# Torque (τ)

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

# Equilibrium

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 m

_{p</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