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1.Does tension force act on string or on the roof when a object is hanged from roof with a string((what is agent causing tension and object on which force acting))?

[I have been told object and agent of force cant be same]

  1. Since tension is contact force does it originate normal reaction also on roof, what is its direction?

  2. Why do we say tension is same throughout string?

  3. What is clue behind using a massless and inextensible string?

  4. If string is massive and extensible how will it impact tension within string?

IN GENERAL I AM ASKING ABOUT FULL WORKING OF TENSION FORCE,,BUT PLEASE MENTION ANSWER TO ALL THESE QUESTIONS..

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When a string carries some load,

  • the bottom-most particle of the string carries the full weight $w$ of the load. This "carrying force" is what we call tension $T$. $$T_1=w. $$
  • The next particle carries the bottom-most particle and the load, $$T_2=w_1+w. $$
  • The next particle carries the two bottom-most particles and the load, $$T_3=w_2+w_1+w. $$
  • Etc.

This propagates through the string all the way to the ceiling so the ceiling carries every single string particle - it carries the whole string - and the load, $$T_\text{top} = \underbrace{w_1+w_2+w_3+\cdots}_{w_\text{string}}+w.$$

Question 1) The "agent" and "object" is each of the neighbouring pairs of string particles.

Question 2) The force from the ceiling of course has to pull upwards since it is carrying the string and the load. I wouldn't call this a normal force, since a normal force typically is understood as a pushing force away from the surface.

Question 3) and 4) In general, as shown above, the tension is not the same throughout the string because the further up you come the more particles have to be carried apart from the load. Often, though, the load might be so much larger that the string's own weight can be considered negligible, $$\require{cancel} T_\text{top} =\cancel{w_\text{string}} +w.$$ This corresponds to the string being massless and then the tension at any point will only have to carry the load - so it is the same throughout.

Question 5) A massless string is an often used idealisation which simplifies the scenario a lot. If we can't make that assumption and must include the mass of the string, then we need to know the mass distribution up along the string. If we can assume it linear (constant) then the tension will gradually increase up along the string as shown above.

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