Physics 195: Energy Exploration
Part 1: Stretching a spring
APPARATUS: Cart with wireless force probe, bluetooth device for wireless force probe, motion detector with accompanying cable support stand, LabPro interface with power supply, computer, string, printer cable, lab jacks (for use in part 3).
1. Set up the track in a level position, with the support stand at one end and the motion detector at the opposite end. Attach one end of the spring to the support stand and one end to the wireless force probe on the cart (use a short piece of string if necessary).
2. Turn on the computer, insert bluetooth device in the back USB port, wait for it to be recognized and then open Logger Pro. Scan for the force probe (Experiment icon). Once the computer has detected your force probe by name, open the experiment file called Stretching Spring (L11E2-2) to display force vs. position axes.
3. Click on the “GO” icon in the upper left hand corner and Disable reverse direction for the motion detector. Motion towards the detector will graph as being in the negative direction with the motion detector as the origin. The spring force will be graphed as positive.
4. Zero the force probe with the spring hanging loosely and the cart at the starting position. Do not zero the motion detector. Write down the starting position on track:
Starting position on track ______m.
5. Begin graphing force vs. position as the cart is moved with constant speed towards the motion detector, until the spring is stretched between 0.5 and 1 m (make sure to keep your hand out of the way of the motion detector). (If you can’t find your data, try Autoscale).
Question # 1: Select a section where the graph is linear (If there is no such place redo or ask) and determine the spring constant, k:______N/m. How did you do this?
Question #2: The spring constant is a positive number. Why is the slope negative?
Question #3: Determine the work done by the spring on the cart. Include sign:
Ws: ______JHow did you do this?
Question #4: Determine the work done by the hand on the cart. Include sign:
Ws: ______JHow did you do this?
Question #5: Determine the change in elastic potential energy of the cart/spring system. Include sign:
∆Us: ______J
How did you do this?
Question #6: Draw an energy bar chart for this cart/spring system. ∆Uint is change in thermal energy and can be ignored for this problem, if we assume friction to be negligible:
PRINT graph, showing any software feature used to obtain the above answers.
Part 2 – Stretching a spring and letting it go
1. Open the experiment file called Work-Energy (L11A3-3).
2. Verify horizontal axes for both graphs show position; if not, change them for both graphs (double click on axis name).
3. Change sample rate to 1000 samples/sec (clock icon).
4. Measure the mass of the cart and enter this value in the formula for kinetic energy. (Column Option menu).
5. Zero the probes with the spring hanging loosely, then pull cart along track so spring is stretched about 1 m from unstretched position. This is the initial position: ______m
6. Begin graphing and release cart, making a valiant attempt to stop cart before it crashes into support stand.
7. Use the features of the software to complete find ∆K and ∆Us . Include sign
∆K = ______∆Us = ______
8. Calculate the percent difference (should be < 5%).
9. When you get a good graph, PRINT and attach to the end of the lab (If you can’t find
your data, try Autoscale).
10. Was mechanical energy conserved (Ignore small loss due to friction) ? Explain and draw the energy bar chart below:
Part 3 Energy of a cart on an inclined ramp
Apparatus
- Low friction cart with force probe acting as extra mass
- Smooth ramp or other level surface
- Lab Jack to elevate one end of track
- Motion detector and Logger Pro software
- scale
1.Record the mass of your cart with any extra mass ______kg
2.Set up the ramp and motion detector as shown, except that the end stop isn’t necessary. Also, set up the ramp with the lab jack so the bottom of the ramp at the 190 cm mark sits 20 cm above the table. This will provide a suitable angle, which can be calculated using trig.
3.Turn off the force probe and cancel wireless if the program prompts. Open the experiment file called Inclined Ramp (l12A1-3) to display graphs for Ug, K and Mechanical energy (Ug, + K). Disable reverse sensing for the motion detector.
4.Edit the Kinetic Energy axis by right clicking on the U/K axis and choosing Column Options. This will bring up the equation used to calculate K. Enter appropriate values.
5.Edit the U axis the same way, only this requires some engineering thinking. Remember that gravitational potential energy is mgy, where y is the height above an arbitrary reference. The motion detector only measures distance along the track from the detector (L). You must express height (y), as a function of position along the track (L). Recall that trig functions such as sine and cosine are actually ratios, so you only need to know the value of the ratio at one place,along the track. If you followed the instuctions in #2, above, you should be able to calculate this ratio. Note: You must use a decimal value to enter a trig function in Logger Pro.
6.Show the calculation you made to obtain the value for the angle of the track:
7.What is the reference position for the potential energy, i.e., the place where the potential energy is zero in this experiment? ______
8.Prediction: As the cart rolls down the ramp, how will the kinetic energy change with position?
9.Prediction: How will the gravitational potential energy change with position?
10.Prediction : How will the mechanical energy change?
11.Draw a bar chart for this situation.
12.Hold the cart at the top of the ramp and release. Plan to catch it at about 40 cm. Think ahead.
13.When you get a good set of data, PRINT your graphs and attach them. On the bottom graph, indicate which of the curves is the kinetic energy and which is the gravitational potential energy.
14.Explain how the graph of kinetic energy agrees (or disagrees) with your predictions. Where is the kinetic energy zero according to your graph?
15.Explain how the graph of potential energy agrees (or disagrees) with your predictions. Is there a place on the graph where is potential energy equals zero on your graph? Where? If not, why not?
16.Explain why the graph of mechanical energy looks as it does (excluding stopping and starting) What does this imply about energy losses in this system?
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