how weight distribution affects speed of rolling object

[alygym]

Is this where we ask the science project questions? Before I registered, I was directed to a site where I asked my specific question & they asked the date my project was due, if I was done with my experiment, etc. Now that I'm registered, it just takes me directly to this site. Anyway, the rough draft of my research paper is due Feb. 4, 2008. Have already done the experiment and am trying to analyze the data.
Question: How does the distribution of weight affect the speed of a rolling object? Put equal am'ts of weight on both ends of a can, around the sides of the inside of the can, and on an axle through the center of the can. As I expected, the speed increased as the weight moved toward the center (like an ice skater pulling in to increase speed of rotation.) Then I put the weighted axle off-center and rolled the can downhill, and it was faster than the centered weighted axle (but only if I started it with the off-center axle at the top of the can; if it was at the bottom, the can wouldn't roll downhill at all - it just rocked back and forth.) Why did the off-center weighted axle go faster than the centered weighted axle? Thank you for your prompt reply.

alygym

[deleted-2574]

Hi alygym,

Weight distribution is also called center of gravity. Doing searched with center of gravity, I got a lot of hits, but none seemed to answer your question. The best one I could find was http://www.three-peaks.net/pinecar.htm

Please see if that page helps. Thanks. Others may be able to help.

[deleted-2574]

Please see if that page helps. Thanks. Others may be able to help.

In rereading my post, "others" is ambiguous. It could mean either other people or other webpages. I had meant other people. (Of course, other webpages also works, but that was not my intent.) The above is all an example of why precision is needed in written language.

[deleted-71487]

I'm not sure about the answer to this question, but you'll want to be careful to define all of your terms precisely, because you may be starting your objects in locations where they have different potential energy (i.e. if all your weight is at the top, most of the mass of the object is starting out higher if you start it at the same position on the slope, and so it starts out with more potential energy).

If you're talking about an ideal object on an ideal slope, no slipping, etc., etc., etc. you should end up with the sum of the motion and rotational kinetic energies at the bottom that strictly depends on the total loss of potential energy. The rotational energy component will depend on the moment of inertia of the object (do research on moment of inertia of rolling objects). Generally, an off-centered object will have a higher moment of inertia, so in theory it should be going slower, which means you have an interesting result here.

Is the cause just the higher potential energy? Is the off-centered weight causing you to get slippage? Is there something else going on? Have I gotten my analysis backwards due to my last physics class being 20+ years ago? All good areas to investigate.

In any case, it might be useful to compare cans started in a position such that their center of mass is at the same height in all cases. That way, only the weight distribution will be different.

It would also be interesting to discover what angle the off-center can has to be started at in order to roll. The off-centeredness of the object creates a kind of "potential energy well" once the weight is at the bottom that has to be overcome by kinetic energy and the force of gravity in order to continue rolling. Those would be interesting areas to investigate.

[deleted-71588]

Two aspects of what you are seeing experimentally is torque and centripetal force and how they interact in addition to the difference in starting potential energy that the previous responder talked about. Look up these forces.

Look at a front mountain bicycle tire mounted on a rim and axle. Where is the mass? Where is the center of mass? The tire is "balanced" so the center of mass is in the center of the axle. If you took out all of the spokes and weighed what you have left, what would you find? My guess is the spokes would be a small part of the total mass, the axle and bearings would be significant; however, the tire, tube, air in the tube, and rim would be the biggest part of the total weight or mass. Note: You don't want to actually do this because putting it back together is extremely time consuming.

How could you get a similar weight distribution on a can? How would that compare?