Understanding Pulley Systems: Forces on Blocks with Unequal Mass

[1MileCrash]

Homework Statement

I can do the math work, but I always overthink these multiple choice concepts.

A block is attached to each end of a rope that passes over a pulley suspended from the ceiling. The blocks do not have the same mass. If the rope does not slip on the pulley, then at any instant after the blocks start moving the rope:

Homework Equations

The Attempt at a Solution

Exerts different forces on both blocks, the lesser force exerted on the smaller block?

I figure that F = ma and the acceleration is the same.

 

[SammyS]

What are ALL the choices?

 

[1MileCrash]

Pulls on both blocks and exerts the same non-zero force.
Pulls on both blocks, exerting greater force on heavier block.
Does not pull on either block.
Pulls on both blocks, exerting greater force on lighter block.
Pulls only on the lighter block.

 

[SammyS]

If you were to solve such a problem, would the tension be the same , or different for the two blocks?

 

[NascentOxygen]

Which options can you strike out right from the word go?

 

[1MileCrash]

Pulls on both blocks and exerts the same non-zero force.
Does not pull on either block.
Pulls only on the lighter block.

Are the options I would strike out. Agree?

 

[1MileCrash]

If you were to solve such a problem, would the tension be the same , or different for the two blocks?

I've always worked problems assuming pulleys were nothing more than "direction changing" and therefore the tension was always equal.

 

[SammyS]

I've always worked problems assuming pulleys were nothing more than "direction changing" and therefore the tension was always equal.

Then, why eliminate: "Pulls on both blocks and exerts the same non-zero force." ?

 

[1MileCrash]

Then, why eliminate: "Pulls on both blocks and exerts the same non-zero force." ?

Because it was my initial choice, marked incorrect.

 

[1MileCrash]

Anyone?

 

[NascentOxygen]

Pulls on both blocks and exerts the same non-zero force.
Does not pull on either block.
Pulls only on the lighter block.

Are the options I would strike out. Agree?

Yes. So that leaves just two to choose from ...

 

[NascentOxygen]

I always overthink these multiple choice concepts.

I don't believe it's possible to overthink this one. It's nasty.

 

[SammyS]

Not an ideal mass-less pulley?

 

[NascentOxygen]

We are told the rope does not slip on the pulley. We are not told that the pulley bearing is frictionless; so expect energy loss there. Nor are we told the pulley has negligible mass. As SammyS points out, when the pulley has mass it too requires a force to speed up its rotation.

I presume that these practicalities must have been discussed in class, for them to appear on a multi-choice test paper.

 

[1MileCrash]

They will appear in class :P

Our assignments are posted ahead of time

So, since the driving force is the heavier block, it preceeds the frictional loss, and pulls on the cord normally, which in turn pulls back on it with a tension.

Meanwhile, the rope on the other end pulls up, but since the oulley has sapped some efficiency, it is with a lesser force.

Correct?

 

[NascentOxygen]

That's the general picture.