Torque is a force that causes an object to turn
Torque must be perpendicular to the displacement to cause a rotation
The further away the force is applied from the point of rotation, the more leverage you obtain, so this distance is known as the lever arm (r)
Direction of the Torque Vector
The direction of the torque vector is perpendicular to both the position vector and the force vector
You can find the direction using the right-hand rule. Point the fingers of your right hand in the direction of the line of action, and bend you fingers in the direction of the force
You thumb then points in the direction of your torque
Note that positive torques cause counter-clockwise rotation, and negative torques cause clockwise rotation
Newton's Second Law: Translational vs. Rotational
Static Equilibrium implies that the net force and the net torque are zero, and the system is at rest
Dynamic Equilibrium implies that the net force and the net torque are zero, and the system is moving at constant translational and rotational velocity
Example 1: See-Saw Problem
A 10-kg tortoise sits on a see-saw 1 meter from the fulcrum.
Where must a 2-kg hare sit in order to maintain static equilibrium?
What is the force on the fulcrum?
Example 2: Beam Problem
Find the beam's angular acceleration
Example 3: Pulley with Mass
A light string attached to a mass m is wrapped around a pulley of mass mp</sub and radius R. Find the acceleration of the mass
Example 4: Net Torque
A system of three wheels fixed to each other is free to rotate about an axis through its center. Forces are exerted on the wheels as shown. What is the magnitude of the net torque on the wheels?
Example 5: Café Sign
A 3-kg café sign is hung from a 1-kg horizontal pole as shown. A wire is attached to prevent the sign from rotating.
Find the tension in the wire
2008 Free Response Question 2