Torque

The cause of motion

Cause of rotation

Cause of rotation

TorqueTorque – ‘momentum of force’ – the ability of a

force to rotate an object around some axis

Torque

Lever Arm – the shortest distance from the axis of rotation to the line of the force.

TorqueLever arm – the perpendicular distance from the axis of rotation to a line drawn along the direction of the force

Another view on it

You can use the component of the applied force that is perpendicular to the line that connects the axis of rotation to the point where the force is applied.

Cause of rotation

Compare the magnitudes of torques

Example

You are installing a new spark plug in your car, and the manual specifies that it be tightened to a torque that has a magnitude of 45 Nm. Using the data in the drawing, determine the magnitude F of the force that you must exert on the wrench.

ExampleThe tendon exerts a force of magnitude 720 N. Determine the torque (magnitude and direction) of this force about the ankle joint

The sign of torqueCW = “—”, CCW – “+”

Net Torque

Torque

Torque vs. Work

Chimp is no wimp

• Adult chimpanzees have muscle mass of only about one third of an adult human male, but have been shown to be over twice as strong is some movements. Explain this phenomenon.

Total equilibrium

• Total equilibrium – translational (the net force = 0) and rotational (the net torque = 0)

• A woman whose weight is 530 N is poised at the right end of a diving board with a length of 3.90 m. The board has negligible weight and is bolted down at the left end, while being supported 1.40 m away by a fulcrum. Find the forces that the bolt and the fulcrum respectively exert on the board

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The system is in translational and rotational equilibrium

The wheels, axle, and handles of a wheelbarrow weigh 60.0 N. The load chamber and its contents weigh 525 N. The drawing shows these two forces in two different wheelbarrow designs. To support the wheelbarrow in equilibrium, the man’s hands apply a force to the handles that is directed vertically upward. Consider a rotational axis at the point where the tire contacts the ground, directed perpendicular to the plane of the paper. Find the magnitude of the man’s force for both designs.

Let’s make the axel the axis of rotation. The rotational equilibrium equation will be:

Let’s make the axel the axis of rotation. The rotational equilibrium equation will be: