physics problem, newtons law?

hey folks, im at my last try for uTexas on this and I was wondering if someone can help me with it.. 

A 22 kg chair initially at rest on a horizontal floor requires a 174 N horizontal force to set it in motion. Once the chair is in motion, a 141 N horizontal force keeps it moving at a constant velocity. The acceleration of gravity is 9.81 m/s .

2

a) What is the coefficient of static friction between the chair and the floor?

011 (part 2 of 2) 10.0 points

b) What is the coefficient of kinetic friction between the chair and the floor?

thanks to anyone that helps!

-James 

2 Answers

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  • 1 month ago
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    OK so first, you want to understand the difference between static and kinetic friction. When the chair is at rest, you have to surpass the static friction to get it moving. Once it's moving, you just have to surpass kinetic friction to keep it moving.

    The force of friction is = μ * Normal Force = μ * m * g. Normal force = m*g here because it is horizontal. If they were asking for this question on a slope, you would use trig to figure out the component of m*g that is the normal force. This problem is easier for you to solve because the chair is on flat ground.

    μs (static coefficient) = Friction force/ Normal Force = Ffriction / (m*g)

    μs = 174 N / (22kg * 9.81 m/s^2) = 0.806, maybe 0.81 if it's rounded to two decimals which I bet it is

    μk = 141N / (22kg * 9.81 m/s^2) = 0.653, most likely 0.65 if it is rounded to two decimals which I bet it is.

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  • 1 month ago

    a) What is the coefficient of static friction between the chair and the floor?

    m = mass of chair = 22 kg

    g = gravitational acceleration = 9.81 m/s²

    W = weight of chair = m x g = 215.82 N

    Fn = normal force = W = 215.82 N

    Ff = friction force = 174 N

    μs = coefficient of static friction = to be determined

    Ff = μs x N

    Ff / N = μs

    μs = Ff / N

    μs = 174 N / 215.82 N

    μs = 0.806227412 ≈ 0.81

    b) What is the coefficient of kinetic friction between the chair and the floor?

    Ff = friction force = 141 N

    μk = coefficient of kinetic friction = to be determined

    Ff = μk x N

    Ff / N = μk

    μk = Ff / N

    μk = 141 N / 215.82 N

    μk = 0.653322213 ≈ 0.65

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