Uniform circular motion q

[Dasypus]

I've been looking over the uniform circular motion section in TPRH, and there's one thing that's never addressed (that I can see).

Drawing: http://i.imgur.com/px0lV.png

If (A) you have a weight on a rope, and you whirl it around in a circle, so that the weight and the circle it forms are both below your hand (the endpoint of the rope), the tension on the rope 'balances' gravity, allowing the weight to stay in its flat trajectory. But what about (B), where you whirl the weight above your head (like a lasso before you throw it)? What's opposing gravity there?

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[MedPR]

I've been looking over the uniform circular motion section in TPRH, and there's one thing that's never addressed (that I can see).

Drawing: http://i.imgur.com/px0lV.png

If (A) you have a weight on a rope, and you whirl it around in a circle, so that the weight and the circle it forms are both below your hand (the endpoint of the rope), the tension on the rope 'balances' gravity, allowing the weight to stay in its flat trajectory. But what about (B), where you whirl the weight above your head (like a lasso before you throw it)? What's opposing gravity there?

Draw free body diagram. When the mass is above your head, gravity is pushing it down, tension in the rope is preventing the mass from hitting you in the face.

When the mass is below your head (closer to the ground than the sky) gravity is still pushing it down, but tension is preventing it from hitting the floor.

In both cases tension in the rope is opposing gravity and gravity + tension = mv^2/r

 

[Dasypus]

Draw free body diagram. When the mass is above your head, gravity is pushing it down, tension in the rope is preventing the mass from hitting you in the face.

When the mass is below your head (closer to the ground than the sky) gravity is still pushing it down, but tension is preventing it from hitting the floor.

In both cases tension in the rope is opposing gravity and gravity + tension = mv^2/r

But that doesn't seem right. The rope can't push, only pull. So it can't be what's preventing the thing from dropping down on me.

 

[MedPR]

But that doesn't seem right. The rope can't push, only pull. So it can't be what's preventing the thing from dropping down on me.

Yea that makes sense.. I've always had an issue with circular motion. I'm glad you made the thread, I can learn from it too

milski will probably have an answer here shortly.

 

[chiddler]

i'm not sure if B is possible, is it?

i'm trying to think outside cartoons. is it possible to keep a rope in such conditions?

i remember my physics prof saying that if you hold a weight at the end of a rope and spin it, the only time it'll reach a perfect 90 degrees verticle angle is if speed reaches infinity. here, you're going higher than 90 degrees!

 

[rjosh33]

nm

 

[milski]

B is not possible. If there's a weight at the end of the rope, you always have situation A - the weight is compensated by the vertical component of the tension.

 

[Dasypus]

i'm not sure if B is possible, is it?

i'm trying to think outside cartoons. is it possible to keep a rope in such conditions?

i remember my physics prof saying that if you hold a weight at the end of a rope and spin it, the only time it'll reach a perfect 90 degrees verticle angle is if speed reaches infinity. here, you're going higher than 90 degrees!

Stupid cartoons! Thanks for the help everyone. I think I was seriously just thinking about this from the perspective of someone who absorbed too much Looney Toons.