Dynamics Track Momentum, Energy, and Collisions

Transcription

1 Dynamics Track Momentum, Energy, and Collisions Student Handout Collisions between objects create some interesting questions about which conservation laws apply. In this lab you will be comparing elastic and inelastic collisions and determining whether momentum and/or kinetic energy are conserved. Equipment (per group): Dynamics Track ibook Computer Cart Launcher Vernier LabPro Interface Dynamics Cart End Stop - Motion Detectors Collision Cart g Masses USB Cable Power cube for LabPro - cables (Motion Detector to LabPro) Cautions: The carts: Never leave a cart on its wheels unless it is sitting on the track, as it will roll quietly off of the table when you least expect it! The motion detector is fragile and must be handled gently. The dynamics cart (the one with the plunger) has two ends. One end has two Velcro tabs on its face, which mate with similar Velcro tabs on the collision cart to provide a totally inelastic collision. The other end has no Velcro, and contains two small magnets behind the plastic end cap. The magnets serve as an excellent frictionless spring to allow perfectly elastic collisions. The plunger on the dynamics cart will not be used in this lab. To get it out of the way, push it in and slightly up until it stays in place in the cart. In this lab, the dynamics cart will be given a repeatable push by the cart launcher this is why we use the launcher. Procedure: 1. Open the ibook computer.. Push the power button near the screen on the ibook (the circle with a vertical line through it). Wait for the computer to come up and for the login to appear. 3. To log in click on Student then on the Log in button. The password is student. 4. A student folder should appear with icons. If not, click on the tab labeled student at the bottom of the screen. 5. Click once on the icon labeled Cart Collisions. 6. Check to see if there is a button near the top of the screen labeled Collect. The presence of this button indicates that the computer is ready to collect data. TOPS Dynamics Track Momentum,Energy,and Collisions with Motion Detectors.doc Page 1

2 Elastic collisions: Collecting Data: 1. Place the middle of the collision cart (cart, red line on graphs) at the 110 cm mark. This does not have to be exact, just approximate. Collision cart = m = 500 g = 0.50 kg. Cock the cart launcher. 3. Place the plunger end of the dynamics cart (cart 1, blue line on graphs) against the cart launcher. Make sure that there is no gap between the cart and the plunger on the launcher. Dynamics cart = m 1 = 500 g 4. Click the Zero button near the top of the screen on the ibook. The motion detectors will buzz momentarily. This determines the start point for the two carts (where x = 0). 5. Click on Zero all Sensors Important note: Any time the motion detectors are clicking, they are looking for motion to record. ANY motion in the cone of sound produced by the detectors will be recorded as motion on the computers, so be sure that no one is moving in front of the detectors while they are in use. In order to collect data, two people must work together to complete the next step. 6. a. Click on the Collect button on the ibook b. Listen for the motion detectors clicking, then pull the launch string to trigger the launcher. Important note: while the experiment is running, be sure to watch the carts and not the computer screen so that you see what is actually happening. Look at the movement of both carts before and after the collision. 7. Examine your graphs, the first run should look very much like the graphs below, subsequent graphs will look slightly different. If the graphs do not look similar, ask the teacher to adjust the motion detectors so that the data being collected is accurate. Saving Data: (check with your teacher to determine if you need to save your data) 1. On the ibook, Click on File. Click on Save As 3. Save this data in the following format: P#G#R#. For example for period 3, group 5 and run 1, the saved file name should look like P3G5R1. 4. After each of the remaining runs save that data in the same format as above, incrementing the run number. TOPS Dynamics Track Momentum,Energy,and Collisions with Motion Detectors.doc Page

