Kinetic Friction Problem Needs Help

[Drstevebrule]

Kinetic Friction Problem Needs Help!

Homework Statement

A truck is traveling at constant acceleration. A box of mass 80kg rests in the bed of the truck. The Kinetic Friction between the bed and the box is 0.5. After traveling for 3 seconds the box falls off the back of the truck, what is the distance the box traveled?

Homework Equations

F=ma
F friction = Uk(Mg)
Kinematic equations.

The Attempt at a Solution

a=Ukg

a=.5(10)

a=5

D=.5(at^2)
D=.5(45)
D=22.5

Yet this distance, 22.5 meters, isn't even close to 4 multiple choice answers given on the test. The highest distance being 5 meters... what am I missing here?

 

[TSny]

I don't see anything wrong with your analysis. If you were also given the acceleration of the truck, then you could calculate the distance the box slides relative to the truck and maybe that would come out less than 5 m. But you aren't given the truck's acceleration, so that can't be it. I don't know. I'm sure you've checked that you haven't misread any of the data given in the problem.

Comments from anyone else?

 

[CWatters]

Looks right to me. I mean I also get 22.5m.

 

[Drstevebrule]

Yes I agree with both of your comments and thank you for posting. My friend took the MCAT this week and he called me after and relayed this problem to me, so I cannot verify that he wasn't mistaken about some of the variables given. He said that he also came to the answer of 22.5 meters but that the answer choices were much lower. The only reasonable explanation I can think of is that he has confused some of the information in the problem or somehow missed something, because with being given 80kg, constant acceleration, .5 coefficient of friction, and 3 seconds, everyone that I have spoken to is getting the same answer.

I just wanted to post on here and see if maybe I was missing something, something about the constant acceleration of the truck was bugging me and I thought the answer perhaps was linked to that information, but so far everyone has come up with the same result of 22.5 so I am afraid he just confused some information in the problem. Thank you for your time.

 

[CWatters]

Just for info..You don't appear to need to know the mass of the box to solve the problem.

u_kmg = ma

Mass cancels.