Tricky Conceptual Question Regarding Mass Rotating Horizontally from StringHELP

In summary, the free body diagram for a point mass swinging in a horizontal circle shows a tension force acting diagonally, with a vertical component balancing the force of gravity and a horizontal component providing the required centripetal force. It is physically impossible for the string to be completely horizontal in this scenario.
  • #1
spikey151
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Homework Statement



Okay, so this is more of a conceptual question. Imagine there is a point mass of mass "m" at the end of a string and the other end of the string is secured.

At first, when the point mass and string is just hanging vertically with the mass at the bottom and the string secured at the top, the free body diagram of the mass would just be a force of gravity downward and a tension force upward, counteracting the gravity force right?

Then, consider that the mass begins swinging around in a circle horizontally so that the string is at an angle [tex]\theta[/tex] from the horizontal. Now if you draw a free body diagram from a side-view, the tension force is acting diagonally and the force of gravity is still acting downward. If tension force = T, then:
[tex]\sum[/tex]Fy = T*sin[tex]\theta[/tex] - mg = 0 and
[tex]\sum[/tex]Fx = T*cos[tex]\theta[/tex] = Fcentripetal

So here's my main question, if the mass is swinging around in a circle fast enough (in other words, if the tangential speed is large enough) then the string would become horizontal correct, also meaning angle [tex]\theta[/tex] equals zero? (correct me if I'm wrong) Now when the free body diagram is drawn, there is a horizontal tension force and a vertical gravity force. The horizontal tension force provides the required centripetal force for circular motion of the mass, but now what force is counteracting the force of gravity? The sum of all the vertical forces, which should be zero because the mass does not fall or have any vertical acceleration, would only involve force of gravity.

Homework Equations



Fcentripetal = m*v2/r

The Attempt at a Solution



I do have one theory, which I'm not very sure about: Is it technically or physically impossible for the string to be completely horizontal? It only seems horizontal because the centripetal force (aka the horizontal component of the tension force) becomes so large that the gravitational force (aka the vertical component of the tension force) becomes negligible. But if this is the case, then when drawing the free-body diagram for the situation, is it physically incorrect if, along with the vertically downward force of gravity, a horizontal tension force is drawn acting on the mass?

Any help would be appreciated, I have the AP physics C mechanics test tomorrow -,-.
 
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  • #2


Does nobody have any idea?? T_T
 
  • #3


Yes, it's physically impossible for the string to be horizontal in this scenario. You can draw a horizontal tension component, but the tension force also has a vertical component.
 

1. What causes an object to rotate horizontally when attached to a string?

The rotation of an object when attached to a string is caused by the tension force in the string pulling on the object and creating a centripetal force that keeps the object moving in a circular path.

2. How does the mass of an object affect its rotation when attached to a string?

The mass of an object has no direct impact on its rotation when attached to a string. However, a heavier object may require a stronger tension force in the string to maintain its circular motion.

3. Can an object rotate horizontally from a string without any external force?

No, an object cannot rotate horizontally from a string without any external force. This is due to the law of inertia, which states that an object at rest will remain at rest and an object in motion will continue moving in a straight line unless acted upon by an external force.

4. How does the length of the string affect the rotation of an object?

The length of the string affects the rotation of an object by determining the radius of the circular motion. A longer string will result in a larger radius and therefore a slower rotation, while a shorter string will result in a smaller radius and a faster rotation.

5. What is the relationship between the speed of rotation and the tension force in the string?

The speed of rotation is directly proportional to the tension force in the string. This means that as the tension force increases, the speed of rotation also increases. Similarly, if the tension force decreases, the speed of rotation will decrease as well.

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