Please Help! Friction Problem

**dciple** · Oct4-05, 02:25 AM

Hi all, this homework problem has been killing me. It is listed as easy but I just can't figure it out. Ill post it word for word:

A boy coasts down a hill on a sled, reaching a level surface at the bottom with a speed of 6.4 m/s. If the coefficient of friction between the sled's runners and snow is 0.060 and the boy and sled together weigh 570 N, how far does the sled travel on the level surface before coming to rest? (in meters)

Since we are looking for the position I started with the equation x = vt + 1/2at^2 but I dont know the acceleration or time. I did figure out the force using the equation f=un (335.16 N).

Please help me! Any help is greatly appreciated!

**Tom Mattson** · Oct4-05, 10:22 AM

Originally Posted by dciple

Since we are looking for the position I started with the equation x = vt + 1/2at^2 but I dont know the acceleration or time. I did figure out the force using the equation f=un (335.16 N).

OK, not a bad start. Since you have the frictional force and you have the mass of the boy + sled, it follows that you do have enough information to get the acceleration (Hint: Think Newton's 2nd law).

It is true that you don't have any information about the time, though. That means that you have selected the wrong equation. You should always list out what you know and what you don't know, and then select the equation that relates those things. In this case you should be looking for an equation that does not have time in it.

**dciple** · Oct4-05, 01:55 PM

Thanks Tom, that really got me started on the problem. I did what you suggested and solved for acceleration (.588 m/s). But i dont know where to go from here, im still missing time, velocity(final), and x(position) so i cant use any of the following equations i know:

v = v0 + at
x = v0t + 1/2at^2
v^2 = v0^2 + 2ax

Where do i go from here?! Thank you guys!

**Tom Mattson** · Oct4-05, 03:41 PM

Originally Posted by dciple

im still missing time,

True, but you don't need it.

velocity(final),

You are not missing that. It is given in the problem statement.

Hint: If you are moving and a force brings you to rest, then how fast are you going?

and x(position)

True you are missing that, but that is because it is the unknown. You were asked to find it.

so i cant use any of the following equations i know:
v = v0 + at
x = v0t + 1/2at^2
v^2 = v0^2 + 2ax

You can indeed use one of these. You just think you can't because you haven't caught where they give you the final velocity.

**dciple** · Oct4-05, 09:38 PM

Tom you are a god. I followed what you said and it worked brilliantly. Thank you so much for your help.