ENERGY  I

In this part of the lab, you will do some qualitative investigations of energy to practice with the concepts of kinetic and potential energy. There are no calculations in this part just thinking. Therefore, you need to write good descriptions of your thoughts in your notebook. Do not allow the cart to crash off the end of the track during your experiments!

Familiarize yourself with the apparatus:

 The track has feet that rest on the table at one end and a clamp that goes to a ring stand on the other end as shown in the Figure 1.  To adjust the height of the track: Loosen the knob that holds the clamp to the track about one turn.  Loosen the knob that holds the clamp to the ring stand and slide the clamp up or down until the track is at the desired elevation.  Tighten the ring stand knob - firmly, but not overly tight.  Tighten the knob that holds the clamp to the track in a similar fashion. The amount of elevation is easily determined by placing a ruler against the table top and measuring to the top edge of the track at each end.  The difference in the two heights is the elevation. The cart has a spring-loaded plunger that pushes outward upon release.  The tension in the spring (and hence the amount of push) can be adjusted by rotating the end of the plunger which causes the spring to be more compressed or less compressed when the plunger is 'cocked'.  Never force the end of the plunger to rotate - it will damage the cart.  This experiment asks you to use 3 different compression values for the spring.  Suggested values are distances of 10.5 cm, 9.8 cm and 9.0 cm between the inside of the black end piece and the end of the spring as indicated in Figure 2.  One fairly reproducible method is: Turn the cart upside down. Rest a ruler against the wheels. Align the 10 cm mark with the inside of the black end of the cart opposite the plunger. Adjust the plunger by rotating the knob at the end until the desired distance is shown (remember - the number on the ruler will be 10 cm larger than the distance). There is no need to be overly precise.  After firing the plunger, the distance may vary by a millimeter  or two. The plunger is 'cocked' by smoothly pushing it straight into the cart and pressing the horizontal metal tab at the end of the cart.  It is released by pressing down on the vertical rod.

### Part One:  Getting in the ballpark

• Elevate the track so that the end without the bumper is higher than the other end by 8 to 10 cm.  Record your values.
• Set the length of the spring as described above to 9.0 cm.
• Cock the plunger and place the cart on the track with the plunger end against the bottom 'end-stop'.  Be sure that the narrow edges on the tires are in the grooves on the track.
• Release the plunger by tapping on the release mechanism. (Using the eraser end of a pencil or a ruler works better than using your hand. Be sure to press straight down so that you don't give the cart an extra shove to the left or right.) Describe what happens.
• If the cart bangs against the screw that holds the sonic ranger to the track, increase the elevation of the track and repeat the previous step.
• Estimate the distance traveled to the highest point in the motion.
• Experiment with the track set to different angles - i.e. different elevations of the track.  Remember to not allow the cart to bang into the screw or to fall off.

What is the nature of the relationship between the angle of the track and the distance traveled? Explain your results in terms of energy.

### Part Two:  How does compression of the spring affect the distance?

Set the angle of the track such that the cart travels more than half of the length of the track. Keep this height constant for the remainder of this experiment.

1. Make a note of how far up the track the cart goes - to the nearest cm is fine.
2. Repeat the experiment with the other two spring compression distances as described above.
3. Explain your results in terms of energy.
4. Now using the 9.0 cm compression length, try different amounts of mass on the cart.
5. Describe what you observe.
6. Again, explain your results in terms of energy.

### Part Three: Getting to the numbers.

The next portion is quantitative where we will have Logger Pro make measurements using the Lab Pro.

• The sonic ranger should already be attached to the  end of the track.
• Using the cart without any extra mass and the 9.0 cm compression length, make distance, velocity and acceleration graphs.
• Release the cart once you hear the sonic ranger start 'chirping'.
• Is the behavior of the graphs as you would have expected?
• Explain what is happening in each part of the three motion graphs.
• Why is the second acceleration peak greater in magnitude than the first one?

Now that you have data you need to use Logger Pro™ to calculate the KE, PEg and ME of the cart. KE is pretty straightforward, but what about PEg?  You measured the position as a function of time, but you need the height in order to find PEg.  The inclination of the track is constant.  By measuring from the desktop to the top of both ends of the track you can determine the sine of the angle of inclination which is the ratio of (hhi-end - hlow-end) to the length of the track.  Recall that the reference for gravitational potential energy is arbitrary.  Use the 'Analyze', 'Examine' function to determine the ME before the cart is released.  Calculate a new column that subtracts that value from the sum of PE+KE.

• Is the energy of the cart conserved for the entire experiment?
• Is the energy of the cart conserved for some time intervals?  If so what intervals?
• Are there regions where the ME slowly decreases? If so What do you think is the cause?
• Are there regions where the ME quickly changes significantly?  If so what is the cause?
•  If ME is not conserved, where does the lost energy go?
• Describe any other interesting physics you noticed in this experiment that was not covered in answering the above questions.