Kinetic Friction of sliding down a pole

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Homework Help Overview

The problem involves a firefighter sliding down a vertical pole, with a focus on understanding the forces of gravity and friction acting on her as she accelerates. The context is rooted in dynamics, specifically applying Newton's laws of motion to analyze the situation.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the application of Newton's second law and the calculation of forces, questioning how to incorporate both gravity and friction into their analysis. There is uncertainty about the coefficient of kinetic friction and its necessity for solving the problem.

Discussion Status

Participants are actively engaging with the problem, with some providing calculations for mass and force. There is recognition that the net force must be expressed in terms of the actual forces acting on the firefighter, and hints are given regarding careful consideration of signs in the equations. No consensus has been reached yet.

Contextual Notes

Participants note the absence of the coefficient of friction and express confusion about how to proceed with the calculations given the forces involved. There is an ongoing exploration of the relationship between the known weight and the forces acting on the firefighter.

lacar213
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Homework Statement


A firefighter whose weight is 812 N is sliding down a vertical pole, her speed increasing at the rate of 1.45 m/s2. Gravity and friction are the two significant forces acting on her. What is the magnitude of the frictional force?


Homework Equations


Fk = ukFn
uk = coefficient of kinetic friction


The Attempt at a Solution


I'm not sure what the coefficient of KF should be so I just tried ..
812*1.45 = 1177.4
I'm not sure how to incorporate the gravity and friction into the equation.
 
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lacar213 said:
I'm not sure what the coefficient of KF should be so I just tried ..
You're not given the coefficient of friction. Luckily you won't need it--you'll figure out the friction force another way.
812*1.45 = 1177.4
I'm not sure how to incorporate the gravity and friction into the equation.
Gravity and friction are the two forces acting on the firefighter. You are given her acceleration. Apply Newton's 2nd law: ∑ F = ma

(What's the firefighter's mass?)
 
OK ...
F = ma
mass -> 812 N/9.8 = 82.8571 kg
F = (82.8571)(1.45)
F = 120.143 N

Is it that simple?
 
lacar213 said:
F = ma
mass -> 812 N/9.8 = 82.8571 kg
Good.
F = (82.8571)(1.45)
F = 120.143 N

Is it that simple?
Not that simple, but almost. The "F" in F = ma stands for the net force acting on the object. Express the net force in terms of the actual forces acting on the firefighter (weight and friction) by adding those forces up. (∑ F means the sum of the forces.) Then you can solve for the unknown friction force.

Hint: Careful with signs.
 
Would the next step be to multiple her weight and gravity? Then add that net force to 120.143 N?

F = (82.8571)(9.8) = 812 N - but that's the weight already given??
120+812 = 932.143
 

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