3 Recording Data: 1. Click once anywhere inside the velocity graph window.. Look at the velocity graph. 3. Find the point in the data where the velocity of cart (red graph) increases from zero. 4. Click and drag to the point when the velocity of cart 1 (blue graph) levels off again. 5. Click once on the title bar of the table window. 6. Scroll down until the highlighted area appears. 7. Find where, in the highlighted area, V just starts to increase from zero and record the corresponding value for V1 in the data table as v 1i. The subscripts used are a 1 for cart 1 and a for cart, an i for initial values (before the collision) and a f for final values (after the collision) 8. Find where, in the highlighted area, V starts to level off, and record an average value for V (eyeball it) in the data table as v f. Determine an average value for V1 in the same area and record it as v 1f. (be sure to record a negative sign if appropriate) 9. Record the appropriate values for v i, m 1, and m. 10. Record a qualitative description of the carts motions before and after the collision in the data table s empty row. 11. Repeat the collecting data and saving data steps for each of the following configurations of masses: g mass on dynamics cart, no mass on collision cart -500 g masses on dynamics cart, no masses on collision cart g mass on collision cart, no mass on dynamics cart Data Table Elastic Collisions: Run # m 1 (kg) m (kg) v 1i (m/s) v 1f (m/s) v i (m/s) v f (m/s) 1 1 Observations: Observations: 3 3 Observations: 4 4 Observations: TOPS Dynamics Track Momentum,Energy,and Collisions with Motion Detectors.doc Page 3

4 Inelastic collisions: 1. Turn the dynamics cart around so that the plunger is facing away from the cart launcher.. Repeat all data collecting, saving, and recording procedures used above for elastic collisions. Notice that now the carts stick together when they collide. Data Table Inelastic Collisions: Run # m 1 (kg) m (kg) v 1i (m/s) v 1f (m/s) v i (m/s) v f (m/s) 5 5 Observations: 6 6 Observations: 7 7 Observations: 8 8 Observations: 3. When finished recording and saving data, click on the file menu then click on quit, then on don t save (as the data has already been saved). 4. To log out of the computer click on special then on logout. Check with your teacher to find out if the computer should be shut down or just put to sleep. If just put to sleep, simply close the cover, if shut down, click on the shut down button then on shut down. TOPS Dynamics Track Momentum,Energy,and Collisions with Motion Detectors.doc Page 4

5 Data Analysis: Now it is time to analyze your data and see how the world really works. Here are the equations that are needed to do the calculations below. Momentum (p) (in kg m/s) p = mv (notice that momentum is a vector, so signs matter) for example: p 1i = m 1 v 1i p tot = p 1 + p Kinetic energy (E k ) (in J) E k = ½ mv (notice that kinetic energy is not a vector) for example: E k1i = ½ m 1 v 1i E ktot = E k1 + E k Elastic Collisions: Momentum calculations: Run # p 1i (kg m/s) p i (kg m/s) p toti (kg m/s) p 1f (kg m/s) p f (kg m/s) p totf (kg m/s) Kinetic Energy calculations: Run # E k1i (J) E ki (J) E ktoti (J) E k1f (J) E kf (J) E ktotf (J) TOPS Dynamics Track Momentum,Energy,and Collisions with Motion Detectors.doc Page 5

6 Inelastic Collisions: Momentum calculations: Run # p 1i (kg m/s) p i (kg m/s) p toti (kg m/s) p 1f (kg m/s) p f (kg m/s) p totf (kg m/s) Kinetic Energy calculations: Run # E k1i (J) E ki (J) E ktoti (J) E k1f (J) E kf (J) E ktotf (J) Discussion questions: Elastic Collisions: How did the total momentum change before and after the collision? How did the total kinetic energy change before and after the collision? TOPS Dynamics Track Momentum,Energy,and Collisions with Motion Detectors.doc Page 6

7 Where did the missing energy go? TOPS Physics How did the relative masses of the two carts affect the final velocity of cart 1? (if cart 1 was more massive than cart what happened, etc ) Inelastic Collisions: How did the total momentum change before and after the collision? How did the total kinetic energy change before and after the collision? Where did the missing energy go? General: For which kind of collisions (elastic, inelastic or both) do the following conservation laws hold: Conservation of momentum. Conservation of kinetic energy. TOPS Dynamics Track Momentum,Energy,and Collisions with Motion Detectors.doc Page 7