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# Potential Energy

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Source:
Author: Janice VanCleave

### Purpose

To determine the effect of distance on the gravitational potential energy of an object.

### Materials

• 1 cup (250 mL) dry rice
• Sock
• Gram food scale
• Calculator
• Meter stick
• Helper

### Procedure

Force weight(N) = mass(g) × 0.0098 N/g
w = fwt × d
1. Pour the rice in the sock and tie the sock.
2. With the food scale, measure the mass of the sock to the nearest gram.
3. Determine the force weight (fwt) of the rice-filled sock in the metric weight unit called newtons (N). Do this by using the calculator and the following equation. Record the weight as the force weight of trials 1 and 2 in an Energy Data table like the one shown.
4. Calculate the work (w) done if the sock was lifted a distance (d) of 0.5 m, using the following equation. The force needed to lift the sock is equal to its weight, which can be called the force weight ( fwt). Record the calculated work in the data table.
5. Ask your helper to hold the meterstick vertically with one end on the floor and raise the sock to a height of 0.5 m above the floor. Hold your hand just above the floor and in line with the sock.
6. Ask your helper to release the sock. Make note of how the sock feels when it hits your hand. Record your observations in the data table.
7. Repeat steps 4 to 6 using a height of 1 m.

### Results

The higher the sock is held, the more work that is done in lifting it and the harder it strikes your hand when dropped.

### Why?

Energy is the ability to do work, which occurs when a force causes an object to be moved. Potential energy is stored energy. When an object is raised above a surface, it is said to have gravitational potential energy. The higher the object is raised, the greater its gravitational potential energy. Gravitational potential energy is also equal to the work done to raise it, which is equal to the work the object can do when it falls from its raised position. As the object falls, its potential energy changes to kinetic energy (energy of a moving object). In this investigation, the sock gained gravitational potential energy when work was done on it by raising it. This stored energy changed to kinetic energy as the sock fell, and by the time it hit the hand, all the potential energy had changed to kinetic energy. The falling sock did work on the hand equal to the work that was done to raise the sock. The higher the sock, the greater the work done to lift it, the greater the amount of gravitational potential energy it has when it's at its height, and the greater the amount of kinetic energy it has when it hits the hand. Work and energy in this investigation are measured in Nm units, which are equal to joules. One joule is the amount of work done when a force of 1 N is applied over a distance of 1 m.

### For Further Investigation

A large rock would hurt more than a small rock if it fell on you. How would more rice in the sock affect its energy? A project question might be, How does mass affect the gravitational potential energy of an object?

### Clues for Your Investigation

1. Repeat the investigation two or more times, using different amounts of rice in the sock each time but keeping the height the same for each test.
2. Record data in a table like the one shown.
3. Prepare a line graph comparing mass and work. Mass will be on the x-axis and work on the y-axis.

### References and Project Books

Ardley, Neil. The Science Book of Gravity. New York: Harcourt Brace Jovanovich, 1992.

Doherty, Paul, and Don Rathjen. The Cool Hot Rod and Other Electrifying Experiments on Energy and Matter. New York: Wiley, 1991.

Franklin, Sharon. Power UP! Glenview, Ill.: Good Year Books, 1995.

VanCleave, Janice.]anice VanCleave's Gravity. New York: Wiley, 1993

Janice VanCleave's Physics/or Every Kid. New York: Wiley, 1991

Wiese, Jim. Roller Coaster Science. New York: Wiley, 1994.

Williams, Brian. Science and Technology. New York: Kingfisher Books, 1993.

Wood, Robert W. Mechanics Fundamentals: Funtastic Science Activities for Kids. New York: Learning Triangle Press, 1997.

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