The Spinning Sprinkler

Research Question:

How does Newton’s third law work?

Throw a ball against a wall, and it bounces off. Did you know that the wall actually pushed back at the ball? In fact, Sir Isaac Newton’s third law of motion is that every action has an equal reaction in the opposite direction. You can demonstrate this law using a milk carton hanging from a tree branch.

Materials:

• Recycled paper milk carton (half gallon)
• Nail
• String (2 feet)
• Tree with horizontal branch

Experimental Procedure:

1. Using the nail, poke a hole into the bottom left corner of one of the sides of the carton. Then punch similar holes in the bottom left corner of each of the other sides of the carton.
2. Poke another hole through the top of the carton, in the center of the small flap sticking up.
3. Thread the string through the hole on top of the carton. Tie one end of the string in a knot to secure it to the carton.
4. Tie the other end of the string around the horizontal tree branch. The milk carton should now be hanging from the branch.
5. Have two helpers stand on either side of the carton. Each helper should place a right index finger on one of the holes in the bottom of the carton. They should then do the same thing with their left index fingers, so that each helper is covering up two of the holes.
6. Fill up the carton with water.
7. Tell your helpers to take their fingers off the holes – and duck! Watch what happens. How does this illustrate Newton’s third law?

Terms/Concepts: Newton’s third law; Equal and opposite reaction; How does a jet propel itself through the air?

References:

• Experiments You Can Do in Your Backyard, edited by Joanna Callihan and Nathan Hemmelgarn. Pp 50-51.