Chapter 11 Rotational Mechanics

Torque

If you want to make an object move, apply a force. If you want to make an object rotate, apply a torque.

Torque = force x lever arm distance

Example: 80 N

3 m

(80 N) x (3 m) = 240 N*m

Lever Arm

The lever arm distance is the distance from the fulcrum to a point perpendicular to the applied force.

Examples of Lever Arms

Find the lever arm in the following examples:

Torque = force x lever arm distance

• Balancing Torques

Larger kid sits closer to fulcrum to balance torque

Inertia

• For objects at rest or in motion, they will tend to stay at rest or in motion unless acted on by an outside force

• Inertia is the tendency of an object to resist a change in its motion

• Inertia is dependent upon mass.

Rotational Inertia

• The same is true for rotating objects:

• Rotational Inertia – tendency of an object to resist a change in rotation

Moment of Inertia: The resistance of an object to changes in its rotational motion

Moment of intertia = (mass)(radius)2

I = mr2

Angular Momentum• Just as an object moving in a straight path has linear momentum, a rotating object has angular momentum.

• angular momentum = rotational inertia x rotational velocity

Angular momentum = I x ω

Angular Momentum: The measure of how difficult it is to stop a rotating object

Angular Momentum = (mass)(velocity)(radius)

L = mvr

Law of Conservation of Angular Momentum:

If no unbalanced external torque acts on a rotating system, the angular momentum of that system is constant.