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lydia141822 · Jan 14, 2014, 05:45 PM

1) A 27 kg chair initially at rest on a horizontal ﬂoor requires a 373 N horizontal force to set
it in motion. Once the chair is in motion, a 344 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?

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

2) Once a 21 kg crate is in motion on a horizontal ﬂoor, a horizontal force of 55 N keeps the crate
moving with a constant velocity.The acceleration of gravity is 9.81 m/s^2.
What is µk, the coeﬃcient of kinetic friction, between the crate and the ﬂoor?

THANK YOU

ebaines · Jan 15, 2014, 12:39 PM

Friction force is modelled as:

$<br /> F = \mu N<br />$

where F is force applied parallel to the surface the obkect is istting on and N is the normal foprce between the object and the surface. In this case N is equal to the object's weigt, so you have

$<br /> F = \mu mg<br />$

If you use the coefficient of static friction $\mu_s$ in this formula it yields a value for F where the object first starts moving from rest. If you use the coeffcient of kinetic friction for $\mu_k$ you get the force required to keep it moving at a constant velocity. You have been given enough informatio to determine these coefficients of friction - post back with your attempt at solving these problems and we'll help if you get stuck.