Page 93 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

74 Making Things Move

ave you ever tried to push open a door, only to realize you’re pushing on the
Hwrong side—the one closer to the hinge—and it’s really hard to open? This
happens because a door needs a certain amount of torque to open, and if you
push too close to the hinge, you have to use a lot more force than if you push at
the handle to create the same amount of torque. I suggest pushing on doors in the
middle to avoid embarrassment.

In order to estimate torque, know where on the door to push, figure out if something
will break, choose a motor, or pick a material for a project, it helps to think about the
world around us in terms of numbers. So, before we get to examples of forces,
friction, and torque, we need to review some math.

Torque Calculations
First, you need to understand the relationship between force and torque (also called
moment). We talked about force in Chapter 1. Just as a force can be thought of as a
push or pull, torque can be thought of as turning strength.

Torque is how hard something is rotated.
More specifically, torque is force multiplied FIGURE 4-1 A picture definition of torque
by the perpendicular distance to the axis of
rotation. This distance is also called the lever
arm or moment arm:

Torque = Force × Distance (⊥)
In the case of the unruly door, the hinge
is the axis of rotation. You can see from
Figure 4-1 and the equation that the
greater the distance from the applied force
to the hinge, the greater the torque. The
force in this case is you pushing open the
door. So when you accidentally push on
the door very close to the hinge, you need
to push with a lot of force to create the
same torque as pushing with just a little
force farther away from the hinge.