A box is initially at rest on a flat table. The force of gravity acting on the b
ID: 2021138 • Letter: A
Question
A box is initially at rest on a flat table. The force of gravity acting on the box is directed down and has a magnitude of 20.0 N. You then exert a horizontal force on the box, gradually increasing the force you apply until the box starts to slide across the table. Once you have the box moving, you then adjust the magnitude of the force you apply until the box moves at constant velocity across the table (the force you apply is always in the same direction, and is horizontal). The coefficient of kinetic friction for the box-table interaction is 0.500, while the coefficient of static friction for the box-table interaction is 0.600.(a) Before you start exerting a force on the box, what is the magnitude of the force of friction acting on the box?
(b) The minimum horizontal force you need to apply to start the box moving has a magnitude of ..
(c) Once the box is moving, the horizontal force you need to apply to keep the box moving with a constant velocity has a magnitude of ..
*all answers in newtons
Explanation / Answer
Ok so first were going to make a data table, I would also suggest that you draw out the scenario.
V0=0
s=0.600
k=0.500
Fg=20N
And thats it. So for part a we need to figure out the force of static friction. We need to use the formula
Ff = n (its easy to memorize this as fun), where n = the normal force (opposite but equal to Fg)
Ff = (0.600)(20N) = 12N
B) Static force is the force required to get an object moving, but the kinetic force is the force needed to keep an object moving. So for a constant velocity we need a net force of 0 So we need a force pulling on the box that is equal to the static frictional force. So we need a force of 12N pulling on the box.
C) Now the frictional force will be different now that the box is moving using the coeeficient of kinetic friction
Ff = (0.500)(20N) = 10N
This is the force needed to keep the box moving at a constant velocity(again need a net force of 0) once it's already in motion.
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