Forces on Identical Bricks: Ranking Magnitudes

[ohphysics]

Three identical bricks are pushed across a table at constant speed as shown. Rank the magnitudes of all the horizontal forces that you identified in your free body diagram. So I identified the horizontal forces of push, friction, system a on system b, and b on a. I understand the force will equal 0 because they are moving at a constant speed and are equal and opposite. I am having trouble conceptualizing how if the force of the push is equal to the force of friction the blocks could possibly keep moving forever. If the force of the push equals the force of friction doesn't that mean it would stop moving because of friction??

 

[Orodruin]

No, it means that there is a push force balancing the friction so that the object does not accelerate. In order to decelerate (slow down) an object there needs to be a net force on it according to Newton's second law and the friction would have to be larger than the push.

 

[ohphysics]

No, it means that there is a push force balancing the friction so that the object does not accelerate. In order to decelerate (slow down) an object there needs to be a net force on it according to Newton's second law and the friction would have to be larger than the push.

Oooooh that makes a lot of sense. Thanks a lot!

 

[Entanglement]

The answes lies in Newton's first law.

 

[HalfLostAndSearching]

Your understanding is correct. In this scenario, the forces of push and friction are equal and opposite, resulting in a net force of 0 and the blocks moving at a constant speed. This may seem counterintuitive, but it is because the force of push is being continuously applied, counteracting the force of friction. This is similar to pushing a box across a floor - as long as you continue to push with the same force, the box will keep moving at a constant speed, even though there is friction between the box and the floor. In this case, the blocks will continue to move because the force of push is being maintained. This can also be explained by Newton's first law of motion, which states that an object will remain in motion at a constant speed unless acted upon by an external force.