Moments of Inertia

Four days... four days!!!!????

I apparently did a number on my karma by basing a problem on dear Mr. Kirpes' suffering.  Going on day four of a fever, long story short, when you can get a flu shot for free, you really should get that flu shot.  Sooooo, uh day four of not being at school, uh...   By now, you've seen a pattern a couple of places whereby the work aka working aka energy transfer can be found by determining the area of a force v. distance graph.  For the spring, that graph is a triangle, and the area equation shakes out to be Ee=1/2k*(x)^2.  If you think about it, when you went up the stairs yesterday, you applied an average force equal to your weight-ish (Fg=mg) over a vertical displacement of delta y.  That average force would be constantish (think about the elevator doing the same thing, little bit more than Fg at the beginning, equal to Fg for the CVPM part in the middle, and a little less than Fg at the end;  long story short, it'd average out to Fg.)  So let's say you want to determine the amount of energy transfer due to friction.  As you'll no doubt recall, friction force depends only on the coefficient of friction, and the Normal force.  It does not vary with velocity.  So consider a graph of Friction force v. delta x for an object sliding to rest.  The area under the graph would be equal to the energy transferred from Ek to Eth by friction.

So apply that idea to the puck in these videos (keep in mind that good suggestion to leave mass as a a variable 'm' when it's not given, because it just might cancel out).  Grab a laptop for your group (or use your own if it's fancy) and a whiteboard for your work.  See if you can determine the coefficient of friction.

If that doesn't work, try this link
If that doesn't work, just go to the dang page and click the links yourself until it does work

What must be true of the initial speed of the puck in this follow-up video based on the distance it slides?

After the presentation of the solutions to the above problems, have at the work and energy clicker questions (Max, can you please hook up the clickers set up for everyone to use? thanks!)

Student Instructions for Wednesday, March 8th

Hopefully you wore comfy shoes today.  You'll be collecting data in one of the stairwells at West High.  Use the measuring tapes to figure out the delta Y between the ground floor, and the top floor.  We'll be looking at how quickly you convert your Ee?Eg?Eth?Ech?Eo? into Eg, assuming you arrive at the top step with a velocity of more or less zero (no Ek at the top).  Power is the time rate of change of Energy Transfer, Delta Energy over Delta Time.  The units are therefore N*m/s or J/s which is commonly referred to as Watts.  746 Watts are equivalent to 1 Horsepower.  Once you've collected your data for walking safely up the stairs, and running briskly, yet safely, up the stairs, record your Hp value on this chart, and we'll see who is the Most Powerful Trojan!

How to find final velocity in a perfectly elastic (bouncy!) collision

Just a heads up, some of my YouTube videos aren't playable over district wifi, so you may have to watch this elsewhere...