# Conservation of energy, momentum

• ripoli85
In summary: The conservation of momentum is still valid but you have to use vector addition of velocities. Also, the balls are identical so they should have the same speed afterwards, not half.In summary, the conversation discusses a system of three pool balls, where one is moving and the other two are stationary. It examines the idea of conservation of momentum and the potential loss of kinetic energy in the system. The conversation also mentions the importance of considering all types of energy changes and the potential for mistakes when using conservation of energy. It also draws a parallel to a similar situation in electricity.
ripoli85
i don´t quite understand:

if you have a system made up of three pool balls. one of them is moving with a speed of 1 meters per second, the two other balls are standing still. if the moving ball hits the two other balls, in a way that the moving ball is standing still afterwards, then the two other balls should have a speed of 0.5 meters per second each, so that momentum is conserved in this system, right?
if that is the case than there is a loss of kinetic energy, because 1 ball moving at a certain speed has double the kinetic energy, than two balls moving at half the speed?
but energy is supposed to be conserved in a system two.
can someone please help get rid of that twist in my head, what am i getting wrong here? is momentum not being conserved in a system?

Yes momentum is conserved. And yes there is a loss of kinetic energy to the environment.

However a rolling ball also has rotational energy so you need to add all types of energy to do an energy balance.

You always have to be careful accounting for all the energy changes when using conservation of energy conservation of momentum can be a safer bet (although less intuitive) this is will come home if you ever study elementary particle physics.

There is an equivalent situation in electricity.

Consider two identical capacitors, one charged, one not charged.

Now connect the discharged one across the charged one.

What is the voltage across the combination and what is the energy state of the system?

ripoli85 said:
i don´t quite understand:

if you have a system made up of three pool balls. one of them is moving with a speed of 1 meters per second, the two other balls are standing still. if the moving ball hits the two other balls, in a way that the moving ball is standing still afterwards, then the two other balls should have a speed of 0.5 meters per second each,

Your conservation of momentum argument is wrong, because the balls can move in different directions, so their speed can be larger, but their momentum can still be equal to momentum of the first ball.

## What is conservation of energy?

Conservation of energy is a fundamental principle in physics that states that energy cannot be created or destroyed, but can only be transferred or transformed from one form to another.

## Why is conservation of energy important?

Conservation of energy is important because it allows us to predict and understand the behavior of physical systems. It also helps us to identify and design more efficient energy systems.

## What is conservation of momentum?

Conservation of momentum is a fundamental principle in physics that states that the total momentum of a closed system remains constant, unless acted upon by an external force.

## How is conservation of energy related to conservation of momentum?

Conservation of energy and conservation of momentum are closely related concepts. In a closed system where there are no external forces acting, the total energy and total momentum will both remain constant.

## What are some examples of conservation of energy and momentum in action?

Some examples of conservation of energy and momentum in action include the motion of planets in our solar system, the bouncing of a ball, and the movement of a pendulum. In all of these cases, the total energy and momentum of the system remain constant.

• Mechanics
Replies
30
Views
1K
• Mechanics
Replies
53
Views
2K
• Mechanics
Replies
2
Views
804
• Mechanics
Replies
3
Views
966
• Mechanics
Replies
14
Views
1K
• Mechanics
Replies
5
Views
785
• Mechanics
Replies
5
Views
684
• Mechanics
Replies
20
Views
17K
• Mechanics
Replies
19
Views
991
• Mechanics
Replies
3
Views
1K