Friction Forces Unite!

Today (A.K.A. yesterday) we started off class by checking over Friday's quiz, and we reviewed the lesson from Friday.

We reviewed the formula:  FF = Fn*µ , that tells us that the force of friction is equal to the normal force (represented by the object's weight) multiplied by the friction coefficient.

We also reviewed that the coefficient for static friction is greater than the coefficient for kinetic friction.

We then took some notes, and learned that friction only exists when a force is being applied We also learned that forces act through the center of the object, so friction has nothing to do with the surface area.

Mr. Banow then did a demo where he attached a force sensor to a wooden object, to show us the idea of limiting static friction. The force peaked just before the object moved, then leveled out as it moved.

The last thing we did was some examples of the Friction Force Formula, fortunately forementioned for frequent and fervorous figuring of Friction Forces (holy alliteration, Batman!). One of these examples was the following question:

The total weight of Physics Boy (on skis) is 500N. If it takes 35N for Calculus Man to start Physics Boy moving (so he can help somebody find the slope of a tangent on a specific point), determine the coefficient of static friction.

The formula is: FF = Fn*µ

• We are trying to find µ.
• FF is 35N, because this is the Limiting Static Friction 
• Fn is 500N, because the normal force is the same as the object's weight.

So we substitute and solve:

                           

35N = 500N*µ

35N/500N = µ

µ = 0.07

The friction coefficient between Physics Boy's skis and the snow is 0.07

There are no units, because  Newtons/Newtons = 1, so they cancel.

Nicole, you're next!