TPR Fluid Statics Question

[sanguinee]

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Is this explanation correct? In my head, gravity and acceleration are going to be in opposing directions, so it would be m(g-a) making the answer 800 instead of 1200. Thanks!

 

[aldol16]

Why do you think it would be T = m(g-a)? That would equate to mg - T = ma. In other words, in that coordinate system, up would be negative and down would be positive. Therefore, g = 9.8 and a = -2. You would get 1000 - T = -200 and therefore T = 1200.

 

[sanguinee]

Why do you think it would be T = m(g-a)? That would equate to mg - T = ma. In other words, in that coordinate system, up would be negative and down would be positive. Therefore, g = 9.8 and a = -2. You would get 1000 - T = -200 and therefore T = 1200.

I agree that the net force (ma) = T - mg. However, I think g and a are in different directions. With down being positive and up being negative, g = +10, while a = -2.
When rearranging ma= T-mg, you get T= m(a+g) = 100(10 + (-2)), which would be 800. Does this make sense?

 

[aldol16]

I agree that the net force (ma) = T - mg. However, I think g and a are in different directions. With down being positive and up being negative, g = +10, while a = -2.
When rearranging ma= T-mg, you get T= m(a+g) = 100(10 + (-2)), which would be 800. Does this make sense?

First, just define your coordinate axis. You're saying that down is positive and up is negative. Fine. But you can't use T - mg. Because by T - mg, you're already making the down direction negative - that's why it's a minus instead of a plus. You're just getting confused with sign conventions. Do something simpler. Forget about signs. Net force is T + mg and is also equal to ma. Thus, T + mg = ma. Now, if you define g as negative, then T + m(-g) = m(+2). Alternatively, if you define g as positive, T + mg = m(-2).

You're confused because you're applying directionality twice. Once when you say T - mg and once again when you assign g to be either positive or negative.

 

[sanguinee]

First, just define your coordinate axis. You're saying that down is positive and up is negative. Fine. But you can't use T - mg. Because by T - mg, you're already making the down direction negative - that's why it's a minus instead of a plus. You're just getting confused with sign conventions. Do something simpler. Forget about signs. Net force is T + mg and is also equal to ma. Thus, T + mg = ma. Now, if you define g as negative, then T + m(-g) = m(+2). Alternatively, if you define g as positive, T + mg = m(-2).

You're confused because you're applying directionality twice. Once when you say T - mg and once again when you assign g to be either positive or negative.

That clears it up. Thank you!!!