Conical Pendulum: Solving the Height Issue

In summary, the conversation discusses the relationship between height and length in a conical pendulum and how it compares to a planar pendulum. It is suspected that the given values for the problem may not allow for a real solution to exist, and the period of the conical pendulum is found to be less than or equal to that of the planar pendulum. The criteria for circular motion is also mentioned, with the understanding that the conical pendulum cannot have a smaller angular frequency than that of a pendulum swing.
  • #1
Spencer25
13
4
Homework Statement
A 3m long string is attached to a 5kg mass and rotates at 5 rpm. Calculate the height of the pendulum.
Relevant Equations
H=g/w2 (Height = gravity divided by angular velocity squared)
= 9.81/ .27415 = 35.78m
So in this instance height is longer than the 3m string which is impossible. Do we just say that when ever H>L, H=L?
 
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  • #2
I suspect that the given values for this problem are incorrectly specified for a real solution to exist. Consider that at 5 rpm the bob would have a period per revolution of 12 seconds. The same setup but swinging like a normal (planar) pendulum would have a period of
$$2 \pi \sqrt{\frac{3 m}{g}} \approx 3.5 \text{ seconds}$$
Pretty sure the period of the conical pendulum should be less than or equal that of the planar version (haven't proved this yet, so it's just intuition at the moment).
 
  • #3
I know the formula is correct as is the math...just must be missing some sort of rule?
 

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  • #4
gneill said:
I suspect that the given values for this problem are incorrectly specified for a real solution to exist. Consider that at 5 rpm the bob would have a period per revolution of 12 seconds. The same setup but swinging like a normal (planar) pendulum would have a period of
$$2 \pi \sqrt{\frac{3 m}{g}} \approx 3.5 \text{ seconds}$$
Pretty sure the period of the conical pendulum should be less than or equal that of the planar version (haven't proved this yet, so it's just intuition at the moment).
Yes, the period of the conical pendulum is ##T=2\pi \sqrt{\frac{h}{g}}## where ##h## is the "height". Since ##h < L##, it is less than that of the planar pendulum unless, of course, the bob isn't moving.
 
  • #5
Makes sense, thank you so much!
 
  • #6
I find that the criteria is

$$L \omega^2 \ge g$$
else r becomes either zero or imaginary.
 
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  • #7
gneill said:
I find that the criteria is

$$L \omega^2 \ge g$$
else r becomes either zero or imaginary.
Yes. Circular motion can be decomposed as a superposition of linear harmonic motions at right angles with a 90-degree phase shift. In the limit of small angles for the conical pendulum, the harmonic motions are just pendulum swings of small amplitude which we know have angular frequency ##\sqrt{g/L}##. You cannot make the conical pendulum have a smaller angular frequency than this.
 
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  • #8
That is great...thanks again.
 
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FAQ: Conical Pendulum: Solving the Height Issue

1. What is a conical pendulum?

A conical pendulum is a type of pendulum that moves in a circular motion rather than a back and forth motion like a traditional pendulum. It consists of a weight attached to a string or rod that is suspended from a fixed point, allowing it to swing in a horizontal circle.

2. How is the height of a conical pendulum determined?

The height of a conical pendulum is determined by the length of the string or rod, as well as the angle at which it is suspended. The weight of the pendulum also plays a role in determining its height.

3. What is the issue with the height of a conical pendulum?

The issue with the height of a conical pendulum is that it is not constant. As the pendulum swings, the height at which it hangs changes, making it difficult to measure accurately.

4. How can the height issue of a conical pendulum be solved?

The height issue of a conical pendulum can be solved by using a laser sensor to measure the height at various points of the pendulum's swing. This data can then be used to calculate the average height of the pendulum. Alternatively, a high-speed camera can be used to capture the pendulum's motion and measure the height accurately.

5. What are the practical applications of a conical pendulum?

A conical pendulum has many practical applications, including in physics experiments and demonstrations. It is also used in devices such as gyroscopes and accelerometers, which are used in navigation and motion sensing technology. Additionally, it can be used to measure the acceleration due to gravity, or to test the effects of air resistance on a pendulum's motion.

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