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Coefficient Of Kinetic Friction Question

by nintendude794
Tags: coefficient, force, friction, gravity, kinetic
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nintendude794
#1
Nov13-10, 04:23 PM
P: 3
1. The problem statement, all variables and given/known data
A 25 kg box slides down a 26 degree ramp with an acceleration of 1.32 m/s2.
The acceleration of gravity is 9.81 m/s2
Find the coefficient of kinetic friction between the box and the ramp.

2. Relevant equations
mass = m = 25 kg
angle = (theta) = 26 degrees
acceleration = a = 1.32 m/s2
gravity = Fg = 9.81 m/s2
coefficient of kinetic friction = Mk = Fk/Fn
Fk = resistance, friction? Force opposite Mass*Acceleration.
Fn = perpendicular to the object's motion, upward
3. The attempt at a solution
F = Fn - COS26(I don't know...)

Attached is a pic of my teacher's process for solving the same problem, only with different values.

I'm clueless. Teacher said this is the toughest of the 14 problems that are due in 6 hours, so I thought I might seek assistance in learning how to do it. Please and thanks. :)
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ehild
#2
Nov13-10, 05:26 PM
HW Helper
Thanks
P: 10,511
The force of kinetic friction is proportional to the normal force Fn, and its acts along the slope in opposite direction as the body moves. Fn, the normal force is the component of mg which is perpendicular to the slope,

Fn=mgcos(theta).

The force of kinetic friction is

Fk=μ mg cos(theta).

The component of mg which is parallel with the slope is

Fp=mg sin(theta),

and it points down the slope. The force of kinetic friction points in the opposite direction. The resultant of these forces will accelerate the body down the slope

ma=mg sin(theta)-μ mg cos(theta).

Given m, a, theta, g, find μ.

ehild
nintendude794
#3
Nov13-10, 09:00 PM
P: 3
Quote Quote by ehild View Post

The force of kinetic friction is

Fk=μ mg cos(theta).

ehild
Here, what exactly is "μ"? What does it mean?

ehild
#4
Nov13-10, 10:47 PM
HW Helper
Thanks
P: 10,511
Coefficient Of Kinetic Friction Question

It is the coefficient of friction. You used the notation Mk.

ehild
nintendude794
#5
Nov13-10, 11:33 PM
P: 3
Quote Quote by ehild View Post
It is the coefficient of friction. You used the notation Mk.

ehild
Thanks. This assignment was due 30 minutes ago, I accepted a zero if even only temporarily because I do not understand. I have printed it out to take it to tutorials. Thanks so much for trying to help me though. :)


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