Will a free falling rod rotate or not?

In summary, the conversation discusses the concept of rotational centers and why the center of gravity is typically chosen as the rotational center for simplicity. It is also mentioned that different points can be chosen as the rotational center and the parallel axis theorem may need to be used. The conversation concludes by mentioning that in the case of bicycles, the net moment can be considered by choosing the point where friction and normal reaction act on.
  • #1
peterpang1994
37
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Will a free falling rod rotate ?

When we consider a bicycle is turning on a flat plane, we know that there is friction, which provide centripetal force on the bicycle. And we know that the bicycle is no longer perpendicular to the flat plane so as to reach equilibrium. What I want to ask why we always choose the center of gravity of the bicycle be the rotational center. Just like when a rod is free falling and parallel to the ground, if I choose the center of gravity as the rotational center the rod will not rotate, because there is no net moment acting on the rod. But if I choose the points other than the center of gravity as the rotational center, there would be net moment due to gravity and the rod will rotate. Will the rod rotate or not? Why we always choose center of gravity as the rotational center?
 
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  • #2


peterpang1994 said:
And we know that the bicycle is no longer perpendicular to the flat plane so as to reach equilibrium.

So no one rides a bike upright?
 
  • #3


peterpang1994 said:
But if I choose the points other than the center of gravity as the rotational center, there would be net moment due to gravity and the rod will rotate. Will the rod rotate or not? Why we always choose center of gravity as the rotational center?

We choose the center of gravity for simplicity. If you use another point in the body for the center of rotation, you have to use the parallel axis theorem.
 
  • #4
But when I choose the other point as rotational centre and chage the moment of inertia by parallel axis therom, there are probabilities for the rod to rotate or not.
 
  • #5


peterpang1994 said:
if I choose the center of gravity as the rotational center the rod will not rotate, because there is no net moment acting on the rod.
Ok
But if I choose the points other than the center of gravity as the rotational center, there would be net moment due to gravity and the rod will rotate.
There would NOT be net moment.
Imagine a massless rod AB. On the extreme B of the rod let's put a weight.
Let's put the rod horizontal and let it fall.
As said before, if I choose B as center of rotation, the rod will not rotate.
If I choose A, the forces acting of B will be [itex]mg[/itex] pointing downwards, and [itex]ma[/itex] pointing upwards. Of course [itex]a=g[/itex]
That's because the body is accelerating, and with accelerating bodies you should always consider the fictious force [itex]ma[/itex].
Again, no net torque.
Will the rod rotate or not?
Of course not. The result must be the same regardless the method you use to study it.
Why we always choose center of gravity as the rotational center?

As you see, you can choose any point you like.
 
  • #6
Thank you very much .In the case of bicycles, how should I consider the net moment acting on the bicycle if I choose the point where the friction and the normal reaction acting on.
 

FAQ: Will a free falling rod rotate or not?

1. Will a free falling rod rotate or not?

The answer to this question depends on a few factors. If the rod is falling in a vacuum, it will not rotate due to the absence of air resistance. However, if the rod is falling in a fluid, it may rotate due to the forces of drag acting on different parts of the rod.

2. What is the relationship between the length of the rod and its rotation during free fall?

The length of the rod does not have a direct effect on its rotation during free fall. However, a longer rod may experience more drag and therefore rotate more than a shorter rod in the same conditions.

3. How does the shape of the rod affect its rotation during free fall?

The shape of the rod can have a significant impact on its rotation during free fall. A rod with a symmetrical shape, such as a cylinder, will experience less drag and therefore rotate less than a rod with an asymmetrical shape, such as a flat sheet.

4. Can the speed of the free fall affect the rotation of the rod?

Yes, the speed of the free fall can affect the rotation of the rod. A faster fall will result in more drag forces acting on the rod, causing it to rotate more.

5. Is there a way to predict the rotation of a free falling rod?

Predicting the rotation of a free falling rod can be difficult due to the many factors at play, such as the shape and speed of the rod, as well as the surrounding environment. However, mathematical models and simulations can help estimate the rotation of a rod during free fall.

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