Pulley Problem

[The Astroman]

I have this problem I need to solve.

 

I am trying the find the tension T in the cable.

 

 

The weight M is 245.25 N and the force P is 147.15 N.

 

If you don't understand, it's a rope attached to the roof, passes down into a pulley and then back up it, with someone pulling with a force of 147.15 N. The weight attached to the pulley is 245.25N.

 

So, what's the tension and how do we find it?

[swansont]

First you need to identify all the forces and write down an equation (or equations) that you can solve.

[mysolipsis]

think of it like this, and i'm not giving you the answer, rather, the way to look at it. tension is a force. and the sum of the forces should equal zero. so you have a force due to gravity (weight) moving in the downward direction and a force P moving in the upward direction. now it should be easy for you to figure it out.

[swansont]

the sum of the forces should equal zero.

 

I don't think you can assume that. It's not given in the problem.

[mysolipsis]

equal and opposite forces, there is aprently no movement in the problem, therefore the sum of all the forces should be zero (vector sum). or am i incorrect with this statement?

[mysolipsis]

to clarify: there is a tension being exherted down from the ceiling and equally there is a tension exerted up from the pully, and a weight which is pulling down from the mass, and a force pulling up from the force P as well as a tension in the rope moving up and moving down. now the tension up and down should be equal on both sides, assuming there is no slack in the rope, or kink in the pully. now. Tension up = Tension down = Weight - P, however, there is a mechanical advantage of the pully therefore 2(Tension up) = 2(Tension down) = Weight - P. therefore T = (Wg - P)/2 . yes or know?

[spunnery]

hai astroman

 

First & obvious thing ,in this problem is it is not in equillibrium.

b'cause a pull of 147 N is not enough to carry a load of 245 N .So if we want to make such a system into existence,either u have to increrase the pull or change the direction of pull ,so that it will have a vertical component to resist the weight.

 

Anyhow ,the tension on string at the instance will be 147/2 N

[swansont]

hai astroman

 

First & obvious thing ,in this problem is it is not in equillibrium.

b'cause a pull of 147 N is not enough to carry a load of 245 N .So if we want to make such a system into existence,either u have to increrase the pull or change the direction of pull ,so that it will have a vertical component to resist the weight.

 

Anyhow ,the tension on string at the instance will be 147/2 N

 

The force P is not the only vertical force. But I agree that the system is not in equilibrium.

[swansont]

equal and opposite forces, there is aprently no movement in the problem, therefore the sum of all the forces should be zero (vector sum). or am i incorrect with this statement?

 

If you assume this, I think you end up with a physically impossible situation, i.e. there will be some point where you can't make the forces cancel, assuming a nonelastic rope.

[YT2095]

from a practical standpoint, the weight on P is half the Total weight of the mass (and the pully), but were you to pull at P and take up say 1 metre of rope, the mass would only raise by Half a metre.

 

it`s the same principal as a Block-and-tackle for lifting car engines out

[m4rc]

The question is not that complicated.

 

The tension is how much force that the rope is applying to the system. Therefore, in this case, the tension is equal to P.

[richard]

I aren't a math person but, I think it would go like this. The total force at rest is shared between the two sides of the pully equaly. That gives 122.625 N. So there is more force on P than is required to hold M. The force on P is147.15 N. There is a difference of 24.525 N which would be used by countering inertia but shared between the two sides of the pully. So the tension on P would balance to 122.625+(24.525/2)=134.8878 N. math

[swansont]

The question is not that complicated.

 

The tension is how much force that the rope is applying to the system. Therefore, in this case, the tension is equal to P.

 

Astroman has not posted again, and this looks like a homework problem, so I am loath to work it for him.

 

But there are two tensions to consider, and if the mass is accelerating, they will be unequal. But I don't think there's enough information given to actually solve the problem.

[Heretic]

A while back I was given a problem by a friend of mine. He told me that is was a supposedly a high school math problem (turned out it was). The solution was incredibly simple if I had the right equation. However since I wasn't taking high school math and didn't ever need to learn that obscure equation the problem seemed impossible to me.

 

That's more then likely what is going on here. You'll need to find that obscure equation that is only useful for this exact problem.

[swansont]

A while back I was given a problem by a friend of mine. He told me that is was a supposedly a high school math problem (turned out it was). The solution was incredibly simple if I had the right equation. However since I wasn't taking high school math and didn't ever need to learn that obscure equation the problem seemed impossible to me.

 

That's more then likely what is going on here. You'll need to find that obscure equation that is only useful for this exact problem.

 

No, you just need to apply F = ma properly.