## Can you give me an example of when the angular acceleration is in a different di…

#### Can you give me an example of when the angular acceleration is in a different direction from the torque applied?

When an object isn’t symmetric, it can rotate in very peculiar ways. If you throw a tennis racket into the air so that it is spinning about an axis that isn’t along the handle or at right angles to the handle, it will wobble in flight. Its axis of rotation will actually change with time as it wobbles. If you were to exert a torque on this wobbling tennis racket, its angular acceleration wouldn’t necessarily be along the direction of the torque.

## Given a lever long enough, could you move the world?

#### Given a lever long enough, could you move the world?

Yes. Of course, you would need a fixed pivot about which to work and that might be hard to find. But you could do work on the world with your lever. If the arm you were dealing with was long enough, you could do that work with a small force exerted over a very, very long distance. The lever would then do this work on the world with a very, very large force exerted over a small distance.

## How can cats turn their bodies around to land on their feet if they fall and how…

#### How can cats turn their bodies around to land on their feet if they fall and how can people do tricks in the air when they are skydiving if you’re supposed to “keep doing what you’ve been doing” when you leave the ground?

Cats manage to twist themselves around by exerting torques within their own bodies. They aren’t rigid, so that one half of the cat can exert a torque on the other half and vice versa. Even though the overall cat doesn’t change its rotation, parts of the cat change their individual rotations and the cat manages to reorient itself. It goes from not rotating but upside down to not rotating but right side up. Overall, it never had any angular velocity. As for skydiving, that is mostly a matter of torques from the air. As you fall, the air pushes on you and can exert torques on you about your center of mass. The result is rotation.

## Is moment of inertia determined only by mass, as inertia is in translational mot…

#### Is moment of inertia determined only by mass, as inertia is in translational motion?

No, moment of inertia embodies both mass and its distribution about the axis of rotation. The more of the mass that is located far from the axis of rotation, the larger the moment of inertia. For example, a ball of dough is much easier to spin than a disk-shaped pizza, because the latter has its mass far from the axis of rotation.

## Shouldn’t the seesaw be completely horizontal in order to be balanced? How can i…

#### Shouldn’t the seesaw be completely horizontal in order to be balanced? How can it be balanced if it’s not horizontal?

A balanced seesaw is simply one that isn’t experiencing any torque—the net torque on it is zero. Because there is no torque on it, it isn’t undergoing any angular acceleration and its angular velocity is constant. If it happens to be horizontal and motionless, then it will stay that way. But it could also be tilted or even rotating at a steady rate.