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Chapter 4 Forces, Friction, and Torque (Oh My!) 75

You can feel torque in action with a simple FIGURE 4-2 Shoulder torque when
exercise. Grab a can of soup from your holding a can with your arm parallel to
floor
pantry and hold it in your right hand, all the
way out to the side so your arm is parallel
to the floor. The strain you feel in your
shoulder is your muscles creating the
necessary torque to support the soup can.
Your shoulder is acting as an axis of
rotation, and the torque is the force of the
can (its weight) multiplied by the distance
the can is from your hand to your shoulder
(see Figure 4-2). If the can weighs 1 lb, and
your arm is 2 ft long (d ), the torque at your
1
shoulder is 1 lb × 2 ft = 2 ft-lbs. FIGURE 4-3 Shoulder torque while
holding a can at an angle
Your shoulder will get tired after a while in
this position, so lower the can about
halfway (see Figure 4-3). Now the torque on
your shoulder is less, even though your arm
is still the same length and the can weighs
the same. Why? Because the force of the
can is still pointed down (gravity always is!),
but the perpendicular distance to the
axis—your shoulder—is smaller (d ).
2
Torque always has units of distance × force
(sometimes written as force × distance).
Unfortunately, there are many ways of
measuring distance and force, so torque
can be in foot-pounds, ounce-inches,
millinewton-meters, and so on. You can go
to www.onlineconversion.com/torque and convert from any measurement you come
across to one you prefer. There are even smart phone apps that do this for you.
For example, if you find a motor that lists its torque in millinewton-meters, and
foot-pounds make more sense to you, convert the motor torque to foot-pounds.

The last bit of math we need to review before going through some examples is the
basic geometry of triangles you probably learned in high school. Remember sine,