1. Unit 2 Forces Chapter 5 Forces in Equilibrium
5.1 The Force Vector
5.2 Forces and Equilibrium
5.3 Friction
5.4 Torque and Rotational Equilibrium

2. Torque
Torque is a measure of how much a force acting on an object causes the object to rotate.
Torque causes objects to rotate or spin.
Torque is the rotational equivalent of force.
The canoe rotates counterclockwise hits both docks if the forces are applied accordingly.

3. Torque The line about which an object turns is its axis of rotation.
Some objects have a fixed axis.
A door’s axis is fixed at the hinges.
Doorknobs are positioned far from the hinges to provide the greatest amount of torque.

4. Torque
Torque is created whenever the line of action of a force does not pass through the axis of rotation.
The line of action is an imaginary line in the direction of the force and passing through the point where the force is applied.
If the line of action passes through the axis, the torque is zero.

5. Calculating torque
The torque created by a force depends on the strength of the force and also on the lever arm.
The lever arm is the perpendicular distance between the line of action of the force and the axis of rotation.

6. Torque and direction
The direction of torque is often drawn with a circular arrow showing how the object would rotate.
Positive torque is counterclockwise, and negative torque is clockwise..

7. Torque and force differ
Torque is created by force, but it is not the same thing as force.
Torque depends on both force and distance.
Torque (N·m) has different units from force (N).
The same force can produce any amount of torque depending on where, and in what direction, it is applied.

8. Torque and force
The same force can create different amounts of torque depending on where it is applied and in what direction.
Some forces produce no torque.

10. Net torque
If more than one torque acts on an object, the torques combine to equal the net torque.
If the torques make an object spin in the same direction, they are added.
If the torques make the object spin in opposite directions, the torques are subtracted.

11. Looking for: … the torque.
Calculating torque
A force of 50 N is applied to a wrench that is 0.30 m. long. Calculate the torque if the force is applied perpendicular to the wrench.
Looking for: … the torque.
Given: … the force (50 N) and the length of the lever arm (0.30 m)
Relationships: Use: τ = Fr
Solution: τ = (0.30 m)(50 N) = 15 N·m

12. Rotational equilibrium
An object is in rotational equilibrium when the net torque applied to it is zero.
Rotational equilibrium is often used to determine unknown forces.
Any object that is not moving must have a net torque of zero and a net force of zero.

13. Applying rotational equilibrium
Triple beam balances used in schools and scales used in doctors’ offices use balanced torques to measure weight.
When using such scales, you must slide small masses away from the axis of rotation until the scale reaches equilibrium.
Moving the mass increases its lever arm and its torque.

14. Architecture: Forces in Equilibrium
Four thousand years ago, the builders of the Pyramids of Egypt understood how a structure must be designed to remain standing.
Egyptians used trial-and-error along with back-breaking effort to refine their design, keeping ideas that worked and discarding those that didn’t.