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Tension Direction Woes

Okay so I always get confused in understanding which direction tension acts on a string, inwards or outwards, and found out that it acts inwards so I was fine until I started doing a load of connected particle questions (which is never a good thing to be doing)

So as demonstrated in this

Spoiler

Why, in two questions which have pretty much the same set-up, does have q22 have tension acting downwards for both parts, and for q23 it acts inwards?

I'm just thoroughly confused :colondollar:
Reply 1
Avatar for Zacken
Zacken
22
Original post by KaylaB
...


What are the questions for Q22 and 23? It is quite possible that they are asking for different things whilst being the same setting?
Reply 2
Avatar for Zacken
Zacken
22
At a quick glance, I'd say that you should most of the time (if not always) have the tensions point in the direction towards the pulley. So for the vertical one, that means pointing upwards and for the horizontal one, it means point leftwards.

Do bear in mind that if you draw your tensions the wrong way around, you will simply get negative answers that are just as valid (in terms of magnitude, but it will be up to you to then go and re-draw it the correct way around and get a positive answer).

But yeah, if you stick to the convention that I described in my first paragraph, it's always worked for me.
(edited 7 years ago)
Original post by Zacken
At a quick glance, I'd say that you should most of the time (if not always) have the tensions point in the direction towards the pulley. So for the vertical one, that means pointing upwards and for the horizontal one, it means point leftwards.

Do bear in mind that if you draw your tensions the wrong way around, you will simply get negative answers that are just as valid (in terms of magnitude, but it will be up to you to then go and re-draw it the correct way around and get a positive answer).

But yeah, if you stick to the convention that I described in my first paragraph, it's always worked for me.


why tension towards pulley? where's the force acting on what???
Reply 4
Avatar for KaylaB
KaylaB
OP
20
Original post by Zacken
What are the questions for Q22 and 23? It is quite possible that they are asking for different things whilst being the same setting?


Q22

Spoiler



Q23

Spoiler



And after I finished q22 I checked the answers and they were right, so it's not like my method was wrong for it :confused:
If its a string, you can imagine that a tension can only provide a pull rather than a push which should allow yiu to get most of them
Reply 6
Avatar for Zacken
Zacken
22
Original post by KaylaB
And after I finished q22 I checked the answers and they were right, so it's not like my method was wrong for it :confused:


I'd have done both questions with the tension pointing towards the pulley. Like I said, that always works (if the direction of the acceleration vertical block is downwards, it wouldn't work if vertical block was accelerating upwards and the horizontal one leftwards).

So, for example: in Q22, I'd resolve downwards to get 0.8g−T=0.8a  ⟺  T=0.8g−0.8a0.8g - T = 0.8a \iff T =0.8g - 0.8a, it wouldn't work if you let TT act downwards as well. Would it? (Also T would act to the right horizontally).

Like samb said, it's because the string is 'pulling' the mass of block vertically upwards so it acts upwards and but the block is falling and pulling the horizontal particle rightwards, so the tension acts rightwards.
(edited 7 years ago)
Reply 7
Avatar for KaylaB
KaylaB
OP
20
Original post by samb1234
If its a string, you can imagine that a tension can only provide a pull rather than a push which should allow yiu to get most of them


Original post by Zacken
I'd have done both questions with the tension pointing towards the pulley. Like I said, that always works (if the direction of the acceleration vertical block is downwards, it wouldn't work if vertical block was accelerating upwards and the horizontal one leftwards).

So, for example: in Q22, I'd resolve downwards to get 0.8g−T=0.8a  ⟺  T=0.8g−0.8a0.8g - T = 0.8a \iff T =0.8g - 0.8a, it wouldn't work if you let TT act downwards as well. Would it? (Also T would act to the right horizontally).

Like samb said, it's because the string is 'pulling' the mass of block vertically upwards so it acts upwards and but the block is falling and pulling the horizontal particle rightwards, so the tension acts rightwards.


Ah okay thank you both very much :h: It now makes Mechanics a bit less of a guessing game :laugh:
Reply 8
Avatar for Zacken
Zacken
22
Original post by KaylaB
Ah okay thank you both very much :h: It now makes Mechanics a bit less of a guessing game :laugh:


No worries. :biggrin:

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