Calculating Work and Friction in a Horizontal Surface: A Practical Example

[johny_doe]

A person pulls her 70 N bag 200 meters at a constant speed.
The force exerted is 40 N at 50 degrees.

I can figure out the work the person does (5142 J).

But I'm having trouble figuring out the 'work done by the force of friction'

I know that Wfriction = delta KE + delta PE
...Since the speed is constant KE should be 0, but I'm not sure what to do about PE.. I know PE = mgy and delta PE = PEfinal - PEinitial but since it's a horizontal surface wouldn't that be 0? I must be doing something wrong because it can't be 0..

 

[whozum]

The net force is ____ because the bag's velocity and speed are _____ which means the force by the ______ equals the force of _______.

 

[johny_doe]

Just to make sure I understand

The net force is _0___ because the bag's velocity and speed are __constant___ which means the force by the _parallel component of force(40N*cos(50))_____ equals the force of _friction______?

 

[whozum]

You've got it.