Friction and Pulleys

  • #26
haruspex
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So,to find in which direction the block will move,I need to know force is larger in which direction,i.e tension +force by truck in forward direction should be greater than force of friction then only block will move in forward direction.
You know which way the block will move relative to the car (since the string neither stretches nor breaks).
And what do you mean by "force by truck in forward direction"? There are only two horizontal forces on the block.
 
  • #27
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I only know force of friction (static in nature) i.e 0.5 multiplied by 30 multiplied by 10=150 N.
 
  • #28
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what do you mean by "force by truck in forward direction"? There are only two horizontal forces on the block.
As truck is moving and block is in contact with the truck will not it exert any force?
 
  • #29
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As truck is moving and block is in contact with the truck will not it exert any force?
Yes, friction. What else?
 
  • #30
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When we board in bus ,we are initially (before getting in bus)at rest,but then start travelling at the speed of bus,As per newton's first law some external must acts on us.What's that?Is it fiction ?
You may find this trivial,but seriously that's why I was thinking that flat car does exert force on block.
 
  • #31
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When we board in bus ,we are initially (before getting in bus)at rest,but then start travelling at the speed of bus,As per newton's first law some external must acts on us.What's that?Is it fiction ?
You may find this trivial,but seriously that's why I was thinking that flat car does exert force on block.
No, it's not fiction, it's friction. The bus is stationary when you board. You are standung, say. When the bus starts, if there were ni friction between your shoes and the bus floor you would remain still, your shoes sliding, until you hit the back of the bus.
 
  • #33
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Is my calculation in post #37 correct?
 
  • #34
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Is my calculation in post #37 correct?
If you mean #27, yes. So,
- which way does that friction force act?
- are there any other horizontal forces on the block other than friction and the string tension?
- what is the sum of horizontal forces on the block?
- what is the acceleration of the block?
 
  • #36
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As tension force never pushes it always pulls .So block should move forward.
 
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  • #37
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Is there any tension force in the end which is tied to wall (I have pointed out that end in my below diagram)

truck.png

I think there should be.
 
  • #38
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As tension force never pushes it always pulls block should move forward.
In particular, forward relative to the car. So friction acts the other way, agreed? What, then, is the sum of forces?
 
  • #39
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Is there any tension force in the end which is tied to wall (I have pointed out that end in my below diagram)

View attachment 81877
I think there should be.
Yes. What is the relationship between the tension there and the tension at the end attached to the block?
 
  • #40
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They should be same.As string and pulley is massless(assumption)
 
  • #41
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Wait,I think I am getting the solution now.
 
  • #42
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They should be same.As string and pulley is massless(assumption)
Right.
 
  • #43
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I wish Mass of flat car was given!
 
  • #44
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I wish Mass of flat car was given!
As I posted previously, you don't care about the mass of the car or the forces on it because you know exactly how it is moving. Concentrate on the forces on the block and its acceleration.
 
  • #45
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Is it correct.
tr.png
 
  • #47
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- which way does that friction force act
Backward.
- are there any other horizontal forces on the block other than friction and the string tension?
No.
what is the sum of horizontal forces on the block?
T-150N =30Kg multiplied by acceleration of the block.
what is the acceleration of the block?
Here I am struggling.
 
  • #48
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Backward.

No.

T-150N =30Kg multiplied by acceleration of the block.

Here I am struggling.
Ok so far.
The string is of constant length. If the car moves 1m, the top part of the string gets how much longer? What does that tell you about the length of the lower part? How far has the block moved?
 
  • #49
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Sorry,actually I am really pedantic.
the top part of the string
Did you mean the part of string indicated?
 

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  • #50
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Sorry,actually I am really pedantic.


Did you mean the part of string indicated?
Yes.
 

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