PHY138Y - Mechanics - Class 12 - Mon. Oct. 25, 2004


"Here arises a puzzle that has disturbed scientists of all periods. How is it possible that mathematics, a product of human thought that is independent of experience, fits so excellently the objects of physical reality? Can human reason without experience discover by pure thinking properties of real things?"

-- Einstein

Pre-Class Quiz #5

MP Problem Set #5

Here are the results of the MP Problem Set due last Friday.

What Number PHY138 Students Answering PHY138 Results The Rest of the World
Colliding Balls
99% correct
Conservation of Momentum in Inelastic ...
98% correct
Girl on a Trampoline
95% correct
Collision at an Angle
78% correct
Ball Hits a Wall Elastically
82% correct
Time to complete
54 minutes
42 minutes

Representative Assembly

This week's Representative Assembly is cancelled.

Change in Curriculum

About the Test

You must bring to the test:

You may bring to the test:

The format of the test will be:

The remainder of this section discusses how to study for this or any PHY138 test.

You may find it useful to review the tips on Problem Solving that we discussed in Class 3.

You can also review the tips on studying Physics that we discussed in Class 5.

If you have been keeping up with the material, especially the concepts, the good news is that you are already pretty well prepared to take the test. If you have not been keeping up, there is bad news: trying to assimilate all the sometimes subtle concepts takes time, and you have only just over a week to do that.

Whether you have been keeping up or not, there are a couple of general principles which are important:

Things that you may wish to look over in preparation for the test include:

Finally, because Physics really is only about a few powerful concepts, the last-minute cram before the test is unlikely to help and in fact can actually just confuse you. Thus, I strongly recommend that:

At least one hour or so before the beginning of the test, stop studying! If the weather is nice, consider going for a walk.

Flash Animation

As part of our discussion of a rolling wheel, we used a Flash animation to illustrate the motion of a point on the edge of the wheel.

In-Class Questions

We asked the text's Quick Quiz 10.5 in class. Initially the class was fairly evenly divided between the hollow sphere and the solid sphere, with only a few thinking they were equal After some small group discussion, about 3/4 reached the correct answer: the hollow sphere has the higher kinetic energy.  
We also asked this question. Initially the class didn't have much of an idea about the answer. After some small group discussion, about 40% of you chose the correct answer: 3) 1 kg, but about half of you thought the answer was 4) 2 kg.

There are at least a couple of ways to arrive at the right answer to this question:

  1. The center of mass of the measuring stick is 0.25 m away from the axis of rotation. So is the 1kg mass. Thus for the total torque to be zero, the mass of the stick must be 1 kg.
  2. The quarter of the meter stick to the left of the axis of rotation is exactly balanced by the quarter of the stick just to the right of the axis. Thus their torques cancel, and we need only consider the 2 pieces to the far right of the stick. Their center of mass is located 0.5 m from the axis of rotation. So if the total torque is zero, the mass of those 2 pieces must be 0.5 kg. Therefore the total mass of the stick is 1 kg.

Supplemental Topic

As an example of equilibrium, we analysed in detail the forces acting on the leg. Notes on this have been prepared in both html and pdf format. They are accessible via the buttons to the right. We did not finish our discussion based on this document; we will finish it next class.



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