Page 97 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

78 Making Things Move

1. Weight (W) The weight of the box is focused at the box’s center of gravity
and points down toward the floor, in the direction of gravity.

2. Normal force (N) This means the floor is actually pushing up with the same
amount of force as the box weighs in a direction normal (or perpendicular) to
the floor. This might seem like a made-up force, but think of what would
happen if you set the box down on a bed of marshmallows. Those
marshmallows would squish until they were compressed enough to support
the weight of the box. The floor doesn’t need to squish, because it’s already
strong enough. (If your floor needs to squish to support a 50 lb box, you
should get yourself a new floor!)

The normal force (N) equals the weight (W) of the box when it’s standing still. The
fancy term for this is static equilibrium, which is when all the forces acting on an
object cancel out so the object doesn’t move.

Since the box is heavy, you decide to sit on the floor and push it with your feet instead
of trying to lift it. How hard do you need to push? You must push just hard enough to
overcome the friction between the 50 lb box and the hardwood floor. This force is a
fraction of the normal force. In equation form, the last sentence looks like this:

Force Due to Friction (F ) = Fraction (μ) × Normal Force (N)
f
The fraction, or coefficient of friction, commonly represented by the funny-looking
Greek letter μ (pronounced miu), is less than 1. This means that you’ll need to push
sideways with less than 50 lbs of force to move the box.
For example, let’s assume the coefficient of friction for our cardboard box on a
hardwood floor is 0.4. Our equation looks like this:

F = 0.4 × 50 lbs
f
So the force due to friction that you’ll need to overcome is just 20 lbs. If you put the
force due to friction and the force you’re pushing with into the diagram, it looks like
Figure 4-6. Friction always acts opposite the direction of movement.

So again, if all the forces cancel out, the box will be in static equilibrium, and it won’t
move anywhere. You need to push with just slightly more force than the force due to
friction in order to get the box out of equilibrium and moving.