1.

A crate of mass 100 kg is at rest on a horizontal floor. The coefficient of static friction between the crate and the floor is 0.4, and the coefficient of kinetic friction is 0.3 . A force F of magnitude 344 N is then applied to the crate, parallel to the floor. Which of the following is true ?

Answer»

The crate will accelerate across the floor at `0.5 m//s^(2)`.
The static friction force, which is the reaction force to F as guaranteed by Newton 's Third Law, will also have a magnitude of 344 N.
The crate will slide across the floor at a constant speed of `0.5 m//s`.
The crate will not move.

Solution :The maximum force that static friction can exert on the crate is `mu_(s)F_(N)= mu_(s)F_(W)= mu_(s) MG= (0.4)(100 kg)(10 N//kg)=400 N`. Since the force APPLIED to the crate is only 344 N, static friction is able to apply that same magnitude of force on the crate, keeping it stationary. [Choice (B) is incorrect because the static friction force is not the reaction force to F, both F and `F_(f("static"))` act on the same object (the crate) and therefore cannot FORM an action / reaction pair.]


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