Why Does Tension Act in Two Directions at a Point in a String?

[Chestermiller]

You mean when I apply Newton's law on the blocks and strings and get the tension, I would get a specific direction for tension? But as I understood it, tension is always acting in 2 directions. So how can I get a single direction? Lastly, why doesn't the string have tension pointing in 2 directions at the point it is pulling the pulley down? (Even though it is unclear as to why it pulls it down..that is another question altogether at the moment).

See posts 31-37.

Chet

 

[andyrk]

See posts 31-37.

Chet

Yes, but I still don't understand as to why does the string pulls the pulley down and not up? I don't think compression as an explanation which I need to go through because it is way beyond what my curriculum demands of me. Could you explain it more high school-ish terms which are easy to comprehend and fall within the realm of logical capacity for high school students?

And the reason you provided for why a point is pulley equally in 2 directions is not rigorous enough. Its too qualitative. Could you provide a more quantitative explanation? Also, why does a massless string have constant tension at all points? Can you explain this quantitatively rather than qualitatively?

 

[Chestermiller]

Yes, but I still don't understand as to why does the string pulls the pulley down and not up? I don't think compression as an explanation which I need to go through because it is way beyond what my curriculum demands of me. Could you explain it more high school-ish terms which are easy to comprehend and fall within the realm of logical capacity for high school students?

It doesn't matter whether you assume that the string is pulling down on the pulley or pushing up. When you set up Newton's 2nd law equations and solve the problem, the sign of the tension will take care of itself.

And the reason you provided for why a point is pulley equally in 2 directions is not rigorous enough. Its too qualitative. Could you provide a more quantitative explanation?

If body A exerts a contact force F on body B in the positive x direction, what force do you think that body B is exerting on body A (at their contact point)?

Also, why does a massless string have constant tension at all points? Can you explain this quantitatively rather than qualitatively?

See post #43. Just apply the same approach over a shorter segment of string.