Physics assignments (urgent !!)

PHYS 150 NAME:

Homework Assignment: Energy Conservation in a Cart-Spring System Due Date: Monday, June 6, 2016, at the beginning of class

You may print this sheet and write your answers on it, or write your answers on separate pages (be sure to clearly label each question

& answer.)

Open a web browser, and navigate to the following web page:

http://serc.carleton.edu/student_videos/index.html#energy

Look for the video called “Conservation of energy in a cart launched by a spring.” Click the DMV link and

start the video. You should see a frame like Figure 1 below:

Figure 1: Screenshot using DMV player

Play the video through at least once in its entirety to see the “big picture” first. Then, using the video controls,

you can advance the video one frame at a time. Note the scales marked on the screen for measuring position.

Using the data available on-screen, answer the following questions. Where calculations are necessary, show

ALL of your steps on this page or separate pages. When counting frames, round to the nearest frame (e.g.

2.5 frames would be rounded up to 3 frames). We will be working with many approximations.

- Our system in this exercise will be the cart, the spring, and the Earth. We will measure gravitational energy

from the track surface.

- Always include appropriate UNITS when you finish a calculation.

1. What is the frame rate for the video? (You need to play the whole video to see this information.)

2. Before the cart is released, what is the maximum amount the spring is stretched? (Answer in meters, to the

nearest 0.01 m.)

3. Before the cart is released, how much elastic energy is stored in the spring? (Include units.)

4. Before the cart is released, what is the system’s kinetic energy?

PEgrav=0

5. Before the cart is released, what is the system’s total mechanical energy?

6. After the cart is released, advance the video frame-by-frame until you notice the string going slack. (In other

words, there is no longer a tension force pulling the cart forward.) Notice that “Einstein” is holding his left arm

up. Use his arm to track the motion of the cart. More specifically, use the yellow dot he holds in his hand.

How many seconds does it take for the cart to coast forward from 0 cm to 20 cm? (Hint: First count the

number of frames, then convert to seconds.)

7. Calculate the average speed of the cart as it coasts from 0 cm to 20 cm. Answer in meters per second.

8. Calculate the kinetic energy of the system as the cart coasts from 0 cm to 20 cm.

9. When the string goes slack, as the cart coasts forward, how much elastic energy remains in the system?

10. Does the gravitational energy of the system change as the cart moves along? Justify your answer.

11. Calculate the total mechanical energy in the system when the string goes slack:

12. Find the percent difference between (a.) the total mechanical energy of the system before cart is released

and (b.) the total mechanical energy of the system when the spring goes slack:

13. Let’s say that, if the percent difference between two numbers is less than 5%, those two numbers are

reasonably close together (close to being the same value). Was the total mechanical energy of the system

before the cart’s release reasonably close to the total mechanical energy when string went slack?

14. If you answered “No” to the question above, explain what may have happened to the “missing” energy. If

you answered “Yes,” explain where all of the cart’s kinetic energy came from after release.