# Can someone tell me if I made an error?

1. Sep 8, 2007

### myelevatorbeat

I have two questions I'm having a little trouble solving. I will post the question, then my work, and then the problem I'm having for each question below.

1) Problem: A dockworker loading crates on a ship finds that a 25 kg crate, initially at rest on a horizontal surface, requires a 74 N horizontal force to set it in motion. However, after the crate is in motion, a horizontal force of 55 N is required to keep it moving with a constant speed. Find the coefficients of static and kinetic friction between crate and floor.

How I solved it:

For the Static Friction Coefficient:

(Fnet)x=ma (a=0)
74 N -fs=0
fs=74 N
usn=74 N
since normal = 25 x 9.8 I plugged that in and got 245

us x 245 N = 74 N
I divided and got 0.302

Kinetic

(Fnet)x=ma (a=0 since it has a constant speed)
55 N-fk=0
fk=55 N
ukn=55N
uk x 245 N = 55 N
and divided and got 0.22

Now my problem:

I am using webassign.net for my homework and when I plug in these values it tells me I'm wrong even though I thought I did it right.

2) Problem:

A hockey puck is hit on a frozen lake and starts moving with a speed of 11.0 m/s. Five seconds later, its speed is 5.10 m/s.
(a) What is its average acceleration?

(b) What is the average value of the coefficient of kinetic friction between puck and ice?

(c) How far does the puck travel during the 5.00 s interval?

I solved for part a and got -1.18 m/s^2
I also solved for part c and got 40.3 m

I tried solving for part B by using my acceleration/gravity. So, -1.18/9.80 = 0.28. However, webassign.net is telling me this answer is wrong and I'm not sure why because I did the EXACT same problem with different values last week and got it right by acceleration/gravity.

If someone could find where I made an error, I would really appreciate it.