Phys121-040* -- Homework 9 Part 1
Due Thursday, November 4.
23. TV on a handtruck
You are working as a staff person on the call-in internet program “Ask Dr. Science.” The following e-mail message comes in and needs a quick answer.
| My wife just called me at the office and asked the following question. We had a large computer monitor delivered to her home office this morning. The delivery person was kind enough to put the box on our hand truck (see attached picture) but he put it on while the truck was lying flat. My wife has some back problems and doesn't want to have to exert more than 50 pounds of force. The monitor in its box weighs about 85 pounds. She's having a business meeting at the house later and would like to get the box out of the front room. What I want to know is, can she stand the truck upright safely without hurting her back? |  |
A schematic diagram of the hand truck with the box on it is shown below. Is it safe for his wife to pull the handtruck upright so she can roll the box into the backroom? Be sure to explain why you think so.
24. Linear and rotational momenta and energies
In the figure below are shown snapshots of four situations in which a set of 4 point masses are moving in various configurations. The masses are all identical and each has a mass m. Their speeds are all identical and have the value v, and each mass is a distance R from the place where the dotted lines cross (the system origin). Use the coordinate directions shown on the right but for each system take the origin to be at the indicated crossing of the dotted lines.
The arrows indicate the direction of motion of the masses at the instant the snapshot was taken. Where a thin ring is shown, the masses are attached to a light (nearly massless) plastic hoop.
Calclulate the values of the total linear momentum, angular momentum, and total energy of motion in the four cases. Rank their magnitudes from largest to smallest. For the case of the energy of motion, discuss how much of the energy you would assign to rotational kinetic energy and why.
(There is no significance to the different thicknesses of the arrows)
25. Comparing conserved quantities
The four objects in the figures below are moving as indicated by the arrows. A curved arrow indicates a rotation about the center. For object (A) use the coordinates shown. For the others, take the origin at the center of the circle. Use the directions associated with the arrows shown. Construct a table with the values of
- the total linear momentum,
- total angular momentum, and
- total energy of motion
at the instant shown for each case. Express your answers in terms of m, v, and R. (Include an indicator of the direction where appropriate.) Which system has the largest and smallest of each of the quantities? Explain your reasoning.
