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Help drawing freeform diagram (pulley problem)

  • Thread starter wraithetc
  • Start date
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"A 20 kg block, m1, is sliding on a 10 kg block, m2. The blocks
are on a 20º slope and are connected by a light string looped
over a pulley. "

All surfaces are frictionless and the string/pulley are massless.

I understand that the 20kg block should slide down since it should have a greater force.

When I draw the diagram I have three forces for each block. The normal force perpendicular to the incline, weight pointing straight down and the tension from the String.

I'm however confused on certain aspects of the diagram.

For instance Tension should always be along the string... so does that mean the tension vector for each block should be in opposite directions?

Also since the 20kg block is sitting a top the 10kg block does that not add to the magnitude of the 10 kg's block weight force?

Thanks for any help in advance.
 

Answers and Replies

Doc Al
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When I draw the diagram I have three forces for each block. The normal force perpendicular to the incline, weight pointing straight down and the tension from the String.
OK. (But the bottom block will experience two normal forces.)

I'm however confused on certain aspects of the diagram.

For instance Tension should always be along the string... so does that mean the tension vector for each block should be in opposite directions?
No. (Strings can't push. :wink:)

Also since the 20kg block is sitting a top the 10kg block does that not add to the magnitude of the 10 kg's block weight force?
It adds to normal force applied to the 10 kg block.

Hint: Consider forces parallel to the incline.
 
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For instance Tension should always be along the string... so does that mean the tension vector for each block should be in opposite directions?
No. strings can only pull and not push. The strings are obviously pulling the same way. I think you try to use action = reaction here and forget the forces that the pulley exerts on the strings.

Also since the 20kg block is sitting a top the 10kg block does that not add to the magnitude of the 10 kg's block weight force?
since the surfaces of the blocks are frictionless, the force of the big block on the small block will be perpendicular to the slope. It will increase the force that the small block exerts on the slope, and therefore it will increase the normal force that the small block feels, but all these forces are perpendicular to the slope, and the strings and won't affect the movement of the blocks
 

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