Force Diagrams & Torque (College Board AP® Physics 1: Algebra-Based)
Study Guide
Written by: Katie M
Reviewed by: Caroline Carroll
Force diagrams & torque
Torques can be described using force diagrams
Similar to free-body diagrams, force diagrams can be used to represent
the relative magnitude of force vectors by the length of an arrow
the relative direction of force vectors by the direction of an arrow
The main difference between free-body diagrams and force diagrams is
free-body diagrams represent objects as point particles
force diagrams represent objects as rigid systems
Force diagrams are very useful for analyzing the torques exerted on a rigid system, because
they depict the location at which forces are exerted on a system relative to the axis of rotation
they allow us to predict the rotational motion of a system
Representing forces on a pulley system
Worked Example
A uniform rod of mass 
and length 
is attached to a vertical support by a frictionless hinge that allows the rod to swing up or down, as shown in the figure.
A block of mass 
is suspended from the right end of the rod. The same point is supported by a cord that makes an angle of 30° with the rod.
On the diagram below, draw and label arrows that represent the forces that are exerted on the rod. Each force in your diagram must be represented by a distinct arrow starting on, and pointing away from, the point at which the force is exerted on the rod.
Answer:
Step 1: Identify the forces acting on the rod
Four forces act on the rod:
= the force exerted on the rod by the hinge, directed up and to the right (angle unknown)
= the tension in the cord, directed up and to the left from the right end of the rod at 30°
= the weight of the rod, directed downward from the center of mass of the rod
= the weight of the block, directed downward from the right end of the rod
Step 2: Draw and label each force vector
The arrows should be drawn such that they originate at the lever arm at the correct point of application
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