A person pushes a box a cross a horizontal surface at a constant speed of 0.5 meter per second...

A person pushes a box a cross a horizontal surface at a constant speed of 0.5 meter per second. The box has a mass of 40 kg , and the coefficient of sliding friction is 0.25. The power supplied to the box by the person is...
The answer is 50W ,but I don't get it.

Ther is no time when Power = work / time and I don't know how speed can help in this equation at all..
Help.

Power = work / time
= (force × distance) / time
= force × (distance / time)
= force × speed

They give you the speed, so you just have to figure out how much force the person is applying. Now, there are TWO horizontal forces acting on the box:
1. forward force due to person
2. backward force due to friction

But we know the box is not accelerating (has "constant speed"), so that means the forward and backward force exactly balance:

force due to person = force due to friction

The force due to friction is just mgμ ("m"=40kg; "μ"=0.25), so

force due to person = mgμ

and:

power = (force due to person) × speed = mgμ × speed.

(40 x 9.8) = 392N. weight.
Friction = (392 x .25) = 98N. force required.
In 1 sec., the box moves 0.5m.
Work performed in 1 sec = (.5 x 98) = 49 Joules, and as that is in 1 sec., answer = 49 Watts.

You must be using 10 as g?? I don't understand why... I used 9.8.