# Centripetal force question

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#1
Context: a metal sphere swings in circular motion about a horizontal bar, in which the sphere is connected to the bar by a rod.

Why is the tension in the rod equal to the centripetal force provided by the rod subtract the weight of the sphere, i don't think there should be tension in the rod in the first place because at the instant the rod is at the top the sphere is perfectly following the circular path so it doesn't need a centripetal force to keep it on its path at that instant.

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2 years ago
#2
(Original post by Freedom physics)
Context: a metal sphere swings in circular motion about a horizontal bar, in which the sphere is connected to the bar by a rod.

Why is the tension in the rod equal to the centripetal force provided by the rod subtract the weight of the sphere, i don't think there should be tension in the rod in the first place because at the instant the rod is at the top the sphere is perfectly following the circular path so it doesn't need a centripetal force to keep it on its path at that instant.

Your reasoning is a flaw. For any object to undergo circular motion, a centripetal force is required.

For the vertical circular motion that you have described, the tension can be zero when the sphere is at the top if the sphere is moving a minimum speed to move in a circular motion.

At the top, both the tension and weight is acting toward the centre to provide for the centripetal force:

If tension is zero at the top, the weight of the sphere provides for the centripetal force:

Note that centripetal force is NOT a real force.
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#3
(Original post by Eimmanuel)
Your reasoning is a flaw. For any object to undergo circular motion, a centripetal force is required.

For the vertical circular motion that you have described, the tension can be zero when the sphere is at the top if the sphere is moving a minimum speed to move in a circular motion.

At the top, both the tension and weight is acting toward the centre to provide for the centripetal force:

If tension is zero at the top, the weight of the sphere provides for the centripetal force:

Note that centripetal force is NOT a real force.
How is the tension acting at the top when it's 0? 🙂
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2 years ago
#4
(Original post by Freedom physics)
How is the tension acting at the top when it's 0? 🙂
The sphere is rotating in a VERTICAL plane. I almost made the same mistake too.
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2 years ago
#5
(Original post by Freedom physics)
How is the tension acting at the top when it's 0? 🙂
I am not sure what are you asking. Reread your question. It is like asking when you are not going to school, how to find you in the school. Does the question make sense to you?
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#6
(Original post by Eimmanuel)
I am not sure what are you asking. Reread your question. It is like asking when you are not going to school, how to find you in the school. Does the question make sense to you?
Oops sorry! I'll make it clearer 🙂: how is the tension in the rod acting on the sphere when the sphere is at the top of the vertical circular path when the tension in the rod at this point = 0 🙂
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2 years ago
#7
(Original post by Freedom physics)
Oops sorry! I'll make it clearer 🙂: how is the tension in the rod acting on the sphere when the sphere is at the top of the vertical circular path when the tension in the rod at this point = 0 🙂
Your question is still the same as post #3.
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#8
(Original post by Eimmanuel)
Your question is still the same as post #3.
You said my question in post three wasn't clear so i made it clearer 🙂
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2 years ago
#9
(Original post by Freedom physics)
You said my question in post three wasn't clear so i made it clearer 🙂
It is no clearer as I said in the previous post.

(Original post by Eimmanuel)
Your question is still the same as post #3.
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#10
I'm honestly not sure why "how is the tension in the rod acting on the sphere when the sphere is at the top of the vertical circular path when the tension in the rod at this point = 0?" isn't clearer than "how is the tension acting at the top when it's 0?" 🙂, please guide me to better clarity 🙂
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2 years ago
#11
(Original post by Freedom physics)
Context: a metal sphere swings in circular motion about a horizontal bar, in which the sphere is connected to the bar by a rod.

Why is the tension in the rod equal to the centripetal force provided by the rod subtract the weight of the sphere, i don't think there should be tension in the rod in the first place because at the instant the rod is at the top the sphere is perfectly following the circular path so it doesn't need a centripetal force to keep it on its path at that instant.

What is making the sphere go round in a circle? If there was no force then the sphere would travel in a straight line - there is an acceleration because the sphere changes direction - therefore there has to be a force.
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#12
(Original post by Muttley79)
What is making the sphere go round in a circle? If there was no force then the sphere would travel in a straight line - there is an acceleration because the sphere changes direction - therefore there has to be a force.
But isn't that force just the weight of the sphere? When the sphere is at the top it would travel at constant (horizontal) velocity if there wasn't a force acting on it, but the only force acting on the sphere is the weight of the sphere since the centripetal acceleration of the sphere is gravity; and because the centripetal force is gravity the tension in the rod is 0 because it isn't needed - if it wasn't 0 it would just be pulling the sphere down which gravity is doing anyway 🙂, i hope i was clear enough, if i haven't been just tell me where i haven't been specific enough 🙂
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2 years ago
#13
(Original post by Freedom physics)
But isn't that force just the weight of the sphere? When the sphere is at the top it would travel at constant (horizontal) velocity if there wasn't a force acting on it, but the only force acting on the sphere is the weight of the sphere since the centripetal acceleration of the sphere is gravity; and because the centripetal force is gravity the tension in the rod is 0 because it isn't needed - if it wasn't 0 it would just be pulling the sphere down which gravity is doing anyway 🙂, i hope i was clear enough, if i haven't been just tell me where i haven't been specific enough 🙂
The system is not in equilibrium .. I think you have some misconceptions you need to discuss with a teacher
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#14
(Original post by Muttley79)
The system is not in equilibrium .. I think you have some misconceptions you need to discuss with a teacher
Ok, thanks 🙂
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2 years ago
#15
(Original post by Freedom physics)
I'm honestly not sure why "how is the tension in the rod acting on the sphere when the sphere is at the top of the vertical circular path when the tension in the rod at this point = 0?" isn't clearer than "how is the tension acting at the top when it's 0?" 🙂, please guide me to better clarity 🙂
Honestly, I don't know what you are trying to ask. How can I guide you?
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2 years ago
#16
(Original post by Freedom physics)
But isn't that force just the weight of the sphere? When the sphere is at the top it would travel at constant (horizontal) velocity if there wasn't a force acting on it, but the only force acting on the sphere is the weight of the sphere since the centripetal acceleration of the sphere is gravity; and because the centripetal force is gravity the tension in the rod is 0 because it isn't needed - if it wasn't 0 it would just be pulling the sphere down which gravity is doing anyway 🙂, i hope i was clear enough, if i haven't been just tell me where i haven't been specific enough 🙂

If the sphere is at the top and the sphere detached from the bar, the sphere will undergo a projectile motion instead of moving at constant velocity.

If the tension is not zero when the sphere is at the top, both tension and weight will act toward the center to cause the sphere to move in a circular motion.
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#17
(Original post by Eimmanuel)

If the sphere is at the top and the sphere detached from the bar, the sphere will undergo a projectile motion instead of moving at constant velocity.

If the tension is not zero when the sphere is at the top, both tension and weight will act toward the center to cause the sphere to move in a circular motion.
Thank you for all the help, i went through it with my teacher and he solved the problem 😄 , it was a problem with the speed of the sphere being above the speed for gravity to equal centripetal acceleration 🙂, and to be completely honest i should've included the whole context of the question because otherwise i don't think you could've given me a satisfying answer anyway 😕; sorry about that...
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