University of Virginia
Physics Department

Energy of a Pendulum

A Physical Science Activity

Student Activity

Where is the mass at the end of a pendulum moving the fastest?

Materials

• String about 1 meter long
• 1-kg mass with hook at the top
• 2-3 books
• Small wooden block approximately (5cm X 5cm X 5cm)
• Metric ruler

Procedure

1. Make a pendulum by tying one end of a piece of string to a hook on a 1 kg mass. This mass is called the pendulum bob. Hang the pendulum over the end of the edge of a desk or table so that it just clears the floor.
2. Tape the upper end of the string to the side of the desk so that the pendulum can swing freely. Swing the pendulum to be sure that it does not touch the floor.
3. Place a block of wood under the pendulum. The bob of the pendulum should hit the block at the bottom of its first swing. Mark the spot where the block will be with a piece of tape.
4. Keeping the string taught, raise the pendulum to a point 30 cm above the floor. You will need a ruler to measure this distance from the floor to the bob. Keeping the bob still, find the spot on the floor directly underneath the bob, mark it, and measure the distance from the spot under the bob to the wooden block. Enter the distance in the 1st data table below.
5. Release the bob and measure the distance that the block moved from the tape. Enter the distance in the table and repeat the experiment three times. You will need to find the average distance and enter it in the 1st data table.
6. Again, hold the bob 30 cm above the floor. Move the wooden block horizontally towards the bob, so that the bob will hit the block at about 1/2 the angle that the bob hit the block previously (see the diagram). Place the wooden block on top of 1 or 2 books so that the bob will still hit the block. Measure the new distance from the wooden block to the bob and enter it in table #2.
7. Release the bob from the 30-cm height and let it hit the block. Repeat the experiment three times and find the average as before. Enter all measurements in table #2.

Data Sheet

Table 1
 Trial Distance From Block to Bob Distance Block Moves 1 2 3
Average Distance Block Moved ____________________________

Table 2
 Trial Distance From Block to Bob Distance Block Moves 1 2 3
Average Distance Block Moved ____________________________

1. What happens to the block of wood when it was hit by the pendulum bob?

2. Kinetic energy is energy of motion and potential energy is stored energy. What kind of energy did the bob give to the block?

3. Where would you place the block for the pendulum bob to push the block the farthest?

4. If the energy of the pendulum is either stored (potential) or moving (kinetic), where in the swing does the pendulum have the most potential energy? Explain how you know.

5. Where in the swing does the pendulum have the most kinetic energy (speed)? Explain how you know.