# physics help...

• Feb 14th 2009, 09:35 PM
maeca
physics help...
i don't know how to solve this problem..

a box (m=30kg) is sliding along a path with initial speed of 5m/s. using the definition of energy and work determine how far the box will go before stopping. (uk=0.04)
• Feb 14th 2009, 10:02 PM
Rincewind
Quote:

Originally Posted by maeca
i don't know how to solve this problem..

a box (m=30kg) is sliding along a path with initial speed of 5m/s. using the definition of energy and work determine how far the box will go before stopping. (uk=0.04)

Lets also assume that gravitational acceleration is g = 10 m/s/s.

So the initial kinetic energy of the box is

$E = \frac{1}{2}mv^2 = \frac{1}{2}\times 30 \times 5^2 = 375$ Nm

The frictional force opposing motion is

$F_f = \mu_k mg = 0.04 \times 30 \times 10 = 12$ N

So the work done by friction to overcome the initial kinetic energy is

$W = F_f d$

where d is the distance oevr which the force has acted. To stop the box the W = E, therefore

$d = \frac{E}{F_f} = \frac{375}{12} = 31.25$ m

Hope this helps.
• Feb 17th 2009, 06:20 AM
maeca
thanks..that a great help