What Force Balances Weight of Bob When Swinging?

In summary, when we swing a bob tied to a thread, completely perpendicular to the vertical wall (that is, parallel to the floor), the tension in the string balances the weight of the bob.
  • #1
newtonrulez
9
0
Hi!

I was just wondering regarding this.

When we swing a bob tied to a thread, completely perpendicular to the vertical wall (that is, parallel to the floor), what force balances the weight of the bob?

I don't think it can be mv^2/r because that centripetal force is perpendicular to the weight of the bob (thus mv^2 cos90 = 0N).

What force balances the weight of the bob?

Thanks!
 
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  • #2
No force balances it - it is accelerating straight downward due to gravity.
 
  • #3
Thanks for the reply!

But then how does it keep moving in a "straight" circle?

Why doesn't it become a cone shaping circle?

Thanks!
 
  • #4
Help?

I am really feeling very anxious...

Could it be air molecules?
 
  • #5
Newtonrulez said:
Hi!

I was just wondering regarding this.

When we swing a bob tied to a thread, completely perpendicular to the vertical wall (that is, parallel to the floor), what force balances the weight of the bob?

I don't think it can be mv^2/r because that centripetal force is perpendicular to the weight of the bob (thus mv^2 cos90 = 0N).

What force balances the weight of the bob?

Thanks!

Are you describing the bob moving in a horizontal circle?If so the string can never get perfectly horizontal so it is the vertical component of the tension in the string that balances the weight of the bob, the horizontal component providing the centripetal force.Suppose you whirled the bob faster and faster ,if so the bob rises and the string gets closer to the horizontal but never quite reaches because this would require an infinite speed.
 
Last edited:
  • #6
Thanks for the replies!

Dadface,

Thus, ((mv^2)/r)sinx = mg.

Even if it is agreed that perfectly horizontal isn't possible (till the speed is infinity), when we swing the bob, it does seem to be close to the eye. Thus, x is small (4-5 degrees at the most?). Thus, isn't x too small for sinx to balance mg?
 
  • #7
Hello Newtonrulez,try the following:
1.Draw a diagram of the arrangement when the string makes an angle theta to the horizontal.
2.Mark in the two forces on the bob,the tension,T,which acts along the line of the string and the weight,mg,which acts vertically downwards.
3.Resolve vertically from which Tsin theta=mg(it is T sin theta that supports the weight)
4.Horizontally there is an unbalanced force which provides the centripetal force.We can write: Tcos theta=mvsquared/r
5.Divide equation 3. by equation 4.This gives tan theta=gr/v squared.
For the string to become perfectly horizontal tan theta must become equal to zero and from the equation v must become infinite or r become zero, both of these being impossible.If the bob is whirled very fast it may seem as if the string is horizontal but it never quite gets there.It is true that sin theta gets smaller as the speed is increased but T gets bigger.
 
  • #8
Thanks!

Wow, it all seems to fall into place now.
 

FAQ: What Force Balances Weight of Bob When Swinging?

How does the force of gravity affect the weight of a bob when swinging?

The force of gravity is the force that pulls objects towards the center of the Earth. This force is what gives objects their weight. When a bob is swinging, the force of gravity is constantly acting on it, pulling it towards the Earth. This force balances the weight of the bob, keeping it in a stable swinging motion.

What other forces are involved in balancing the weight of a bob when swinging?

Aside from the force of gravity, there are two other forces involved in balancing the weight of a bob when swinging. These are the tension force and the centrifugal force. The tension force is the force exerted by the string or rope that is holding the bob while it swings. The centrifugal force is the force that pushes the bob away from the center of its circular motion. Together, these forces balance the weight of the bob.

How does the length of the string affect the force balance of a bob when swinging?

The length of the string affects the force balance of a bob when swinging in two ways. First, a longer string will allow the bob to swing through a wider arc, which means it will experience a greater range of centrifugal force. Second, a longer string will have more tension, which means it can counteract the greater centrifugal force. Ultimately, the length of the string will determine the balance between these two forces and the stability of the swinging motion.

What happens to the force balance of a bob when swinging at different speeds?

When a bob is swinging at different speeds, the force balance is constantly changing. As the bob swings faster, the centrifugal force increases, requiring more tension from the string to balance it out. At some point, the centrifugal force may become too great for the tension force to counteract, causing the bob to lose its balance and stop swinging. Similarly, if the bob swings too slowly, the centrifugal force may decrease to the point where it is no longer balanced by the tension force, causing the bob to fall towards the center of its circular motion.

How does the force balance of a bob change when swinging on different planets?

The force balance of a bob will change when swinging on different planets due to the different gravitational forces on each planet. On a planet with a stronger gravitational pull, the bob will experience a greater force of gravity, which will require more tension from the string to balance it out. Conversely, on a planet with a weaker gravitational pull, the bob will experience a weaker force of gravity, which will require less tension from the string. This can impact the stability and range of the swinging motion of the bob.

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