How forces work (normal and friction)

[mystmyst]

This is a photo from a question

[PLAIN]http://img209.imageshack.us/img209/6963/forcefrictionquestioni.png

Then it separates the two masses and shows us the forces acting on each mass:

[PLAIN]http://img842.imageshack.us/img842/3320/masssmall.png

This I understand but the next mass, I don't understand why the red vectors are there and why they are facing their directions:

[PLAIN]http://img225.imageshack.us/img225/6720/masslarge.png

Why is there a friction force on the large mass and why is it facing the left direction? It was used for the smaller mass while facing the other direction.

And why is there a normal force facing downwards? N_m was used for the smaller mass. Why is it being used for the large mass?

Thanks!

 

[PhanthomJay]

This is a photo from a question

Why is there a friction force on the large mass and why is it facing the left direction? It was used for the smaller mass while facing the other direction.

And why is there a normal force facing downwards? N_m was used for the smaller mass. Why is it being used for the large mass?

Thanks!

These pictures are Free Body Diagrams of each mass 'isolated' from the system, that shows the internal and external forces acting on that particular part of the system, that is, on each mass. For the red arrows, think about Newton 3. I find it confusing, though, when these forces are not shown on the proper faces on which they act.

 

[mystmyst]

I still don't get it: What does friction have to do with the large mass? The friction makes the small mass move or not move, but it doesn't do anything to the large mass. The weight of the small mass is what makes the large mass move slower

 

[tatiana]

If you were to look at each box individually you would see that with the red arrows, the N_m is the normal force of the larger massed box in combination with the mass or force of gravity acting down from the smaller box onto the larger box.. As for the statics friction red arrow, in order from both massed boxes to be moving in the same directional force they should not have the same frictional force because the smaller block is working to remain on the larger block as it is meing moved in the opposte dirction? does that kinda make sense?

 

[PhanthomJay]

I still don't get it: What does friction have to do with the large mass? The friction makes the small mass move or not move, but it doesn't do anything to the large mass.

sure it does. if there was no friction between the 2, the larger mass would accelerate at a faster rate, and leave the smaller block not moving at all.

The weight of the small mass is what makes the large mass move slower

Not true. It's the friction that makes it move slower. Read up on Newton 3...if there is friction acting between the 2 masses, the friction force that the larger mass exerts on the smaller mass is equal and opposite to the friction exerted by the smaller mass on the larger mass. Note the directions of the friction force in the respective free body diagrams.

 

[mystmyst]

Not true. It's the friction that makes it move slower.

Ok, I think I get it.

if there is friction acting between the 2 masses, the friction force that the larger mass exerts on the smaller mass is equal and opposite to the friction exerted by the smaller mass on the larger mass. Note the directions of the friction force in the respective free body diagrams.

But let's say the bodies were moving at the same acceleration rate. How would that change the dynamic of the free body diagram? Would I just say the friction on the right side equals the friction on the left side and they cancel each other out? Would it be possible to calculate that friction?

 

[PhanthomJay]

Ok, I think I get it.



But let's say the bodies were moving at the same acceleration rate. How would that change the dynamic of the free body diagram? Would I just say the friction on the right side equals the friction on the left side and they cancel each other out? Would it be possible to calculate that friction?

Which free body diagram? If you look at the 2 blocks together, the only external force acting horizonally is F, so the net force in that direction is F and the mass of that system is (m + M). Calculate the acceleratiion accordingly. Now to find the friction between the 2 surfaces , look at the free body diagram of the smaller mass, per your 2nd sketch. What is the only force acting in the horizontal direction? Then since you know it's acceleration, use Newton 2 to find it. Then, to check your work, you can look at your 3rd sketch of the free body diagram of the large mass, if you wish. You should apply Newton 2 to this block and see if you get the same values.

 

[mystmyst]

Awesome