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Chapter 4 Forces, Friction, and Torque (Oh My!) 79

FIGURE 4-6 Forces on a box when moving

In the previous example, we just assumed a coefficient of friction. But what if you
don’t know the coefficient of friction and need to measure it? Let’s use the same 50 lb
cardboard box, but this time on a piece of plywood. Put the box in the middle of the
plywood, and then start lifting up one edge of the plywood until the box just starts to
slip. Use a protractor or other angle-measurement tool. The coefficient of friction is
just the tangent of the angle where the box starts to slip. In equation form, it looks
like this:

μ = tan
The location of angle is shown in Figure 4-7. If the box starts to slide down the
plywood at an angle of about 22°, that matches the coefficient of friction of 0.4 we
assumed earlier.

FIGURE 4-7 Forces on a box tilted at an angle