Why Did I Get the Pulley Tension Forces Wrong?

[Krashy]

Homework Statement

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Homework Equations

F = m*a

The Attempt at a Solution

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Hello, i think i understand d and e so only the first three tasks are important.a) a = (m2 - m1)/(m1 + m2) *g -> (5kg - 2kg)/(2kg+5kg) *10ms^-2 = 4.29 ms^-2

T = m1 (a+g) -> 2kg (4.29ms^-2 + 10ms^-2) = 28 N

So i know the answer is 20N but I am not sure why i got it wrong. Is it because block B stands on the ground and because of that the acceleration is 0?b) The upward force should be the same magnitude as the downward force on block B.

Downward force = weight = 5kg * 10ms^-2 = 50N

Upward force = only normal force? = 50N


My problem here lies in my mistake from a i think. So does block B has tension force on it? I am kinda confused because block B rests on ground, so does it even have tension then? I mean if the acceleration from block A is really 0, then shouldn't the tension be zero as well?c) total downward force of the pulley = weight from block A and B + weight of the pulley

20N + 50N + 15N = 85NThese should be the only forces acting downwards on the system but apparently this is wrong too. It should be 55N but i´m not sure why it should be this value. The tension force should be pointing upwards and the weight downwards, even if i subtract the tension force i won't get 55N.Sorry for so much text. I hope someone can clarify this to me. Thanks for every answer.

 

[Orodruin]

a) a = (m2 - m1)/(m1 + m2) *g -> (5kg - 2kg)/(2kg+5kg) *10ms^-2 = 4.29 ms^-2

Nothing is accelerating. B is supported by the ground below ...

 

[Krashy]

All right thanks, what's what i thought so a should be clear, but then is there tension force on block B?

 

[CWatters]

It's a statics problem so.
For a) Draw a free body diagram for block A. What two forces act on block A? What do they sum to?
For b) Draw a free body diagram for block B.

 

[Krashy]

I drew one for both blocks. For block a i have the weight downwards = 20N and now i know the tension upwards = 20N , so there's no resultant force on A.
For block b i have the weight downwards = 50N and the normal force upwards which should also be = 50N i think

 

[CWatters]

I drew one for both blocks. For block a i have the weight downwards = 20N and now i know the tension upwards = 20N , so there's no resultant force on A.

Correct.

For block b i have the weight downwards = 50N and the normal force upwards which should also be = 50N i think

Yes weight is 50N but there is another force on B in addition to the weight and normal force.

 

[Krashy]

I see, i guess its tension force = 20N. So the normal force should be 50N - 20N = 30N
all right thanks i thought because block B is touching the ground there wouldn't be any tension.

 

[Orodruin]

I see, i guess its tension force = 20N. So the normal force should be 50N - 20N = 30N
all right thanks i thought because block B is touching the ground there wouldn't be any tension.

The tension in the rope cannot change from one point to the other (in the absence of other forces in the direction of the rope).

 

[Krashy]

Ah all right thanks for the information now i know how to calculate a and b, but why is c wrong? I don't really get why the total downward force takes something else into account

 

[Orodruin]

Did you try drawing a FBD for the pulley?

What are the individual contributions based on what we just said?

 

[Krashy]

i think there are 3 forces acting on the pulley, the weight of the pulley acting downwards and the two tension forces acting upwards.
So the only force that's acting downward from the pulley should be just its weight, right?

 

[Orodruin]

How are the tension forces acting upward? What are the third law pairs of the tension forces?

 

[Krashy]

all right i think i understand now, One is the tension acting from the pulley on the masses and the other is the tension acting from the masses on the pulley and because they are a third law pair they are identical in size and act in the opposite direction. So the total downward force is the 15N weight from the pulley + 2 times the 20N tension forces of the pulley on the masses, so the total downward force is 15N + 20N + 20N = 55N
Thanks for the help!