Which forces does work as a car brakes?

[okgo]

Homework Statement

Which forces does work?
http://www.screencast.com/users/ntrinh3/folders/Jing/media/3f09a85d-142c-4f73-a8a8-1ad506d5c2b8

Homework Equations

W=fdcos(theta)

The Attempt at a Solution

I always thought normal and friction does no work because it is perpendicular to motion. What gives?

 

[rock.freak667]

yes the normal force does no work

But the frictional force is opposing motion (anti-parallel to the motion), so it must do work to slow the mass down.

 

[okgo]

When a car brakes, isn't it in motion and using kinetic friction to stop, and not static friction?
Static friction causes rolling motion in wheels, but there is no rolling motion so it skids. So shouldn't kinetic friction do work and not static? I'm a bit confused on the two concept.

 

[rock.freak667]

When a car brakes, isn't it in motion and using kinetic friction to stop, and not static friction?
Static friction causes rolling motion in wheels, but there is no rolling motion so it skids. So shouldn't kinetic friction do work and not static? I'm a bit confused on the two concept.

Had the car been stationary, static friction would be acting, but the car in motion, so kinetic friction is doing work. There is relative motion between the wheels and the ground.