Return to Physics Index
Sandi Sorkin Newton Bateman
4220 N. Richmond
Chicago IL 60618
The main objective of this mini-teach is to demonstrate the concept of momentum.
The lesson is designed for grade levels 3, 4, and 5.
3 dynamics carts
block of wood
marked off measurements
4 empty pop cans
small fire cracker
1. Hold the cart in your hand and demonstrate how the cart works by releasing
the spring. Ask the students, "What happens when I release the spring? Why
doesn't it go?"
2. Take the cart and place it next to a wooden block. Ask the students "What do
you think will happen when I release the spring?" Get their responses. Ask,
3. Take 2 carts back to back and place on a wooden board (platform). Ask the
class what will happen? Release the cart and measure the distance. What
4. Compare the results of the equal carts with the cart against the block. Did
they do the same thing?
5. Double the weight of one cart by placing a cart on top of another cart. Ask
class what will happen. Get response and then release the spring.
6. Place a block at each end of a flat board. Place a single cart back to back
with a double cart. Ask the class how the carts should be placed so that they
will hit the blocks at the same time. "What can we say?" Write on the board
that "lighter" is faster and "heavier" is slower.
7. Place 2 single carts on a balance board or teeter totter. What will happen
when the spring is released? Will they stay balanced?
8. Place 2 unequal carts on teeter totter and ask how you should move carts so
they reach end of board at the same time and the board remains balanced.
Explain that when the heavier cart is half as far from pivot the board is always
balanced. This only happens if the speeds are 2:1.
9. Take out the Jensen Bar and ask if you move the heavier weight on one hook
where do you move the lighter weight to balance? Compare this information with
where the carts land on the teeter totter. Ask class if the distance is the
same ratio. Jensen Bar shows 2 mass x 1 distance = 1 mass x 2 distance.
Momentum shows that 2 mass x 1 speed = 1 mass x 2 speeds. When the total
momentum is zero the teeter totter is balanced.
10. Place 3 carts back to back and release the springs. What happens?
11. Have 2 children on skates face to face and ask what is going to happen if
they push off of each other. Have the children push off and observe.
12. Have the skaters facing the same direction. What will happen when the
skater in back pushes from behind? Have skaters do this.
13. Using 4 or 5 skaters lined up - ask what is going to happen when skater at
the end pushes skater in front and then each one every 2 seconds pushes the
other. Have the skaters enact this. For variety have skaters randomly change
their order and see what happens.
14. Take 2 empty pop cans of equal weight and place a fire cracker taped between
them. Ask what will happen. Set off the fire cracker. Now repeat this, but
this time have some melted wax in only one can, so that it is twice the mass of
the other, but do not let the students know.
Do not tell students that the weight in one of the cans is heavier. As an
assessment ask what happened when the 2 cans were set off and explain why.