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Friction and Work (check please)

  • #1

Homework Statement


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!
 

Answers and Replies

  • #2
201
51
The process appears to be correct (didn't check your arithmetic) . There seems to be an extra minus sign before [itex] F_k [/itex] in the last line ?!
 
  • #3
So...just check the numbers?
 
  • #4
201
51
So...just check the numbers?
You could do that with a calculator, the last one may be wrong though , incidentally did you take the downward direction as -ve?
 
  • #5
what does -ve stand for?
 
  • #6
201
51
  • #7
yes, should i have?
 
  • #8
201
51
yes, should i have?
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.
 

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