PURPOSE: To show Newton's Third Law; action and reaction.
DESCRIPTION: This demonstration uses two large skateboards. NEVER STAND ON THE BOARDS. They are specifically made to be very low friction. They are unsafe for standing. ALWAYS KNEEL OR SIT ON THE BOARDS.
This demonstration provides an excellent opportunity for student participation. Get two student volunteers of either equal weight or of differing weight. Have them kneel on the carts, as shown in the photos. Do NOT let them stand on the carts. They MUST kneel.
Three tests can be done to show action/reaction and conservation of momentum and energy:
1) Students face each other and both push equally in a Paddy-cake fashion, as shown in the photo above left, pushing on each others' hands. After showing this, consider what might happen when one student does not cooperate, ie, one student turns his/her back to the other so that only one set of hands is pushing. What will happen when?
2) The two students separated by about 10 ft. and each one grasping the end of a rope,
as shown in the photo above right. Have one student pull on the rope while the
other student just holds the rope. What will happen when one student reels the other
student in?
3) With the carts close together, ask the two students throw a heavy weight back and forth
to each other (not shown in the photos). What happens?
EQUIPMENT: Two large skateboards, rope and punching bag.
SETUP NOTES: Description below.
Human Dynamics Carts
Materials
2" x 20" x 30" piece of board
4 skate board wheels (hard rubber, 100 durometer)
4 ungreased race bearings, remove the grease by soaking in acetone.
4 bolts to fit the bearings
4 Aluminum blocks threaded for the bolts
16 1 ¼" wood screws
Instructions
1. Cut boards to specified lengths (although the exact lengths are not
critical).
2. Attach the threaded blocks to the 2" x 4" boards using the 1 ¼" screws.
Updated by JZ in 6/6/2000