Work of pushing box horizontally - including force of friction

[avsj]

Homework Statement

A man slides a 100.0 kg box along the floor for a distance of 4.0m. If the coefficient of kinetic friction is 0.250, and the man does the job in 3.6s, what is his power output in watts?

Homework Equations

W=Fd
P=W/t

The Attempt at a Solution

I don't know how to calculate the force used to push the box. Fn should equal Fg so Ff = 0.25 x 980 = 245. But from what force do I subtract this

 

[andrevdh]

Well, the force needs to be at least equal to or larger than the frictional force doesn't it?

 

[ank_gl]

. Fn should equal Fg so Ff = 0.25 x 980 = 245.

thats the force you are looking for

 

[Gear300]

heheh, *(that drum thing)*...well, its obvious as to what to do next, so this was just a random a post.

 

[marlon]

I don't know how to calculate the force used to push the box. Fn should equal Fg so Ff = 0.25 x 980 = 245. But from what force do I subtract this

Well, you have been given the kinetic friction force. This is the friction as long as the box is moving !

marlon

 

[avsj]

The answer is 2.7 x 10^2 W.

So if my Ff = 245, I need to subtract that from the horizontal force to get Fnet which I can then use to get W= Fd and to me it is obvious from there. But how do I find the horizontal force of moving the box?

Thanks

 

[avsj]

Oh I think i understand. The net force must equal the force of friction as it is moving so we use the Ff as the force. Thanks :D

 

[ank_gl]

bingo, now u get it

 

[GoldPheonix]

Homework Statement

A man slides a 100.0 kg box along the floor for a distance of 4.0m. If the coefficient of kinetic friction is 0.250, and the man does the job in 3.6s, what is his power output in watts?

I don't know how to calculate the force used to push the box. Fn should equal Fg so Ff = 0.25 x 980 = 245. But from what force do I subtract this

Just walk yourself through the math, okay? It's nothing bad.

$$\overline{P} = \frac{ \Delta E}{ \Delta t}$$

$$\Delta E = F \Delta x$$

You've calculated force, right? So, what's the box's position at t=1? 0, right? So, time 1 = 0, right? It's your starting out point. Go from there.