# Normal Reaction Force

For question b, the video mentions that the force exerted on B by A is the Normal Reaction Force of A.

Could someone please explain why it is the normal reaction force of A instead of it being the weight of A?
I'm not sure as to why the upwards force of A is the one being exerted onto an object below A.
(edited 1 month ago)
Original post by Dhshsnsksnskxn
For question b, the video mentions that the force exerted on B by A is the Normal Reaction Force of A.
Could someone please explain why it is the normal reaction force of A instead of it being the weight of A?
I'm not sure as to why the upwards force of A is the one being exerted onto an object below A.

Look at box A.

It has only two forces acting on it. Its weight, and the normal reaction force.

The normal reaction is the force exerted by B on A.

If the lift was stationary, this normal force would be equal to the weight of A. However as the lift is bringing all boxes up, this normal force must overcome the weight of A for motion to take place.

Hence, force exerted by B on A is the is the normal reaction force, and by Newton's 3rd law, it is also the force exerted by A on B. So the force diagram for B would contain this normal force acting down.
(edited 1 month ago)
Isn’t this just a statics question where g happens to be 9.8+0.5? The words and descriptions make the whole situation sound more complex than it is?