Question on Princeton Review AP Physics Example

TheMichiganian
Fri, 23 Nov 2012 01:11:11 +0000

On example 4.2/4.3 in PR’s AP Physics C book (same problem in their Physics B book), it reads:

A 15kg crate is moved along a horizontal floor by a warehouse worker who’s pulling on it with a rope that makes a 30 degree angle with the horizontal. The tension in the rope is 200N and the crate slides a distance of 10m. How much work is done on the rope?

Then, the next problem:

In the previous example, assume that the coefficient of kinetic friction between the crate and the floor is 0.4. How much work is done by the normal force, and how much work is done by the friction force?

For the second problem, the answer to the second question doesn’t make sense to me. It says that the strength of the normal force is mg, which is then multiplied by the coefficient of friction and the distance. My question is why it’s mg, considering the crate has an upward component of the force in addition to the horizontal force.

Not sure if it’s an error, or I’m just not getting it, but any help would be appreciated. Thank you!