the real data for the ball toss. Explain any discrepancies.

5. Print the energy graph. (Page 2 only!) (1 point) Label the section of the graph where the ball was in free fall. (3 points) Color code or in some way indicate which line is PE, KE, and TE. (2 points) Does the total energy of a ball remain constant while in free fall? How can you tell? (Discuss the shape of the graph.)

6. (2 points) Is the total energy of the ball constant when NOT in free fall? How can you tell? (Discuss the shape of the graph.)

(1 point) For the portion of the graph just before the free fall section, is the total energy increasing or decreasing?

(2 point) What is causing this change in total energy?

Physics with Computers 16 - 5