I have the following problem:

Ball 1 and Ball 2 are rolling toward each other at the same speed, 5
m/s. Ball 1 has a mass of 8kg, and ball 2 has a mass of 2kg. After
collision, ball 1 is observed to move with a velocity of 2 m/s in the
same direction as v1 (to the right). What is the velocity of Ball 2
after collision?

So I can solve for v2 final, which is 7 m/s. This other question asks
if the collision was elastic or not. At first I thought it was elastic
because it's two balls bouncing off each other, which is always elastic
unless there is mention of loss of energy somewhere. But it's actually
inelastic, and I was supposed to examine the kinetic energies before
and after. Is the main reason why the collision was inelastic is
because they forcefully assigned ball 1 with a velocity?

Thanks,
- Eli

"Eli Luong" wrote in message
oups.com...
I have the following problem:

Ball 1 and Ball 2 are rolling toward each other at the same speed, 5
m/s. Ball 1 has a mass of 8kg, and ball 2 has a mass of 2kg. After
collision, ball 1 is observed to move with a velocity of 2 m/s in the
same direction as v1 (to the right). What is the velocity of Ball 2
after collision?

5*8 - 5*2 = 50
50-8*2 = 34
34/2 = 17m/s = v2


So I can solve for v2 final, which is 7 m/s. This other question asks
if the collision was elastic or not. At first I thought it was elastic
because it's two balls bouncing off each other, which is always elastic
unless there is mention of loss of energy somewhere. But it's actually
inelastic, and I was supposed to examine the kinetic energies before
and after. Is the main reason why the collision was inelastic is
because they forcefully assigned ball 1 with a velocity?

no. always assume it is inelastic, unless they specify it, as it absorbes
energy

it is difficult to make the problem simple when you throw in a loss. then
Angle hits.......
..


Thanks,
- Eli

"Eli Luong" wrote in message
oups.com...
I have the following problem:

Ball 1 and Ball 2 are rolling toward each other at the same speed, 5
m/s. Ball 1 has a mass of 8kg, and ball 2 has a mass of 2kg. After
collision, ball 1 is observed to move with a velocity of 2 m/s in the
same direction as v1 (to the right). What is the velocity of Ball 2
after collision?

So I can solve for v2 final, which is 7 m/s. This other question asks
if the collision was elastic or not. At first I thought it was elastic
because it's two balls bouncing off each other, which is always elastic
unless there is mention of loss of energy somewhere. But it's actually
inelastic, and I was supposed to examine the kinetic energies before
and after. Is the main reason why the collision was inelastic is
because they forcefully assigned ball 1 with a velocity?

It seems like you're trying to find the answer to the
question in the words rather than the numbers. Always
check the numbers.

Momentum is always conserved, but kinetic energy may or
may not be. If kinetic energy is conserved the collision
was elastic, otherwise it was not. So if you're asked
whether or not a collision was elastic, check the KE.

On Wed, 10 Jan 2006, Eli Luong wrote:

I have the following problem:

Ball 1 and Ball 2 are rolling toward each other at the same speed, 5
m/s. Ball 1 has a mass of 8kg, and ball 2 has a mass of 2kg. After
collision, ball 1 is observed to move with a velocity of 2 m/s in the
same direction as v1 (to the right). What is the velocity of Ball 2
after collision?

So I can solve for v2 final, which is 7 m/s. This other question asks
if the collision was elastic or not. At first I thought it was elastic
because it's two balls bouncing off each other, which is always elastic
unless there is mention of loss of energy somewhere. But it's actually
inelastic, and I was supposed to examine the kinetic energies before
and after. Is the main reason why the collision was inelastic is
because they forcefully assigned ball 1 with a velocity?

In a generic 1D collision problem, you know the initial velocities and the
masses. Since you have conservation of momentum, you have one equation.

If you don't know either final velocity, that leaves you with 2 unknowns,
and you need a second equation. There are 2 common possibilities used in
problems of this type: assume that the collision is elastic so that
conservation of energy gives you another equation, or assume that the
collision is totally inelastic (ie the two objects stick together), so
both final velocities are the same.

For something inbetween those two cases, the usual thing is to specify one
of the final velocities, leaving you with only one unknown quantity.
Sometimes you might see something like "assume that the collision is
inelastic, with 10% of the energy being lost".

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/...,_Timo_A..html
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