Question

1. Why does vertically falling rain make slanted streaks on the side windows of a moving auto?

2. What's the difference between a rotational and a circular motion???

Answer needs to be in your own words, short and simple. thank you :).

Answer 1

1

Its all a matter of air resistance isn't it. The water droplets cannot oppose the air resistance as well as the car can. If you stick a piece of string out the window, the string won't hang vertically, it will hang slightly horizontally even though its horizontal velocity with respect to the car is 0. Thats only because it can't oppose the air resistance as well as the car. If you hang a piece of paper with the same mass as the string, it won't hang as all, it will be completely horizontal because its affected by air resistance to a much greater extent than the string.

Also in the case of water droplets, a major factor to be considered is the intermolecular forces between the water and the window. If it was raining hexane, I bet the streaks would be much more horizontal because there would be much weaker intermolecular forces between it and the glass. Hanging a piece of string out the window is a better example because you don't have to consider intermolecular forces attaching the second object to the car.

2

Rotational motion and circular motion are similar indeed.

In rotational motion, a rigid body of significant size is studied, and it is rotating about one particular axis. Speed varies significantly throughout the body's fibers.

Circular motion is usually concerned with objects of negligible size traveling in circular paths.

In your example of a fly sitting on a rotating disc, the fly could be studied by methods in circular motion, whereas the disc is best studied by methods in rotational motion.

About your center of mass question, the center of mass of ANY rigid body never changes its configuration relative to the fibers of the body. This is unless the body re-distributes its mass, in which case it wouldn't be a rigid body. Relative to an inertia coordinate system in the background, yes center of mass does change. The center of mass in circular motion travels in a circle just like the rest of the body. Same is true with rotational motion, unless the body is rotating about a fixed axis which passes through the center of mass.