# 2 blocks, on ontop of the other

## Homework Statement

block A is ontop of block B.
block A more mass than B

Which force is greater magnitude, Force of gravity (mg (weight) on block B, or force of block A on B?
Wouldnt answer have to be Faonb because both blocks are subject to gravity?

## The Attempt at a Solution

PhanthomJay
Homework Helper
Gold Member
Indirectly, your answer is correct, but you need to be a bit more specific in your reasoning. If you believe the force of gravity on A is equal to the force of A on B, you must state why.

Sorry, let me add a little more (important) info
Both blocks are stationary on level ground.

So lets say i want to know which force, on B has the greatest magnitude.
I have 3 forces acting On block B.
1) W (on b by earth)
2) F (on b by a)
3) N (on b by s)

Objects are not accerating (or moving) so I know Fnet=0
specifically,
Fxnet = 0 and
Fynet = 0

Also, there are no forces in x direction

For y I have
SumFynet = NyBS - mgyBE - FyBA= 0

So just purely from above equation, largest force would have to be the normal force of suface on B.
But how do i know which force is bigger, force on B by Earth (FyBE), or force on B by A (FyBA)??
thanks for any help

PhanthomJay
Homework Helper
Gold Member
Good FBD of block B. But now to answer your question, you need to make a FBD of block A, then use Newton 3.

mujadeo, you clearly understand the process, so just finish your thought. The weight of B is the force from gravity. Since A is on top of B, and nothing is moving, the weight of A, would be the force acting on B. You know that A has greater mass than B, gravity is the same for both blocks, so which force is greater?

"You know that A has greater mass than B, gravity is the same for both blocks, so which force is greater?"
yeah thats how i was visualizing it, so thanks for the confirmation.
Obviously A would have the greater Force (gravity times its mass, (which is greater than B))

One other querstion though: Necause system is stationary (Fnet=0) that just means that all the forces must add to zero, right?
OK so does that mean when i do FBD for block A, that should = 0
and FBD for B that should be 0
and FBD for system A and B, that should 0 also, is that rght? ---thanks

PhanthomJay