Lever Arm
The “lever arm” for a force is the closest distance from the fulcrum to the line of that force. Then, the
torque provided by a force is the force times the lever arm.
Consider Figure 10.9 , which represents the same situation as Figure 10.7 . Instead of breaking F into
components, continue the line of the force. The torque is F times the lever arm shown in the diagram. By
trigonometry, you can see that the lever arm is equal to x sin θ . No matter how you look at it, the torque
provided by F is still (F sin θ )x .
Figure 10.9 Force F acting on a bar at an angle θPractice Problems
1 . A 50-g mass is hung by string as shown in the picture above. The left-hand string is horizontal; the
angled string measures 30° to the horizontal. What is the tension in the angled string?