Experiment 19 The of two carts on a track can be described in terms of conservation and, in some cases, energy conservation. If there is no net external force experienced by the system of two carts, then we expect the total of the system to be conserved. This is true regardless of the force acting between the carts. In contrast, energy is only conserved when certain types of forces are exerted between the carts. Collisions are classified as elastic (kinetic energy is conserved), inelastic (kinetic energy is lost) or completely inelastic (the objects stick together ). Sometimes s are described as super-elastic, if kinetic energy is gained. In this experiment you can observe most of these types of s and test for the conservation of and energy in each case. OBJECTIVES Observe s between two carts, testing for the conservation of. Measure energy changes during different types of s. Classify s as elastic, inelastic, or completely inelastic. MATERIALS LabPro or Universal Lab Interface II Logger Pro two Vernier Motion Detectors dynamics cart track two low-friction dynamics carts with magnetic and Velcro bumpers Physics with Computers 19-1
Experiment 19 PRELIMINARY QUESTIONS 1. Consider a head-on between two billiard balls. One is initially at rest and the other moves toward it. Sketch a position vs. time graph for each ball, starting with time the and ending a short time ward. 2. Is conserved in this? Is kinetic energy conserved? PROCEDURE 1. Measure the masses of your carts and record them in your data table. Label the carts as cart 1 and cart 2. 2. Set up the track so that it is horizontal. Test this by releasing a cart on the track from rest. The cart should not move. 3. Practice creating gentle s by placing cart 2 at rest in the middle of the track, and release cart 1 so it rolls toward the first cart, magnetic bumper toward magnetic bumper. The carts should smoothly repel one another without physically touching. 4. Place a Motion Detector at each end of the track, allowing for the 0.4 m minimum distance between detector and cart. Connect the Motion Detectors to DIG/SONIC 1 and DIG/SONIC 2 of the LabPro or PORT 1 and PORT 2 of the Universal Lab Interface. 5. Click the play button to begin taking data. Repeat the you practiced above and use the position graphs to verify that the Motion Detectors can track each cart properly throughout the entire range of motion. Call CPT Han if you need to adjust the position of one or both of the Motion Detectors. Part I: Magnetic Bumpers 6. Reposition the carts so the magnetic bumpers are facing one another. Click the play button to begin taking data and repeat the you practiced in Step 3. Make sure you keep your hands out of the way of the Motion Detectors you push the cart. 7. From the velocity graphs you can determine the velocity and the for each cart. To measure the velocity at a specific time, use the stylus to click on the graph across the interval. 19-2 Physics with Computers
8. Repeat Steps 6 and 7 two more times with the magnetic bumpers, recording the velocities in the data table. Part II: Velcro Bumpers 9. Change the by turning the carts so the Velcro bumpers face one another. The carts should stick together. Practice making the new, again starting with cart 2 at rest. 10. Click the play button to begin taking data and repeat the new. Using the procedure in Step 6 and 7, measure and record the cart velocities in your data table. 11. Repeat the previous steps two more times with the Velcro bumpers. DATA TABLE MAGNETIC BUMPERS: COLLISION Mass of cart 1 (kg) Mass of cart 2 (kg) x x Run number cart 1 cart 2 cart 1 cart 2 of cart 1 (p 1 ) of cart 2 (p 2 ) Percentage of preserved (p1/p2)x100 (m/s) (m/s) (m/s) (m/s) (kg m/s) (kg m/s) % 1 0 2 0 3 0 Average x x x x VELCRO BUMPERS: COLLISION Mass of cart 1 (kg) Mass of cart 2 (kg) x Run number of cart 1 of cart 2 of cart 1+2 Total Total Percentage of preserved (p1/p2) x100 (kg m/s) (kg m/s) (kg m/s) (kg m/s) (kg m/s) % 1 0 2 0 3 0 Physics with Computers 19-3
Experiment 19 ANALYSIS 1. Determine the ( mv ) of each cart the, the, and the total and the. Calculate the percentage of the total that was preserved the. Enter the values in your data table. 2. If the total for a system is the same and the, we say that is conserved. If were completely conserved, what would be the percentage of the total the to the total the? 3. For your six runs, inspect the percentages. Even if is conserved for a given, the measured values may not be exactly the same and due to measurement uncertainty. The percentage should be close to 100%. Provide at least two possible explanations as to why your data did not show 100% conservation of. Collision Practice Problems : Attach a separate sheet of paper showing all work 1) A 1000kg car moving 10m/s collides with a stationary 2000kg truck on ice (assume this is frictionless). The cars bounce off each other, and the 1000kg is at rest following the. What is the speed of the 2000kg truck following the? 2) A 1000kg car moving 10m/s collides with a stationary 2000kg truck on ice (assume this is frictionless). The car somehow got hooked on to the trailer hitch of the truck and the two stick to each other. What is the speed of the car and truck following the? 19-4 Physics with Computers
3) A 1000kg car moving 10m/s collides with a stationary deer. The is forceful enough to completely stop the car, as it applies a 20000N force to the deer. What is the contact time between the car and the deer? 4) A truck collides with a deer. The truck applied a 40000N force for 0.03s. A police officer near the site of the crash measures the speed of the deer with a radar gun and finds that it flies off the bumper at 15m/s.What is the mass of the deer? EXTENSIONS (TO BE COMPLETED INDEPENDENTLY) 1. (On the track with the magnetic bumpers) Using a cart with a spring plunger, create a super-elastic ; that is, a where kinetic energy increases. Ask CPT Han for instructions on how to set up the spring plunger. The plunger spring should be compressed and locked the, but then released during the. Measure and the and record the data on a separate sheet of paper. Is conserved in this case? 2. (On the track with the velcro bumpers) Perform a with the carts with velcro bumpers. Using the time and velocity data, plot your results on the graph below. After getting the data, ask CPT Han for instructions on which data points to include. Physics with Computers 19-5