Why does rotating a ball on a string faster makes it horizontal

In summary, the conversation discussed the forces at play when rotating a ball on a string. The only forces involved are centripetal, gravity, and the tension in the string. As the ball's speed increases, the horizontal component of the tension also increases to balance the centripetal force. The ball cannot reach a completely horizontal position due to the need for an infinite tension force. The concept of centrifugal force was also brought up, but it was explained that it is only an apparent force and is not present in the observer's reference frame.
  • #1
musik132
11
0
Hi suppose i had a ball on string and started to rotate it in a circle around my hand. When i increase the speed of the ball it becomes more and more horizontal.
The only forces i can think of at play is centripetal and centrifugal forces and the force exerted by my hand and gravity. Is somehow the centripetal force's verticle component greater than the centrifugal's or is it something else?
 
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  • #2
Actually - as you are looking at it, there is no centrifugal force.
There is the tension in the string which has radial and vertical components, and gravity, which is only vertical.

The vertical component of the tension has to be equal to gravity.
The horizontal component has to be the centripetal force.
The centripetal force is related to the angular velocity of the ball ... so what is happening is that to go faster the ball need more centripetal force - but only the same lift against gravity to hold it up. Since both these forces come from the same place (the tension in the string) then the string gets more horizontal.

It can never get completely horizontal though.--------------------
From the POV of an ant-physicist on the ball, there is a gravity force pointing down and a centrifugal force pointing horizontally to it. There is also a tension force in the string which exactly balances the other two. The the string were at 45deg then the centrifugal force would be equal to the gravity force ... the faster the ball goes, the bigger the centrifugal force and so the bigger the horizontal component of the tension has to be to balance it... so the angle has to be less than 45deg.
 
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  • #3
so just to check my thinking:
Ft = tension Fg = gravity Fc = centripetal
Ftcosθ = Fg and Fc=mrω^2=Ftsinθ
If ω increases the angle must go up but if the angle goes up doesn't Ftcosθ become less which would mean an increase in tension in order to counteract gravity? So an increase in ω changes both Ft and sinθ

and why isn't there a centrifugal force?
there is a "apparent outward force that draws a rotating body away from the center of rotation. It is caused by the inertia of the body as the body's path is continually redirected" wikipedia
 
  • #4
musik132 said:
If ω increases the angle must go up but if the angle goes up doesn't Ftcosθ become less which would mean an increase in tension in order to counteract gravity?

Indeed. To reach 90 degrees ω and the tension would need to be infinite.
 
  • #5
musik132 said:
and why isn't there a centrifugal force?
there is a "apparent outward force that draws a rotating body away from the center of rotation. It is caused by the inertia of the body as the body's path is continually redirected" wikipedia
That's exactly it - it is only an apparent force. The reference you used also explains.

It did not belong in your initial description because "you" doing the observing are not rotating with the ball. No reason for you to have an inertial correction. You do feel the ball pull back on the string but recall - you are providing the applied force to the center. This is an unbalanced force resulting in acceleration towards the center which you see as circular motion.

The ant in my description has a different POV. He sees no centripetal forces, but it does make sense to talk about his experience in terms of the centrifugal pseudoforce.

In Newtonian physics, the appearance of a pseudoforce in your physics is a clue that you are in an accelerating reference frame.
 

Related to Why does rotating a ball on a string faster makes it horizontal

1) Why does rotating a ball on a string faster make it horizontal?

Rotating a ball on a string faster causes it to become horizontal due to the effects of centripetal force. As the ball spins faster, the centripetal force acting on it increases, pulling it towards the center of the circle. This force counteracts the downward force of gravity, causing the ball to rise and eventually become horizontal.

2) What is centripetal force and how does it affect a rotating ball?

Centripetal force is the force that acts on an object moving in a circular path, pulling it towards the center of the circle. When a ball is rotating, the centripetal force increases as the speed of rotation increases, causing the ball to rise and eventually become horizontal.

3) Can a rotating ball on a string become completely horizontal?

Yes, a rotating ball on a string can become completely horizontal if the speed of rotation is fast enough to counteract the downward force of gravity. However, this requires a high amount of centripetal force and is not possible with just a simple string and ball setup.

4) What happens if the speed of rotation is not fast enough to make the ball horizontal?

If the speed of rotation is not fast enough, the centripetal force will not be strong enough to counteract the downward force of gravity. As a result, the ball will not rise and will remain at an angle or even fall towards the ground.

5) Can other factors affect the horizontal position of a rotating ball on a string?

Yes, other factors such as the length and weight of the string, the mass and size of the ball, and external forces like air resistance can all affect the horizontal position of a rotating ball on a string. These factors can alter the amount of centripetal force acting on the ball and may prevent it from becoming completely horizontal.

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