Newton's second law for rotations

  • #1
pedrovisk
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What is the minimum force to start rotation in a bar?
EDIT: I forgot about Second Newton's law for rotations and this led to a mistake. Anyway, thanks for the people who answered it and remembered me about law of inertia.

I was thinking about how to "make" things to move without rotate the object, then i tried to calculate the minimum force to start a homogeneous bar to rotate in a x axis. I attached a link.
Could someone check if my answer is correct?

 
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  • #2
:welcome:

What force is resisting rotation?
 
  • #3
PeroK said:
:welcome:

What force is resisting rotation?
Hi! The bar is at static equilibrium. What I meant with this force F is what "push" I need to do in order to "win" the moment of inertia.
 
  • #4
pedrovisk said:
Hi! The bar is at static equilibrium. What I meant with this force F is what "push" I need to do in order to "win" the moment of inertia.
I don't know what "win" means in this context. In the absence of a resisting force, any force (no matter how small) will move and rotate the bar.
 
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  • #5
It is like asking what force is required to make an object accelerate when there is no resisting force. F = ma tells you that any net force will result in an acceleration, just as ##T = I\alpha## tells you any net torque will result in an angular acceleration.
 
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  • #6
Orodruin said:
It is like asking what force is required to make an object accelerate when there is no resisting force. F = ma tells you that any net force will result in an acceleration, just as ##T = I\alpha## tells you any net torque will result in an angular acceleration.
Can't believe i forgot about this. Basic Newton's first law for rotations.

I think I was confused when I was trying to rotate a bar like that. I probably just forgot about the friction.

Anyway, now I'm curious. Is there any meaning for the pic I attached?
 
  • #7
PeroK said:
I don't know what "win" means in this context. In the absence of a resisting force, any force (no matter how small) will move and rotate the bar.
Thanks, you are absolutely right. I was having trouble to understand rotations as a extension of Newton's laws.

Asking the same question I did to Orodruin, what is a possible interpretation for the pic? Did it have a meaning or I should just torn it apart the paper and throw it to the trash? The development of the equations looks so smooth.
 
  • #8
pedrovisk said:
Thanks, you are absolutely right. I was having trouble to understand rotations as a extension of Newton's laws.

Asking the same question I did to Orodruin, what is a possible interpretation for the pic? Did it have a meaning or I should just torn it apart the paper and throw it to the trash? The development of the equations looks so smooth.
You have a torque of ##F\frac l 2##. Somehow you have an opposing torque of ##\frac{mlg}{4}##.
 
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  • #9
PS gravity acts through the centre of the bar, so it should not produce a torque on the bar about its centre.
 
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  • #10
pedrovisk said:
I was thinking about how to "make" things to move without rotate the object,
Apply all force through the center of mass, otherwise make sure that the torques cancel.
 

FAQ: Newton's second law for rotations

What is Newton's second law for rotations?

Newton's second law for rotations states that the net torque acting on a rotating object is equal to the moment of inertia of the object times its angular acceleration. Mathematically, it is expressed as τ = Iα, where τ is the torque, I is the moment of inertia, and α is the angular acceleration.

How does moment of inertia affect rotational motion?

The moment of inertia is a measure of an object's resistance to changes in its rotational motion. A larger moment of inertia means the object is harder to accelerate or decelerate angularly. It depends on the mass of the object and how that mass is distributed relative to the axis of rotation.

What is the relationship between torque and angular acceleration?

The relationship between torque and angular acceleration is direct and proportional. According to Newton's second law for rotations, the net torque applied to an object is equal to the product of its moment of inertia and its angular acceleration (τ = Iα). Therefore, for a given moment of inertia, a larger torque results in a larger angular acceleration.

Can Newton's second law for rotations be applied to non-rigid bodies?

Newton's second law for rotations is primarily formulated for rigid bodies, where the distribution of mass does not change. For non-rigid bodies, the law can still be applied, but it becomes more complex because the moment of inertia may change as the object deforms. In such cases, additional considerations of internal forces and deformations are required.

How do you calculate the moment of inertia for different shapes?

The moment of inertia depends on the shape and mass distribution of an object. For simple geometric shapes, standard formulas can be used. For example, the moment of inertia of a solid cylinder about its central axis is (1/2)MR², where M is the mass and R is the radius. For more complex shapes, the moment of inertia can be calculated using integral calculus, summing the contributions of infinitesimal mass elements.

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