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# Calculating tension in a wire/string - AS Physics (Mechanics) watch

1. How do you do tension calculations in AS physics? Seriously stuck on exam questions to do with it, particularly this one right now but also in general:

2. Okay, first just think about the mass in the left hand side.

We know it is in equilibrium, therefore all forces have to be balanced.

Now let's think what those forces are. Obviously the mass experiences the downwards pull of it's weight (W = mg), and it will also experience and upwards pull from the wire.

If we define up to be positive, and remember that all forces are balanced, we get the equation:

T - W = 0
T = W
T = mg
m = 2.0, g = 9.81
Therefore T = 2.0 * 9.81
3. If you wouldn't mind replying to this thread and quoting me so that I get a notification, I'll write a short paragraph on how to tackle the questions generally when I get back to my room later
4. (Original post by Darth_Narwhale)
Okay, first just think about the mass in the left hand side.

We know it is in equilibrium, therefore all forces have to be balanced.

Now let's think what those forces are. Obviously the mass experiences the downwards pull of it's weight (W = mg), and it will also experience and upwards pull from the wire.

If we define up to be positive, and remember that all forces are balanced, we get the equation:

T - W = 0
T = W
T = mg
m = 2.0, g = 9.81
Therefore T = 2.0 * 9.81
Ah okay yeah that was easy, should have got that on my own tbh. Guess I was just confused by the 1.6m and 30 degree angle?

Next part of the question asks: "Show that the mass of the bar is approximately 3.5kg." How would I go about answering this one?

Thanks for your help by the way
5. Have you done anything on taking moments before? If so, take the moment around B and that'll give you an equation you can use to find out mass
6. (Original post by Darth_Narwhale)
Have you done anything on taking moments before? If so, take the moment around B and that'll give you an equation you can use to find out mass
We have but I didn't really understand them. Would this be right?:

1.6 x 9.81 x m = 19.62 x cos(30)

m = (19.62 cos(30)) / (1.6 x 9.81)
7. Never mind it's definitely not
8. (Original post by NiamhM1801)
Never mind it's definitely not
Haha, remember moment = force * perpendicular distance from force to pivot
9. (Original post by Darth_Narwhale)
Haha, remember moment = force * perpendicular distance from force to pivot
19.62 x cos(30) x 1.6 = m x 9.81 x 1.6 ?
10. (Original post by NiamhM1801)
19.62 x cos(30) x 1.6 = m x 9.81 x 1.6 ?
Almost. Weight acts through the centre of mass of an object, which here is gonna be... (I'm letting you figure thiss out for yourself obvs)
11. (Original post by Darth_Narwhale)
Almost. Weight acts through the centre of mass of an object, which here is gonna be... (I'm letting you figure thiss out for yourself obvs)
0.8

so it'll be:

19.62 x cos(30) x 1.6 = m x 9.81 x 0.8

m = (19.62 x cos(30) x 1.6) / (9.81 x 0.8)

which gives me 3.46
12. (Original post by NiamhM1801)
0.8

so it'll be:

19.62 x cos(30) x 1.6 = m x 9.81 x 0.8

m = (19.62 x cos(30) x 1.6) / (9.81 x 0.8)

which gives me 3.46
exactly
13. (Original post by Darth_Narwhale)
exactly
Thanks for the help

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