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Lecture 17
•
Torque
•
Rotational form of Newton’s 2
nd
law
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A body can be set in rotational motion by the action of a
torque.
A rotating (spinning) body will continue to rotate
unless it is acted upon by a torque.
hinge
P
u
s
h
Q: Where on a door do
you push to open it?
A: Far from the hinge.
3
Torque
F
r
τ
×
=
Torque is a vector.
It’s magnitude is
θ
sin
rF
τ
=
It’s direction is found from a righthand rule: point your
fingers in the direction of
r
and curl them into the direction
of
F
.
Your thumb will point in the direction of the torque.
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Torque method 1:
⊥
=
rF
τ
Hinge
end
F
⊥
F

F
θ
r = the distance from the rotation axis (hinge) to the point
where the force F is applied.
F
⊥
is the component of the force F that is perpendicular
to the door (here it is Fsin
θ
).
Top view of door
5
The units of torque are Nm (not joules!).
By convention:
•
When the applied force causes the object to rotate
CCW then
τ
is positive
•
When the applied force causes the object to rotate
CW then
τ
is negative
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Torque method 2:
F
r
⊥
=
τ
r
⊥
is called the lever arm and F is magnitude of the applied
force.
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This note was uploaded on 07/04/2011 for the course PHYS 121 taught by Professor Staff during the Spring '11 term at ASU.
 Spring '11
 Staff

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