Friction and Work (check please)

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Homework Help Overview

The problem involves a bear sliding down a tree and examines the changes in gravitational potential energy, kinetic energy, and the average frictional force acting on the bear. The subject area includes concepts of energy conservation and friction in physics.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • The original poster attempts to calculate changes in energy and frictional force using formulas related to gravitational potential energy and kinetic energy. Some participants question the arithmetic and the use of negative signs in the calculations.

Discussion Status

Participants are engaged in verifying the calculations presented by the original poster. There is a suggestion to check the arithmetic, and some express uncertainty about the directionality of the forces involved. The discussion reflects a mix of checking numerical accuracy and clarifying assumptions.

Contextual Notes

There is a mention of potential confusion regarding the sign convention for direction, particularly whether downward motion should be considered negative. This indicates an area of assumption checking within the problem context.

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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!
 
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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 ?!
 
So...just check the numbers?
 
GalacticSnipes said:
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?
 
what does -ve stand for?
 
GalacticSnipes said:
what does -ve stand for?
negative
 
yes, should i have?
 
GalacticSnipes said:
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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