1. TORQUE – Rotational Force
General Physics
November 28, 2016

2. Focus Questions – write each question and leave spaces for the answers
Define Torque using only two words.
What does “net” Torque produce?
Torque depends on what two quantities?
What else does Torque depend on?
What is the equation for Torque?
What are the units for Torque?
What are the important conversions for Torque?

4. Net torque produces rotation
Imagine a cat trying to enter a house by pushing perpendicularly on a cat-flap door. The picture to the right shows a cat-flap door hinged at the top.
In this configuration, the door is free to rotate around a line that passes through the hinge. This is the door’s axis of rotation.
When the cat pushes at the outer edge of the door with a force that is perpendicular to the door, the door opens.
The ability of a force to rotate an object around some axis is measured by a quantity called TORQUE.

5. Torque depends on a force and a lever arm.
If a cat pushed on the door with the same force but at a point closer to the hinge, the door would be more difficult to rotate.
How easily an object rotates depends NOT only on how much force is applied but also on where the force is applied.
The farther the force is from the axis of rotation, the easier it is to rotate the object and the more torque is produced.
The perpendicular distance from the axis of rotation to a line drawn along the direction of the force is called the LEVER ARM, or some textbooks call it the moment arm (based on the moment of inertia).

6. Torque Diagram
FORCE
AXIS OF ROTATION
DISTANCE OF THE LEVER ARM

7. Torque also depends on the angle between a force and a lever arm.
Forces do not have to be perpendicular to an object to cause the object to rotate.
What would happen if a dog pushed on the door at angle to the door, rather than perpendicular to it as the cat did in the earlier picture?
The door would still rotate, but not as easily!

8. torque = force(lever arm)
Torque Equation
The symbol for torque is the Greek letter tau (t), and the magnitude of the torque is given by the following equation:
torque = force(lever arm)
t = FL

9. Units for torque The SI units for torque is Newton meters = Nm.
The English units for torque is Pounds feet = lbs ft
To convert from SI to English or vise-versus you would need to use the following important conversions:
1 Newton = pounds
1 foot = meters