Hello!

I'm using the "AP Physics Essentials: 1" book to study for my Work, Power, and Energy exam (chapter 5.) I need clarification on question 5.10; how do we decide on the equation to use? I'm so lost on how to make that determination.

 

Thanks!

On a problem like this, where you're not sure where to start, I'm always a fan of going back to your basic tools -- free body diagrams, listing your "givens" and "finds," and writing, in many cases, associated Newton's 2nd Law equations.  

 

In this case, it's specifically asking you to find the work done by friction, and with a diagram showing forces, right away I'd start thinking of finding the force of friction and the distance over which it's applied.

 

Using Newton's 2nd law, you can then find the force of friction (200 N), then use that to find the work done.  Does that help at all?

 

Quite regularly in physics, when you see a problem, the answer won't be immediately obvious.  It's perfectly normal to have to struggle with a couple different avenues, find one that doesn't work, go back to the starting board and try something else.  It looks nice and neat in the book because I only put the final answer in there, but that doesn't mean it was that easy when I originally went to solve it.

 

Good luck!