# Angular Momentum:Calculating Omega with Energy Preservation Law

• vlio20
In summary, the conversation discusses the use of the angular momentum and energy preservation laws to find the angular speed of two sticks that are clashing together. One person suggests using the energy preservation law, but another points out that it cannot be used in this case because the collision is inelastic. They discuss the difference between elastic and inelastic collisions and conclude that the energy equation cannot be used in this scenario.
vlio20
Hi to all,

the question is:

http://ufu.co.il/files/dyzh41xkp5amsfjpv4dk.png

the two sticks clash and stick one to the other, the plane is smooth friction.
the momentum of one stick (in it's own center of mass is 0.5*L^2*m).

the length of the sticks is L and the mass is m, the vertical's stick speed is v.

I have to find the angular speed (w- omega) that both sticks will rotate in, I have manged to do this with the angular momentum preservation law, and I would like to know how to this with energy preservation law.tnx

## The Attempt at a Solution

the energy is not the same before and after the clash- it is not an elastic clash

welcome to pf!

hi vlio20! welcome to pf!
vlio20 said:
… I would like to know how to this with energy preservation law.

you can't , it isn't!

(mechanical) energy is never conserved unless the question says so

in this case, it obviously isn't conserved, because the two bodies stick together, so it's a perfectly https://www.physicsforums.com/library.php?do=view_item&itemid=38"

https://www.physicsforums.com/library.php?do=view_item&itemid=53" and energy are enough to solve a problem … any extra equation (like a geometrical constraint) means that something has to go, and that's always the energy equation

Last edited by a moderator:
oops!

oops!

yes I have understand my mistake after I posted it.
the collision is a plastic and not elastic

tnx

## 1. What is angular momentum?

Angular momentum is a measure of the rotational motion of a system. It is the product of an object's moment of inertia and its angular velocity.

## 2. How is angular momentum calculated?

Angular momentum is calculated by multiplying an object's moment of inertia with its angular velocity. The moment of inertia is a measure of an object's resistance to rotational motion, and the angular velocity is the rate at which the object is rotating.

## 3. What is the energy preservation law?

The energy preservation law, also known as the law of conservation of energy, states that energy cannot be created or destroyed, but can only be transformed from one form to another. In the context of angular momentum, this means that the total angular momentum of a system remains constant unless acted upon by an external torque.

## 4. How is angular velocity (omega) related to energy preservation law?

According to the energy preservation law, the total angular momentum of a system must remain constant. This means that the angular velocity (omega) of an object will change if its moment of inertia changes. If the moment of inertia decreases, the angular velocity will increase, and vice versa.

## 5. What are the practical applications of calculating angular momentum?

Calculating angular momentum is applicable in various fields, such as physics, engineering, and astronomy. It is used to analyze the motion of rotating objects, such as wheels, turbines, and planets. It is also used to understand and predict the behavior of gyroscopes, which are commonly used in navigation systems and stabilizing devices.

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