Physics 105 - How Things Work - Fall, 1998

Problem Set 3

• Due Monday, November 9, 1998, in class
• If you need to turn in any assignment early (or late), put it in the padlocked box outside physics room 133.
• Please answer each problem as thoroughly and precisely as possible. You should think of this as a chance to show off your knowledge of physics.
• The more concise and exact you are with your answers, the more credit you'll receive.
• You may discuss the problems with one another but you must write them up separately and in your own words.
• Write your name and ID# clearly and legibly on the top of each page that you turn in, and staple them together. If you do not, you may not receive full credit.
• If your name and ID# are written very clearly, you will receive 5 bonus points.

• Case 3, page 247, all parts. (This case requires some calculations.)
• Exercise 8, page 223. You must actually try this, if you are able. If not, watch a friend and report on the results. A drawing will greatly help the reader understand your answer to this question.
• Read exercises 15 and 16, page 245, but instead of answering those exercises, try this on a shoe of your own, as follows:

1. Loosen up the laces somewhat on a shoe you are wearing, and spread the two halves of the shoe apart. Try bringing the two halves close together by pulling on them directly. Is this difficult?
2. Make a sketch of your shoe, showing how many eyelets there are on each side of the shoe.
3. With the two halves spread apart, and very little slack in the laces, mark each lace (e.g. with a pencil or tape) at the point where they emerge from the top eyelets (the ones closest to your ankles). Also measure how far apart the two halves of the shoe are. Now pull enough lace from each side so that the two halves move about a half-inch closer together. This will probably take a few repetitions of pulling the ends of the laces, and taking up slack elsewhere. When you're done, how far did you have to pull the end of each lace, and how much closer together are the two halves? Does the stretchiness of the laces play a role here?
4. Use this data to estimate the force exerted on each end of the lace versus the force exerted on each side of the shoe, as the two halves are drawn together. Does your answer seem reasonable? [Don't worry about being extremely exact with this experiment.]

• Exercise 1, page 262.
• Exercise 6, page 274.
• Exercise 12, page 274.
• If you know the air pressure in the tires of a vehicle, and the area of each tire that is in contact with the pavement, you can estimate the weight of the vehicle. For this problem, you will try this for either a car (empty or not) or a bicycle (with at least one person on top).

1. Measure the area of contact of each wheel. The shape of contact will approximate a rectangle, so just measure each side of the rectangle, and calculate the area. Make a sketch of your vehicle, showing the number of passengers, and label each tire with your calculated contact areas. Measure everything in inches, so your areas will be in square inches.
2. Use this information, along with the tire pressures in psi (pounds per square inch), to estimate the weight of the vehicle. If you can't measure the tire pressures, use 30 psi for automobile tires, and 100 psi for bicycle tires. However it's better if you can measure the tire pressures of your actual vehicle.
3. Does your calculated weight seem reasonable?