S5.6 Torque
[A] Torque
- A torque is a vector that describes the ability of a force to change the angular velocity of a body.
Key Point 5.14
Torque. Applying a
force, magnitude F to a point a distance r from point O,
results in a torque about O of magnitude
τ=rFsinθ.
The units of torque are N m. |
There are two equivalent ways of computing the torque:
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[B] The torque vector
Key Point 5.15
Torque vector: The torque about a point O is the vector defined by![]() where |
Worked example
Reveal
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Worked example
A small 0.75kg ball is attached to one end of a 1.25m long massless rod and the other end of the rod is hung from a pivot. When the resulting pendulum is 30∘ from the vertical, what is the magnitude of the torque from the pivot
Solution
The torque is provided by the component of the ball’s weight acting perpendicular to the rod. At 30∘ from the vertical this is
Ft=mg sin30∘
Hence the magnitude of the torque is
τ=rFt=1.25×mg sin30∘=4.7N m
Learning Resources
![]() | HRW Chapter 10.8 and 11.6 |
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