Physics scale on an elevator problem: acceleration of the elevator and reaction force between blocks

In this physics scale on an elevator problem, we are given the reading on the scale on an elevator, and we are given the masses of two blocks on the elevator.

In the first part of the problem, we calculate the acceleration of the elevator by applying Newton's second law to the entire mass of the blocks. We diagram the forces on the two blocks: the force of gravity pointing down, and the scale exerting a force upward on the blocks. We note the Newton's third law reasoning here: the bottom block pushes down on the scale with a given force, but the scale pushes up on the block with exactly the same force.

Next we apply Newton's second law on the combined mass of the blocks: F_net=ma, and we have the net force on the blocks, allowing us to solve for the acceleration of the blocks. The acceleration of the blocks is the same as the acceleration of the elevator, so we're done.

In the second part of the problem, we compute a reaction force between blocks: we want the normal force on the top block exerted by the bottom block. Again, we start by diagramming the forces on the block: the weight vector pointing down, and the normal force pointing up. We use Newton's second law to write n-mg=ma, and we know the acceleration of the top block (same as the acceleration of the elevator). This allows us to solve for the unknown normal force on the block, and we've got the interaction force between the blocks!

[nubia]

this was so helpful!!! i got a 101% on my physics final because of this! thank you so much!!!

[prathamhebbar]

The second part of the problem is weirdly worded. Can you please explain what it is asking?