Multiple choice question regarding Momentum (1 Viewer)

[Gunner_Fly]

I need to clarify something. I have a question saying
"A ball rebounds inelastically from a wall; is..."
Its momentum convserved but not its kinetic energy
Its kinetic energy conserved but not its momentum
Both its kinetic energy and momentum are conserved
Neither ""

I understand in an inelastic collision, momentum is conserved and not kinetic energy. But does this apply to the whole system, or just the ball itself. Because I have a feeling the ball would change momentum and kinetic energy.

 

[Dumbledore]

I need to clarify something. I have a question saying
"A ball rebounds inelastically from a wall; is..."
Its momentum convserved but not its kinetic energy
Its kinetic energy conserved but not its momentum
Both its kinetic energy and momentum are conserved
Neither ""

I understand in an inelastic collision, momentum is conserved and not kinetic energy. But does this apply to the whole system, or just the ball itself. Because I have a feeling the ball would change momentum and kinetic energy.

a) Its momentum convserved but not its kinetic energy
thats an inelastic collision, momentum is always conserved
in elastic collisions both are conserved

EDIT: didn't read the last part, its the whole system

 

[Dumbledore]

.

 

[Gunner_Fly]

But say it was a 1kg ball travelling at 50 ms
then when it rebounds, it changes direction and its new speed is -50
so initial momentum (50) doesnt equal its final momentum (-50)

So how is its momentum conserved?

 

[k02033]

conservation of momentum does not mean the ball's final momentum has to equal to its initial momemtum.

conservation of momentum says that the system's total momentum must be a constant, if that system does not experience external forces.


let say our system consist of 2 objects, one object is the ball and the other say a box, and no external forces like gravity are acting on our objects. then sum of the system's momentum has to remain constant. other way to say this is that the momentum of the ball and the box can change, but they must change in a way so taht the sum of the balls' momentum and the box's momentum must be the same before and after a collision.

ball's initial momentum = 50kgms^-1
ball's final momentum = -50kgms^-1 after some form of interaction with the box

change in bal's momentum is -50-50=-100kgms^-1
so the box's change in momentum must be 100kgms^-1 (so if box had initial momentum of 10kgms^-1, its final mommentum must be 110kgms^-1)

such that total change in teh system's momentum is zero
say if we replace teh box with the earth in our system, so we are say bouncing this ball on the ground, the earth must indeed gain 100kgms^-1 of momentum, but the earth can easily gain this momentum with very little change in its velocity, becuase of its huge mass

this is actually the case with walking, every time u push on the earth to move, the earth gains equal and opposite momentum, so its pretty easy to move the earth.

 

[k02033]

oh and to answer the question, it depends if the wall is firmly in place, if the wall is in space, then the system of wall and ball is experiencing no external forces, then the momentum of the system is conserved, and kinetic energy is lost to deform the ball and is not conserved since its inelastic.
but if the wall is firmly in place, then system of wall and ball is experiencing external forces, (whatever force that is holding the wall in place when its hit) and so conservation of momemtum doesnt apply, and nothing is conserved in the system of wall and ball.