Relatively simple physics question

[Hiatussin]

I just got a bit confused and can´t seem to find the answer in my book. I´m not used to doing this in English.

Dynamics-

On a flat plain with no friction, two objects approach each other directly at 180 degrees of one another and collide.

No energy is transferred into heat and neither attains internal energy from the collision.

From what I understand, the "amount of motion" in the total system of the sum of the two objects has to be equal before and after the collision.

m1 . v1 + m2 . v2

=

m1 . v´1 + m2 . v´2

Sounds nice enough. But then what about the Kinetic energy?

Should not also (Ekin1 + Ekin2) = (Ekin´1 + Ekin´2)?
kinetic energy is a function of v^2 and "amount of motion" or "impulse" is a function of v.

How is this possible?

 

[milo hobgoblin]

Damn you make me miss college.. I actually liked this crap.

Im so doomed to nerdom

 

[KillahBee]

I could read and study that for 5 hours straight and know less than what I do now.

 

[Hiatussin]

Don´t get nostalgic. answer me

 

[Hiatussin]

bump

 

[Scotsman]

Yes if no energy is dissapated via heat, or internalization then the kinetic energy must be the same after collision as before. Though some energy may be transferred from object 1 to object 2 give that object one is of greater mass. Therefore the system doesn't lose energy due to conservation of energy and mass, but on object may retain greater kinetic momentum after the impact. Therefore the less massive object will reverse course and move faster than it's original velocity, and the more massive object will move in the opposite direction at a reduced velocity from the original. Unless the object of greater mass doesn't have enough energy imparted from the lesser object to reverse direction of inertia. Then the more massive object will continue on it's original course at a reduced velocity.

Cheers,
Scotsman

 

[Hiatussin]

Yes if no energy is dissapated via heat, or internalization then the kinetic energy must be the same after collision as before. Though some energy may be transferred from object 1 to object 2 give that object one is of greater mass. Therefore the system doesn't lose energy due to conservation of energy and mass, but on object may retain greater kinetic momentum after the impact. Therefore the less massive object will reverse course and move faster than it's original velocity, and the more massive object will move in the opposite direction at a reduced velocity from the original. Unless the object of greater mass doesn't have enough energy imparted from the lesser object to reverse direction of inertia. Then the more massive object will continue on it's original course at a reduced velocity.

Cheers,
Scotsman

I don´t think you answered my question. Without internalisation or loss to heat, do BOTH the kinetic energy and the "amount of motion" stay equal?

Doesn´t that contradict whenever the masses are different?

 

[Scotsman]

I don´t think you answered my question. Without internalisation or loss to heat, do BOTH the kinetic energy and the "amount of motion" stay equal?

Doesn´t that contradict whenever the masses are different?

Yes I did answer. And yes the kinetic energy stays the same and the amount of motion stays the same.

No it doesn't contradict to have different mass objects as long as you are looking at the total system and not the sum of it's parts.

Cheers,
Scotsman

 

[Hiatussin]

the total system and not the sum of it's parts

eureka

 

[spongebob]

if theres no friction how in the hell do you even have get two objects in motion?

 

[gonelifting]

The answer is 5.

 

[Scotsman]

if theres no friction how in the hell do you even have get two objects in motion?

It's theoretical to teach the base principles of physics. Later on you add friction and other detractive forces like inclines.

Cheers,
Scotsman

 

[HumorMe]

The answer is 5.

That's what I was going to say.

 

[spongebob]

It's theoretical to teach the base principles of physics. Later on you add friction and other detractive forces like inclines.

Cheers,
Scotsman

you physics geeks kill me.

 

[Scotsman]

The answer is 5.

No the answer is 42.

Cheers,
Scotsman

 

[UA_Iron]

A mass system is said be conservative if it does not lose energy by virtue of internal friction forces which do not do negative work or by virtue of inelastic members which dissipate energy upon cycling.

In other words it needs to be a completely elastic equation meaning that material properties and distortion energy must be fully recoverable. This defines the coefficient of restitution, e. When e=1: the capacity of two particles to recover equal their tendency to deform. When e=0: where particles will cling together after impact.

Some materials and their coefficients of restitution sorted in descending order:

e~.9 Glass on Glass
e~.65 Steel on steel
e~.1 Lead on lead

 

[blut wump]

If you consider the simple case of two balls of equal mass coming together with equal speed then the momentum in the system is zero but energy is not. Without the conservation of energy, the balls would simply stop on collision due to the velocity cancellation.