Section 8.4Torque

TorqueHow do I calculate torque?

TorqueHow do we cause a rotational

motion?

Apply a force

The position and direction of the force make a difference

Lever ArmLever Arm: The perpendicular

distance from the axis of rotation to the point the force is applied

Lever ArmThe

longer the better

RelationshipsIf F is constant and r increases, the

amount of angular acceleration increases

Angular acceleration is the product of forces times the lever arm

product is called the moment of the force or torque ()

Lever Arm

FA: = Distance from hinge to knob

FD = zero

FC: trig

Determining Torque EquationTorque:

is the lever arm

Or we can resolve the force into components and say

Torque Equation

is the angle between the force and the line r

Line r is drawn between the axis of rotation and place of force application

Units are m·N

Example

The biceps muscle exerts a vertical force on the lower arm, bent as shown. For each case, calculate the torque about the axis of rotation through the elbow joint, assuming the muscle is attached 5.0 cm from the elbow as shown.

Example

The biceps muscle exerts a vertical force on the lower arm, bent as shown. For each case, calculate the torque about the axis of rotation through the elbow joint, assuming the muscle is attached 5.0 cm from the elbow as shown.

ExampleTwo thin disk-shaped

wheels or rA = 30 cm and rB = 50 cm, are attached to each other on an axle that passes through the center each. Calculate the net torque on this compound wheel due to the two forces shown, each of magnitude 50 N.

HomeworkRead Section 8.4Do Problems # 22 – 26, page 220