I’ve got a couple quick demos that are great for demonstrating momentum in the classroom. These take very little time to set up, but can still be very effective in showing how momentum is conserved throughout a system!
The conservation of momentum states that the total momentum before a collision occurs is equal to the total momentum after the collision, as long as no outside forces are interfering. In the classroom, we discussed how this applies to car crashes and similar events. However, I did not want my students to actually be crashing cars in order to understand these concepts! So these are two simple activities students can perform on their own in order to grasp the concept further!
Here is what you need:
- Ruler (should be a solid wood or plastic one. Super thin rulers will not work well).
- A Dime
- A Quarter
- 2 meter sticks
- 5 marbles
Coins and Ruler
To set up these demonstration, you need to place the ruler on a flat surface with the dime placed right at the edge of the ruler. Place the quarter at the other end of the ruler, however, slide it back and “shoot” it toward the ruler so it hits the end with force. The momentum should cause the dime at the opposite end to move away from the ruler. The more force used with the quarter, the farther the dime will go!
Next, have students try it the opposite way. If you place the quarter on the edge and try to slide the dime into the ruler, the quarter may move, but not very far. Why? Because the quarter is more massive and will not travel as far of a distance. Newton’s 2nd law explains this through the equation force = mass x acceleration.
If you want to take it a step even further, you can have students calculate the mass of each coin and the distance they travel. Their ratios should be equal!
Newton’s Meter stick Cradle
The next demonstration uses the meter sticks and the marbles. Placing the two meter sticks side by side on a flat surface, you can create a small space opening as a “track” for the marbles. Start with resting two marbles next to each other on this track. Roll a third marble towards the two, and watch what happens! When the marbles hit, the collision causes the outside marble to roll away. If you place three marbles on the track and roll two towards them, two of the originally resting marbles will roll away. This is very similar to watching a Newton’s Cradle in action. Students can experiment with rolling different numbers of marbles and watching what happens. The total momentum will always be conserved – how every many marbles are rolled, that is how many of the resting marbles will begin moving! Newton’s 3rd law states that for every action, there is an equal and opposite reaction. Students will see that clearly here!
These two quick and easy demos should take no time to set up and students still love them! Quick, easy, but still great at explaining the concept of momentum!