# M2 energy and momentum

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#1
If Kinetic energy is lost during a collision how can total momentum be conserved?

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2
5 years ago
#2
(Original post by anoymous1111)
If Kinetic energy is lost during a collision how can total momentum be conserved?

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they are different quantities
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#3
(Original post by TeeEm)
they are different quantities
Can you explain how they are different?

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5 years ago
#4
(Original post by anoymous1111)
If Kinetic energy is lost during a collision how can total momentum be conserved?

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Good question! Momentum is conserved as a result of Newton's third law. During the interaction, both bodies exert equal in magnitude but opposite forces on each other. So whether the interaction is elastic or inelastic, the bodies will have to exert equal forces in opposite directions on each other.
Hope this is convincing 1
5 years ago
#5
(Original post by anoymous1111)
Can you explain how they are different?

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

they are different quantities
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#6
(Original post by TeeEm)
write down the definition of momentum and the definition of kinetic energy

they are different quantities
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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5 years ago
#7
(Original post by 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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#8
(Original post by 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
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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5 years ago
#9
(Original post by 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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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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#10
(Original post by 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
Ok so it is in the instant of collision conservation of momentum would be true then

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5 years ago
#11
(Original post by anoymous1111)
Ok so it is in the instant of collision conservation of momentum would be true then

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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)
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#12
(Original post by 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)
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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5 years ago
#13
(Original post by 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 am sorry You are not following my explanations.
I think you need to discuss this with your teacher on a one to one.

All the best
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