# M2 energy and momentum

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If Kinetic energy is lost during a collision how can total momentum be conserved?

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(Original post by

If Kinetic energy is lost during a collision how can total momentum be conserved?

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**anoymous1111**)If Kinetic energy is lost during a collision how can total momentum be conserved?

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**different**quantities

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**anoymous1111**)

If Kinetic energy is lost during a collision how can total momentum be conserved?

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Hope this is convincing

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#5

they are different quantities

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(Original post by

write down the definition of momentum and the definition of kinetic energy

they are different quantities

**TeeEm**)write down the definition of momentum and the definition of kinetic energy

they are different quantities

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#7

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Yes but velocity is a component of momentum and if kinetic energy is lost then velocity will decrease so surely momentum will also decrease

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**anoymous1111**)Yes but velocity is a component of momentum and if kinetic energy is lost then velocity will decrease so surely momentum will also decrease

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**definitely not**

in the system some velocities decrease some may increase,

The momentum in the absence of external forces stays the same.

The kinetic energy will decrease unless the collision is elastic, ie the coefficinet of restitution is 1

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(Original post by

in the system some velocities decrease some may increase,

The momentum in the absence of external forces stays the same.

The kinetic energy will decrease unless the collision is elastic, ie the coefficinet of restitution is 1

**TeeEm**)**definitely not**in the system some velocities decrease some may increase,

The momentum in the absence of external forces stays the same.

The kinetic energy will decrease unless the collision is elastic, ie the coefficinet of restitution is 1

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#9

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I think what I've being missing is that the momentum will be the same the instant before collision and the instant after but anytime after this, friction could act on the individual particles and reduce the momentum in the system? Is this true?

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**anoymous1111**)I think what I've being missing is that the momentum will be the same the instant before collision and the instant after but anytime after this, friction could act on the individual particles and reduce the momentum in the system? Is this true?

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Now momentum is not even conserved as there is an external force (and neither is energy)

What happens after is a different question

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(Original post by

this is a different matter altogether.

Now momentum is not even conserved as there is an external force (and neither is energy)

What happens after is a different question

**TeeEm**)this is a different matter altogether.

Now momentum is not even conserved as there is an external force (and neither is energy)

What happens after is a different question

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#11

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Ok so it is in the instant of collision conservation of momentum would be true then

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**anoymous1111**)Ok so it is in the instant of collision conservation of momentum would be true then

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the momentum of the system is conserved

the kinetic energy of the system is reduced, unless the collision is perfectly elastic (restitution coefficient is 1/never in real life)

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(Original post by

At the instant of collision,

the momentum of the system is conserved

the kinetic energy of the system is reduced, unless the collision is perfectly elastic (restitution coefficient is 1/never in real life)

**TeeEm**)At the instant of collision,

the momentum of the system is conserved

the kinetic energy of the system is reduced, unless the collision is perfectly elastic (restitution coefficient is 1/never in real life)

a) velocity is a component of kinetic energy (1/2mv^2 + 1/2nx^2 where n and x are the mass and velocity of the second object in the collision)

b) therefore if kinetic energy has fallen, given that mass stays the same, velocity must have fallen in magnitude

c) momentum is the sum of the mv and nx (mass X velocity of both objects)

d) if kinetic energy has fallen and therefore totally velocity has fallen in magnitude then the sum of mv and nx must be smaller than the initial sum (therefore momentum of the two objects added together is not conserved as kinetic energy has fallen).

I just don't see how what I say isn't true? Can you tell me where I'm going wrong? Is it only the 2 objects that are involved in the collision that are considered in the system?

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#13

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Ok then I really still don't get this. Isn't it true that:

a) velocity is a component of kinetic energy (1/2mv^2 + 1/2nx^2 where n and x are the mass and velocity of the second object in the collision)

b) therefore if kinetic energy has fallen, given that mass stays the same, velocity must have fallen in magnitude

c) momentum is the sum of the mv and nx (mass X velocity of both objects)

d) if kinetic energy has fallen and therefore totally velocity has fallen in magnitude then the sum of mv and nx must be smaller than the initial sum (therefore momentum of the two objects added together is not conserved as kinetic energy has fallen).

I just don't see how what I say isn't true? Can you tell me where I'm going wrong? Is it only the 2 objects that are involved in the collision that are considered in the system?

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**anoymous1111**)Ok then I really still don't get this. Isn't it true that:

a) velocity is a component of kinetic energy (1/2mv^2 + 1/2nx^2 where n and x are the mass and velocity of the second object in the collision)

b) therefore if kinetic energy has fallen, given that mass stays the same, velocity must have fallen in magnitude

c) momentum is the sum of the mv and nx (mass X velocity of both objects)

d) if kinetic energy has fallen and therefore totally velocity has fallen in magnitude then the sum of mv and nx must be smaller than the initial sum (therefore momentum of the two objects added together is not conserved as kinetic energy has fallen).

I just don't see how what I say isn't true? Can you tell me where I'm going wrong? Is it only the 2 objects that are involved in the collision that are considered in the system?

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I think you need to discuss this with your teacher on a one to one.

All the best

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