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western50 · Oct 2, 2010, 11:14 PM

Block A (mass = MA) is stacked on top of Block B (mass = MB) in an elevator that is accelerating upwards with acceleration a as shown below. The next two questions pertain to this situation. The symbol g has the value +9.8 m/s2.

https://online-s.physics.uiuc.edu/cg...06/01/fig3.gif

Which of the following statements concerning F (A on B), the force that Block A exerts on Block B, is true?

F (A on B) < Mass of A*g
F (A on B) = Mass of A*g
F (A on B) > Mass of A*g

What is the value of F (B on A), the force that Block B exerts on Block A,

F (B on A) = Mass of A*g
F (B on A) = Mass of A*a
F (B on A) = Mass of A*(a+g)
F (B on A) = Mass of B*a
F (B on A) = Mass of B*(a+g)

Unknown008 · Oct 2, 2010, 11:34 PM

You need to apply Newton II to get the value of the forces in 1. and Newton III to deduce the forces on the other bodies.

On your diagram, draw all the forces that are acting on both boxes.

You have a force $F_A$ that A exerts on B.

What is the resultant acceleration of A on B? It's (a+g)

So, use $F_A = M_A(a+g)$

This is the force that A exerts on B.

Can you now deduce the answer for the first part?

western50 · Oct 5, 2010, 12:58 PM

Why it is (a+g), not (a-g)

Unknown008 · Oct 6, 2010, 02:33 AM

Hm... I'm not sure how is the 'official' way to explain it, but I'll start with everyday examples.

When you get into an elevator, and the elevator moves up, you will feel for some time that you weight heavier, right?

Now, you might have seen cartoons where the elevator rises up very quickly, so that the character seems pressed on the floor of the elevator.

When you're in an accelerating car, and quickly go round a sharp corner, you will feel pushed towards the door of the car, away from the centre of the arc that the car makes. There is the centripetal force acting towards the centre of the arc the car does, but your inertia which pushes you to the other side. But when it's like this, you feel a little squished.

I think this has to do with relative acceleration. Just like, if you are standing on the bus stop and see a car coming towards you at a certain speed, you will see it coming with that speed. However, if you were in a car going in an opposite direction to the car coming towards you, you see that car coming at a higher speed!

Acceleration is the same. You experience it as being g, downwards, but if you are yourself accelerating upwards, the g will appear to be greater than before.