PHYSICS 123 EXPERIMENT NO. 5
CONSERVATION OF MOMENTUM
Purpose:
Conservation laws are very powerful tools in understanding physical phenomena. We have already examined some aspects of energy conservation laws in the laboratory. This week's work is devoted to observing momentum conservation.
What happens in collisions of objects is governed by momentum conservation. We use the air track to study collisions in one dimension. The goal is to see that momentum really is conserved in these relatively simple processes [Simple because no external forces act in the horizontal direction since there is no appreciable friction].
Two photogates connected to the computer are available for determining the velocities of the air track carts before and after collisions. Be sure to obtain the initial and final velocities of both carts.
You should obtain results which verify momentum conservation to better than 10%, with reasonably careful measurement technique.
Procedure:
1. Weigh air track carts having different masses m1 and m2.
2. The photogates in this experiment are connected to the grey box with the computer set to "Collision Timing Mode”. The computer determines the time that the light falling on the photocell is interrupted by the opaque "flag" which is attached to the cart. The lab instructor will show you how to use the timing equipment. The velocity of a cart that passes through the photogate is found by dividing the width of the "flag" () by the time determined by the computer.
For good results it is important to accurately measure .
Position the cart so that the front edge of the flag just begins to interrupt the light beam. Record the position of the front edge of the cart, then move it until the back edge of the flag begins to move out of the light beam. Again record the position of the front edge of the cart. The difference of these position readings gives the flag width .
Go through the same procedure for the other photogate with the other cart. Now you are ready to take some data!
3. Level the track.
4. Study elasticcollisions:For the first collision, you will collide the small glider with the big glider, which is at rest (the target). Turn on the air track and make sure that the non-velcro ends of each glider, the ends with the springs, are facing each other. Place the big glider in between the photo gates, not far from the second (downstream) photo gate and gently (you want to avoid an up-down wobble of the glider which will cause friction) launch the small glider toward the resting big glider. Observe the directions that the two gliders take after the collision, and enter your observations in your lab notebook.
After the collision, you should see a table of various times t on the screen similar to the one below:
t1 / t2’
t1’ / 0
Primes indicate time after collision.
Repeat the process with the bigger cart colliding with the smaller one. From your data, compute the initial and final momenta in each case. Also compute the initial and final kinetic energies.
5. Study an inelasticcollision. Install the "Velcro” strips at the ends of the carts so they stick together. Do the same computations you did in step 4 above.
Analysis:
1. Calculate the total initial momentum and the total final momentum for all 4 collisions (2 elastic and 2 inelastic). Find the percent difference between the initial and final for each collision. Question 1: Is momentum conserved in all collisions? If not, which ones don’t conserve it?
2. Calculate the total initial kinetic energy and total final kinetic energy for all 4 collisions. Find the percent difference for each collision. Question 2: Is kinetic energy conserved in all collisions? If not, which ones don’t conserve it?
Question 3:
The air track reduces friction, but does not entirely eliminate it. Air resistance alsoaffects the motion. Would these forces cause the momentum to increase or decrease? Why?
What you need:
- Masses of both carts
- Widths of flags on both carts
- Initial and final times for both carts in 4 collisions
- Initial and final momentums for 4 collisions
- Percent difference between initial and final momentums for 4 collisions
- Initial and final kinetic energies for 4 collisions
- Percent difference between initial and final kinetic energies for 4 collisions
- Answers to 3 questions