Max Static Friction: Questions Answered

[MIA6]

I know that static friction happens only when the object doesn't move. And if the maximum static friction=applied horizontal force, the object still remains at rest. I have already posted a topic about friction yesterday. But there is a problem here:
Two crates are stacked on top of each other on a horizontal floor. The coefficient of static friction between Box A and Box B is 0.6. The coefficient of static friction between Box A and the floor is 0.2. A force of 100 N is applied parallel to the floor on Box B. The boxes move together along the floor. What's the ratio of the mass of Box A to Box B.
My teacher said if there is net force 0 acted on an object, the object is either at rest or moves with constant velocity. So she said the force 100 = the static friction for Box B. I have two questions here: If an object is at rest, then zero net force will still make it rest, right? But is there a possibility that it will move with constant velocity according to what my teacher said? Second, if the applied force = static friction, then the object is not supposed to move? Only if the force is larger than the static friction, then it will move? I have few confusions, hope you can help me out.

 

[Kurdt]

If a net force actos on an object then yes it will either remain at rest or continue at a constant speed depending on its state of motion before hand. Box B will not move on top of box A if the force is equal to the threshold force of static friction. The system of the two boxes together could move though because the coefficient of static friction is less between box A and the ground. However it will be accelerating.

 

[MIA6]

If a net force actos on an object then yes it will either remain at rest or continue at a constant speed depending on its state of motion before hand. Box B will not move on top of box A if the force is equal to the threshold force of static friction. The system of the two boxes together could move though because the coefficient of static friction is less between box A and the ground. However it will be accelerating.

The two boxes can move is that because the force of 100 N is bigger than the static friction of two boxes with the floor? How do we find the static friction? normal force of two boxes times the coefficient ot static friction between Box A and the floor 0.2? Then, why my teacher said 100N= static friction for Box B?

 

[Kurdt]

Well if you want to move the two together when applying a force to box B then the max force you can use is the threshold force for the static friction between A and B. If you used more force then B would just slide off A.

 

[MIA6]

Well if you want to move the two together when applying a force to box B then the max force you can use is the threshold force for the static friction between A and B. If you used more force then B would just slide off A.

Okay, so the applied force 100 N must be equal to maximum value of static friction for box B so that box B wouldn't slide off A (btw, can 100N smaller than static friction here?). Moreover, did you mean that 100 N must be bigger than than whole static friction for two boxes all together so that they were able to move? (as I did before) In other words, the reason two boxes move together is because 100 N is the force giving to both boxes, or only box B, but box A moves, too because its coefficient?

 

[Kurdt]

Yes the force can be less than the maximum for static friction, just as long as its not over it. And yes for the two to slide along the floor the 100N must be greater than the maximum force of static friction for the two boxes on the floor. I will ask however, have you given the question exactly as stated?

 

[MIA6]

Yes the force can be less than the maximum for static friction, just as long as its not over it. And yes for the two to slide along the floor the 100N must be greater than the maximum force of static friction for the two boxes on the floor. I will ask however, have you given the question exactly as stated?

Yes, I did. There was a controversy in my class for this question because the teacher said it was in uniform motion since it didn't tell us it is accelerated and it didn't mention the kinetic friction. Btw, since if the maximum for static friction can be larger than the force it applied, why we do them 'equal'? Is that because it is easier to solve the problem?

 

[Kurdt]

Yes they are set equal as its easier to solve.