# Rotational Motion

#### Moose352

Imagine a rod in space. If I exert a force at one end, will the rod translate, rotate, or both? How do I determine what it will do?

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#### FZ+

It will rotate about it's centre of mass, and it's center of mass will move in a translation. Essentially, you are applying a moment and an unbalanced force to the body.

#### Moose352

But what will be the acceleration of its rotation and translation?

#### Integral

Staff Emeritus
Gold Member
Decompose the external force vector into components at the point of application. One componet which passes through the CM of the body the other perpendicular to it. The component through the CM will become a translational acceleration the other component times the distance to the CM will be the torque which cause rotation.

#### Moose352

So does that imply that when a force is acted upon a lever, like a seesaw, not all of the force acts in the rotation?

Okay, if I had a seesaw of 2 kg, which was 10 meters long, with a child (4kg) at each end, then what would be the force exerted on the fulcrum. Would it not be 2g + 4g + 4g?

What would be the force be if the seesaw became to rotate (that is, the net torque is not 0).

#### russ_watters

Mentor
Originally posted by Moose352
Okay, if I had a seesaw of 2 kg, which was 10 meters long, with a child (4kg) at each end, then what would be the force exerted on the fulcrum. Would it not be 2g + 4g + 4g?
Yes, it would be.
What would be the force be if the seesaw became to rotate (that is, the net torque is not 0).
Say for example one kid weighs 2kg (small kid). Then the balanced force acting on the fulcrum is 2+2+2 and the rotational force (moment) is 2.

#### Moose352

So the translation force is equal to: net force - rotational moment = balanced force? In that case, in the earlier experiment, would the translation force be 0 since the force is not balanced?

"Rotational Motion"

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