# Friction and Work (check please)

1. Jun 7, 2016

### GalacticSnipes

1. The problem statement, all variables and given/known data
A 25 kg bear slides, from rest, 8 m down a lodgepole pine tree, moving with a speed of 6.4 m/s just before hitting the ground

(a) What change occurs in the gravitational potential energy of the bear-Earth system during the slide?
W = F*D
W = (m*a)*D
W = 1960

(b) What is the kinetic energy of the bear just before hitting the ground?
KE = 0.5*m*v^2
KE = 512

(c) What is the average frictional force that acts on the bear?
Ei = m*g*h = 1960
Ef = 0.5*m*v^2

W = -Fk*h
Ei + W = Ef
1960 + (-Fk*h) = 512
-Fk = 120.67

Could someone check to see if i solved this correctly?
Thanks!

2. Jun 7, 2016

### Mastermind01

The process appears to be correct (didn't check your arithmetic) . There seems to be an extra minus sign before $F_k$ in the last line ?!

3. Jun 7, 2016

### GalacticSnipes

So...just check the numbers?

4. Jun 7, 2016

### Mastermind01

You could do that with a calculator, the last one may be wrong though , incidentally did you take the downward direction as -ve?

5. Jun 7, 2016

### GalacticSnipes

what does -ve stand for?

6. Jun 7, 2016

negative

7. Jun 7, 2016

### GalacticSnipes

yes, should i have?

8. Jun 7, 2016

### Mastermind01

You can take it whatever you want. Your method is correct then , there is no extra negative sign. Do check the arithmetic for c) I suspect it's wrong.