Name: ______Perfectly Inelastic Collisions Class: ______

Pre-Lab Questions Page Roster#______

1. List two possible units for momentum. Instructor:______

2. Define the term “perfectly inelastic collision”.

3. During inelastic collisions which of the following statements are true:

I. The kinetic energy is conserved. II. The momentum is conserved.

a. Statement I b. Statement II c. Both I and II d. Neither I or II

4. State the “Principle of Conservation of Linear Momentum.”

5. A 2000 kg truck is moving at a rate of 20 m/s South, calculate the linear momentum and the kinetic energy of the truck.

1 1 ______PERFECTLY INELASTIC COLLISIONS

OBJECTIVE: To verify the principle of conservation of linear momentum and compute the amount of kinetic energy converted to heat energy for inelastic collisions.

APPARATUS: Air track with computer timer and two photogates Small glider with Velcro bumper Large glider with Velcro bumper Flag (1) Balance

INTRODUCTION:

Perfectly inelastic collisions (when the two objects stick together after the collision) are produced between a projectile glider and a stationary glider. Perfectly inelastic collisions are also referred to as completely inelastic collisions. In perfectly inelastic collisions the momentum of the system is conserved but there is a change the kinetic energy of the system. Both the projectile speed before the collision, v1, and the combined speed after the collision, v2, are measured. Conservation of momentum implies:

m1 v1 = (m1 + m 2) v2 Equation 1

where m1 is the projectile mass and m2 is the target mass. Therefore the final speed is ⎛ m ⎞ ⎜ 1 ⎟ v2 = ⎜ ⎟v1 Equation 2 ⎝ m1 + m2 ⎠

Equation 2 implies that a graph of the values of V2 versus the values of V1 would

m1 yield a straight line with slope . This slope is compared with the ratio m1 + m2 of the directly measured masses using the digital scale.

The Kinetic energy before and after the collision should NOT be equal or 1 1 m v2 ≠ ()m + m v2 2 1 1 2 1 2 2 Equation 3

PROCEDURE: (Make sure the air is on before you begin)

2 2 1. Place the timing gates approximately 30 cm apart and at the proper height so that the flag on top of the projectile glider (but not the glider itself) will trigger the photogates.

2. Level the track so that the target glider (small glider) will rest stationary between the gates.

3. Place the flag firmly in the slot on top of the projectile glider (large glider) near the front (Velcro end). Only one glider has a flag.

4. Turn the computer on, if it is not. Double click the “Physics Lab” folder and then double click the “Inelastic Collisions” icon.

5. When ready, click on the COLLECT button.

6. Position the target glider (small glider) between the two photogates, near the second gate. The projectile glider (large glider) should be placed to the left of the first photogate. The flag on the projectile glider must pass completely through the first gate before the collision and completely through the second photogate after the collision. The velcro ends of the gliders must face each other.

7. Launch the projectile glider from a point before the first photogate while holding the target gate at rest. Release the target glider before impact. Stop the coupled gliders before they rebound from the other end of the air-track. *If your data is not plotted on a graph of V2 versus V1 please ask the lab assistant for help.

8. Repeat step 7 for six different launch speeds. You can vary the launch speeds by pressing the launch glider against the rubber band launcher and displacing the rubber band various lengths.

9. Click on the STOP button. Save this data and graph to a disc, then exit the program.

10. Measure and record the mass of each glider. Be sure to include the mass of the flag on the projectile glider.

CALCULATIONS:

1. Take your disc to the computers in the main lab room, SM252. Select the “Physics Lab” folder, then open the file that contains “Logger Pro.”

2. Open the file for this lab. Go to the Analyze menu and select Linear Fit. Print the graph, make sure the title of the graph is “V2 vs V1”.

m1 3. Calculate the actual ratio from the values measured using the m1 + m2 digital scale. Compare the slope of the graph to this computed ratio, use the %error equation.

3 3

A − E x100 Recall: %error = A

4. Pick any one of the six collisions and verify conservation of momentum by computing the momentum after the collision and comparing this value to the momentum before the collision (use Equation 1). Use the %difference equation to do the comparison.

V −V Recall: %difference = 1 2 x200 V1 +V2

5. Using the same collision point as in part 4 of this section, compute the kinetic energy of the gliders after the collision (use Equation 3) and compare this value to the kinetic energy before the collision. This % difference is the portion of the kinetic energy transformed into heat during the collision process.

QUESTIONS:

1. If the target glider had an initial velocity, other than zero, what affect would that have on the final velocity of the system?

2. If the target glider had an initial velocity, other than zero, what affect would that have on the final momentum and final kinetic energy of the system?

3. The air track is used in a number of motion experiments, what is the purpose of using an air track as it relates to friction?

4. How is the momentum affected by the presence of friction?

5. How is the kinetic energy affected by the loss of energy due to heat?

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