Newton's Laws of Motion : Block Problem

  • #1

Homework Statement



In the arrangement shown, all surfaces are frictionless, mass of block M = 30 kg and m = 20 kg.
If F = 180 N , Find accelerations of both the blocks
attachment.php?attachmentid=65490&stc=1&d=1389379517.jpg


2. Doubt

What amt of force will act on larger block M, just T(coz of horizontal string around the pulley) or 2T(coz of both the strings around the pulley)
 

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Answers and Replies

  • #2
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Imagine that the pulleys were not attached to anything else except the string. In which directions do you think they would be pulled?
 
  • #3
tiny-tim
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hi rajumahtora! :smile:

when you consider the forces on a body, you must consider the whole body …

the block M is attached to the pulleys, so you must consider the external forces on the block-plus-pulleys :wink:
 
  • #4
tiny-tim
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Homework Statement


In the arrangement shown, all surfaces are frictionless, mass of block M = 30 kg and m = 20 kg.
If F = 180 N , Find accelerations of both the blocks

attachment.php?attachmentid=65514&stc=1&d=1389452404.jpg



2. Doubt

How much force will be applied on the larger block M by the pulley(coz of tension)?
1. T towards right
2. T towards up
3. T√2 45° to the right
4. Any other answer


3.Reason for doubt

If the total for exerted on the T i(cap) + T j(cap) i.e. Fnet on Bock M due to tension of string on pulley is T√2,
then there will be a force which will tend to pull block M upward......and if the accln of string is much higher, then the tension in the string will cause the block to accelerate up.

[To Jilang and tiny-tim,
After reading ur answer, i had this doubt too...so i asked again]

If anything else, please answer.....
Thanks

(please don't start a new thread)

as i said before, so you must consider the external forces on the block-plus-pulleys (as a single body), not just the block on its own
 
  • #5
haruspex
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rajumahtora, please clarify your question. There are two pulleys attached to M. Are you asking what the net force on M is from the two pulleys, or what force each pulley exerts on the block separately?
(What do you think the two separate forces are?)
 

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