Page 104 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 4 Forces, Friction, and Torque (Oh My!) 85

• All of the contact forces on the object:

• Friction always acts in the opposite direction of motion.
• The applied force is just what it sounds like—the force you apply to
something. This could be a push or a pull (or a kick or a yank or a …). This
can also be a force that something else applies to your object, such as the
upward force that chains apply to the seat on a playground swing.
• The normal force acts perpendicular to the surface of contact. This is what
stops you, or your chair, from falling through the floor and travelling
through the center of the earth.

• Drag is the force that impedes an object when moving through air, water,
or other fluids. Drag increases as an object moves faster. We generally
ignore drag for things moving slowly in air.

• All of the noncontact forces on the object:
• Gravity acts on all objects on the earth, and pulls them toward the center
of the earth. This shows up in free body diagrams as the weight of the
object.
• All of the moments (torques) on the object

For the object to be in static equilibrium, three conditions must be met:

1. All sideways (horizontal) forces must cancel out.

2. All up and down (vertical) forces must cancel out.

3. All moments (torques) must cancel out.

If these three conditions are not true, you have a moving object.
Let’s walk through a real example to put this all in context. One of my former students
built a graffiti-drawing robot that suspended a spray paint can from two motorized
spools, as shown in Figure 4-10.