Work and Energy

Research Questions:

• What is energy? What is work? How is it measured?
• How does the incline of a board affect how far a jar can roll down the board? Does a steeper incline necessarily mean that the jar will roll further?
• How does the weight of the jar affect how far it will roll? Does a heavier jar roll further? What happens when the jar is filled with water?

In these experiments, you will measure the distance that a jar rolls down an incline. You will vary the degree of the incline and weight of the jar to learn about energy. As perplexing as it seems, if you start rolling the jar from the same distance above the ground it will roll the same distance, regardless of the incline of the board.

This odd result can be explained by looking to the amount of energy it requires to push the bottle down the incline. The distance that the jar rolls depends upon how much energy it has when it reaches the bottom of the incline. That energy can be measured by looking to the weight of the jar and the height of the incline. For the experiments done here, energy can be measured according to the following formula:

 E = HW. 

In this formula, E stands for energy, H stands for height above the ground and W stands for weight of the jar. If your jar is measured in pounds and the height above the ground is measured as a fraction of a foot, the units of your energy will be in foot-pounds. Unlike varying the degree of the incline, varying the weight by using a lighter or heavier jar will produce very different outcomes. Varying the weight of the jar by filling it with water produces unpredictable outcomes because energy is lost by the sloshing of the water in the jar.

Materials:

• Large jar and lid. The jar should have a wide mouth and smooth sides. Large peanut butter or mayonnaise jars work well.
• Small jar and lid. Like the large jar, this should also have a wide mouth and smooth sides. The important consideration is that the small jar weighs appreciably less than the large jar.
• A large sturdy board, at least 1 foot wide and 4 feet long. The depth of the board does not matter. The board needs to be smooth, so as to cause as little friction as possible.
• Stack of books or blocks of various sizes for propping up the board
• Water
• Marking pencil
• Camera
• Masking tape

Experimental Procedure:

1. Using your stack of books or blocks, place several under the board to create an incline. The stack of books should be sufficient to raise the top of board exactly 3 inches off the ground directly underneath the books. Draw a line across the board exactly where the height is 3 inches off the ground. 
2. Stick a piece of masking tape so it runs the up and down one side of the jar from top to bottom. When you roll the bottle down the incline, be sure to align your tape with the line you drew in step one.
3.  Roll the large jar down the incline 10 times, being sure to align your tape with the line on the board that marks 3-inch height. For each roll, measure the distance that the jar rolled using the bottom of the incline as a starting point, and the center of the bottom of the resting jar as an end point. Average these distances together.
4. Increase the incline so that the board forms a steeper angle. Once again, identify the part of the board in which the top of the board is three inches from the ground and draw a parallel line across the board at this place. Roll the bottle ten times, starting the roll from the line you just drew. Repeat ten times and average together. Does changing the incline matter? What if you start rolling further from the ground?
5. Switch to using smaller jar that weighs less and repeat steps 2 – 4. Do your results make sense with respect to the equation appearing in the Introduction?

References:

• Hollinan, Kelly Logan. Isaac Newton and Physics for Kids: His Life and Ideas with 21 Activities (For Kids series). Chicago Review Press (2009)
• Keller, Rebecca. Real Science 4 Kids Physics Level I. Gravitas Publications, Inc. 2005
• Physics for Kids: Motion http://www.physics4kids.com/files/motion_intro.html