Circular Motion: Ball on String

In summary: Sorry for the late reply. Hate to say it but that was all the information that was given in the problem.If you have ever swung a mass at the end of a string, you would know that the faster you swing the mass around, the harder the string pulls on your hand. Tension depends on speed. If the speed is not known, the tension cannot be known. You might wish to ask for clarifications from whoever asked you to do this problem.
  • #1
physicaled
10
0

Homework Statement


A 1.5 kg ball is tied to a string that is .75 m long and whirled in a verticle circle.
A) Find the tension in the string at the top of the circle
B) Find the tension in the string at the bottom of the circle

Homework Equations


Fc=mac
ac= v2/r

The Attempt at a Solution


A)
Fnet= Ft + Fg
Fnet = Ft + (1.5)(10)

B)
Fnet = Ft - Fg
Fnet = Ft - (1.5)(10)

As you can see, I did not get very far in my calculations as I am very confused where to go from here. Both equations would require velocity which is not given and I don't know how to find it.
 
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  • #2
Do you know how fast the ball is moving?
 
  • #3
kuruman said:
Do you know how fast the ball is moving?
... or any other piece of information?
 
  • #4
Perhaps the spin rate is 'just enough' to keep it going. That would happen when, at the top, it is just fast enough to overcome gravity acceleration. So what is Ft + Fg at the top of the circle?
 
  • #5
scottdave said:
So what is Ft + Fg at the top of the circle?
Fg is mg and you know what that is. You are asked to find the tension and that depends on the speed. If the speed is just enough for the ball to go around the circle, what do you think the tension ought to be? I think you should revisit the problem and see if you forgot to post some needed information.
 
  • #6
kuruman said:
Fg is mg and you know what that is. You are asked to find the tension and that depends on the speed. If the speed is just enough for the ball to go around the circle, what do you think the tension ought to be? I think you should revisit the problem and see if you forgot to post some needed information.
Scottdave is not the originator of the thread.
 
  • #7
haruspex said:
Scottdave is not the originator of the thread.
Oops, I missed that. Thanks and sorry Scottdave for directing my reply to you instead of the OP.
 
  • #8
kuruman said:
Oops, I missed that. Thanks and sorry Scottdave for directing my reply to you instead of the OP.
It's an easy mistake when posters don't use the reply or quote buttons, so it is not obvious whom they are addressing (hint @scottdave)
 
  • #9
kuruman said:
Oops, I missed that. Thanks and sorry Scottdave for directing my reply to you instead of the OP.
Sorry for the late reply. Hate to say it but that was all the information that was given in the problem.
 
  • #10
If you have ever swung a mass at the end of a string, you would know that the faster you swing the mass around, the harder the string pulls on your hand. Tension depends on speed. If the speed is not known, the tension cannot be known. You might wish to ask for clarifications from whoever asked you to do this problem.
 

FAQ: Circular Motion: Ball on String

How does the radius of the circle affect the speed of the ball?

The radius of the circle does not directly affect the speed of the ball. However, a smaller radius will result in a tighter and faster circle, while a larger radius will result in a wider and slower circle.

What is the relationship between the mass of the ball and its centripetal force?

The mass of the ball has a direct relationship with the centripetal force. As the mass of the ball increases, the centripetal force required to keep it in circular motion also increases.

How does the angle of the string affect the ball's motion?

The angle of the string, also known as the angle of elevation, affects the direction of the ball's motion. A higher angle of elevation will result in a steeper and faster circular motion, while a lower angle will result in a wider and slower circular motion.

What is the difference between centripetal force and centrifugal force?

Centripetal force is the force that keeps an object moving in a circular path, directed towards the center of the circle. On the other hand, centrifugal force is a fictitious force that appears to push an object away from the center of a circle due to its inertia. It is not a real force, but rather a result of an object's tendency to continue moving in a straight line.

How does the speed of the ball change as it moves in a circular path?

The speed of the ball remains constant throughout its motion in a circular path. This is because the centripetal force is constantly changing its direction to keep the ball moving in a circular path, but the magnitude of the force remains the same, resulting in a constant speed.

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