Physics Core/AP 1 and 2

Physics Lab

# Physics Lab: Investigating Elastic Collisions

## Goal: Investigate conservation of momentum in elastic collisions.

Ideally, you would do this with pucks on an air table (such as the kind used for air hockey). If you don't have access to one, use the instructions below to create "ice pucks" to use on a glass surface.

#### Materials

1. Pendulum
• Fixed point for attaching string.
• String (measure length!)
• Bob of known mass; should be equal to or less than the mass of the ice pucks.
• Ruler to measure height to which you raise bob.
2. Six tuna cans or other cylindrical mold
3. Distilled water
4. Glass plate, large mirror, or other flat, very smooth surface
5. Cardboard barriers to run around the edge of the flat surface, except where the pendulum bob will hit the puck.

#### Procedures

1. Set up the pendulum and glass surface as shown in the diagram.
2. Puck Creation.
• Fill each tuna can with the same amount of distilled water and freeze it, making sure that the cans are set on a level surface.
• When you are ready to perform the experiment, take out the number of cans you need for that experiment. Set each briefly in a pan of hot water to loosen the "ice pucks" and pop them from the cans. If necessary to get a smooth sliding surface, partially melt one of the circular sides until it is flat.
• Weigh each ice puck and record its weight.
3. Wipe the glass surface clean.
4. Pendulum-single puck collision.

• Based on the masses of the puck and bob, predict the collision behavior of both. Will the bob bounce back? Will the puck move?
• Raise the pendulum bob so that when you let go, it will collide with the puck. Record puck elevation, then let go.
• Record the behavior of the puck and pendulum.
5. Puck collision

• Based on the masses of the bob and both pucks, predict the behavior when the bob mass hits the first puck, and when the first puck hits the second puck.
• Perform the collision, recording the height of the pendulum bob.

#### Data Analysis

1. Create a table that contains columns for bob mass, pendulum height, potential energy stored in the pendulum, puck mass, velocity of pendulum puck after collision, velocity of ice puck after collision. If there is a second puck involved, include columns for its mass and post-collision velocity.
2. Include one final column with a conclusion about whether your observations support the calculated velocities.