Lab: Energy conservation
This is a single experiment that tests the principle of mechanical energy conservation. You'll set up a simple device called Atwood's machine, consisting of two slightly different masses (mi > m2) or loads connected by a string run over a fixed pulley. When you release the loads from rest, one rises and one falls, and you're going to make measurements to determine whether the system's mechanical energy is conserved in the interval from release to (just before) impact.
Lab Data
You'll need string, scissors, and a large flat object such as a book. From the kit, you'll need a pulley, a stopwatch (or stopwatch app), digital mass scale, measuring tape, and two loads (mi, m2).
The first step is to fmd two objects that will serve as loads. These should each be about 150 g each, though larger masses are fine. One idea would be to put quarters or other coins in the cloth pouch in your lab kit, and use a small plastic bag to hold coins for the other load. The heavier load will need to be able to repeatedly hit whatever you choose as the landing pad without damage to itself or the pad.
Setup
1. Create two loads you can easily attach to the ends of your string, with at least 150 g of mass each
2. Measure the total mass of each load: container and coins or other objects inside (the masses should differ by 10-20 g)
- m1 is the heavier load (falling)
- m2 is the lighter load (rising)
- m1 > m2 + 10-20 g, approximately
3. Hang the pulley from a firm support ut least 1.5 m above the floor
4. Place a book (or other flat-topped object) directly below the pulley (landing pad for mi; launching pad for m2)
5. Cut a length of string about 15-20 cm longer than the distance between the Pulley and the pad
6. Tie mi (the heavier load) to one end of the string, then place m, on the pad
7. Thread the suing over the top of the pulley
8. Tie m2 (the lighter load) to the free end of the string, so it hangs as close to the pulley as possible
9. Pull m2 straight down to the launching pad, checking to see if the two loads can move past each other without hitting; if necessary, change one or both loads (re-measure masses later)
10. Hold m2 at rest on the pad, with its string as vertical as you can make it: check for both left/right and front/back slants
11. Mark this position on the pad with masking tape and/or marker so you can launch m2 from the same spot each trial
12. Test run: when both masses are at rest, release m2 from the marked launch position
- if m2 doesn't accelerate smoothly upward, increase the mass difference (make m1 heavier or m2 lighter) and try again
- if you can't prevent the two loads from hit each other while moving, you may need to choose a different object for one or both loads
13. Place in, on the book, choosing its position so the string is as vertical as you can make it (check for left/right and front/back slants)
14. Measure the distance h between the bottom of m2 and the marked launch position (see Fig. 6.1)
When measuring it, make sure the meter stick or measuring tape is completely vertical. Once you've nished steps 1-14, your Atwood's machine is ready for data-taking.
Lab Questions
1. Would it be most reasonable to say that the mechanical energy of the Atwood's machine was or wasn't conserved in this experiment? Explain.
2. Choose either load 1 or load 2 for this question, not both: was the mechanical energy of this load conserved, or not? Explain, and support your answer by calculating the initial and final ME for this load. Show your work.
As always, answers should be explicitly supported with actual data.