# Accelerating a wheel of two different materials.

1. Feb 10, 2013

### racer123

We all remember the experiment where two wheels, one made of metal that is hollow and one of solid wood, of equal mass, circumference, and width are raced down a ramp, how the wooden wheel accelerates faster.
If you cut the wheels in half and used one part of each to create another wheel, half metal and half wood, would they balance? What would happen if rotated? And, if the rotation was accelerated and decelerated, would it wobble or would it act in a uniform manner?
Thanks

2. Feb 10, 2013

### Naty1

Your first paragraph answers the second....different accelerations would tend to cause vibrations at higher rpm....

3. Feb 10, 2013

### racer123

That's my understanding as well. I wanted to get the opinion of people that have a far better understanding of physics than I do to confirm it.

4. Feb 10, 2013

### Staff: Mentor

Its center of mass would not be at its geometric center, so it would wobble around if you let it roll.

5. Feb 10, 2013

### Naty1

The concept is the same concept as when a weight is clamped on the rim of a wheel for dynamic balancing on an automobile....

6. Feb 10, 2013

### AlephZero

The center of mass of a solid semicircle is at distance $\dfrac{4r}{3\pi} = 0.424 r$ from the center. The COM of a semicircular arc is $\dfrac{2r}{\pi} = 0.637 r$ from the center.

The "hollow" metal half is not exactly a semicircular arc, but it should be clear the COM's of the two equal masses of wood and metal wlll be at different distances about the center, and the wood+metal disc will not be balanced.