# Homework Help: Help on essay question

1. Nov 21, 2005

### 9danny

What is the difference between tangential and radial acceleration for a point on a rotating body? :uhh:
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Also, and this is mainly for a personal inquiry trying to get a "physics-oriented" answer...
I've noticed that the lower the tire pressure the greater the contact area between the tire and the pavement... why?

2. Nov 21, 2005

### mezarashi

Draw a circle. Now label the tangential and radial directions. Remember, orthogonal (different by 90 degrees) components cannot affect each other directly.

About tire-pavement surface area versus tire pressure: there probably isn't a "physics-oriented" answer. It's just a simple observational fact that, if you lower the pressure, the tire tends to 'slump' more and because of that, there is more of it touching the ground.

3. Nov 21, 2005

### NateTG

Actually:
For an air-pressure based tire - like normal bycicle - the area of the tire that touches the ground is goint to be equal to the weight that the tire is carrying divided by the pressure in the tire.

Modern car tires use a somewhat different tenchology where a non-trivial amount of the weight is carried by the side walls of the tire, and, as a consequence, the area that makes contact with the ground is affected less by the air pressure in the tire.

4. Nov 21, 2005

### 9danny

also related to rotational motion... a concept that I can't grasp entirely...
can a simple force applied to a body change both its translational and the rotational motion?
I thought they were the same thing....

5. Nov 21, 2005

### NateTG

Rotational and translational motion are not the same thing.

Consider:
A stationary bycicle wheel has no rotational or translational motion.
If you lift it by the axle, a bycicle wheel could be spinning in place with rotational, but no translational motion.
A bycicle wheel on the back of a moving truck will have translational, but not rotational motion.
And, if you're riding the bycicle, it will have both rotational, and translational motion.