# Physics- Working with Common Forces, Thursday Febrauary 12, 2009

In today’s class we reviewed some examples of common forces encountered in problems and took note of some of the things to keep in mind when working with these forces.

The common forces that we covered included:

• Gravitational force
• Frictional forces
• Normal force
• Applied forces

One of the key elements that we discussed today was the force of friction. In the past, you have learned that the force of friction is found with the following formula:

Ff=μFN

This is not always the case. Consider:

A) A box is sitting on the floor, no force is being applied.

The net horizontal is composed of the Ff and the F applied, all of which equals zero.

If no force is applied and the net force is zero, the Ff is equal to zero as well, not to μFN.

B) A box is on the floor and someone is pushing it, but no movement is seen.

The net horizontal force is still zero but not our F applied does not equal zero.

Rearranging the equation, we can see mathematically that in this case, the force of friction is equal to the negative of the force applied:

Fapp + Ff = 0

Fapp = -Ff

The formula Ff=μFN expresses the maximum frictional force that is experienced by the mass.