Re: Calculating Newtons in Joules and Joules/s


Dennis B wrote:
Randy Poe wrote:
Dennis B wrote:
Since the 2kg mass and the 4kg mass have the same momentum, wouldn't
both of the 3 kg masses have the same momentum as well?

Momentum and energy are separately conserved. Due to
those requirements, your hypothetical momentum transfers
can't both happen.

If the 3 kg mass were to get all of the momentum of the
2 kg mass, it would be moving at 2/3 of the velocity and
would therefore have (3/2)*(4/9) = 2/3 of the KE that the
2 kg mass originally had. That is possible in the right
inelastic collision, with the remaining 1/3 being lost to
heat.

What does the (4/9) represent?

(2/3)^2.

KE1/KE2 = (m1/m2)*(v1/v2)^2 = (3/2)*(2/3)^2

I found it more convenient to analyze this problem in terms
of ratios rather than work out the actual velocities.



If the 3 kg mass were to get all of the momentum of the
4 kg mass, it would be moving at 4/3 of the velocity and
would have (3/4)*(16/9) = 4/3 of the KE that the 4 kg mass
originally had. That is impossible.

What does the (16/9) represent?

(4/3)^2



It is not possible for a 4 kg mass to transfer all of its momentum
to a 3 kg mass in a collision.

- Randy

Make it two 4 kilogram masses instead of two 3 kg masses.

Originally you had a 2 kg and 4 kg mass with the same
momentum, each of which is striking a 3 kg mass. There
were no "two 3 kg masses". So what is the situation now?
What are the moving masses and what mass are they
striking?

And make the
collision totally inelastic, so that all of the momentum is
transferred.

"Totally inelastic" means that the objects stick together
after the collision, both moving together as one mass.
Not that one comes to a halt and the other gets all
the momentum.

Then what would the results of a transfer of momentum be?
I don't see why the velocities would be any different. Although the 2kg
and 4kg masses differ, the 2kg mass travels at a greater velocity to
make up for the difference so that the momentum of the two is the same.
Therefore, I would expect that the 2kg mass should have just as much of
an impact as the 4 kg mass.

Your expectations, based on your intuitions, continue to
be at odds with the laws of physics and experimental
results. Yet when told such things, you continue to insist
that your beliefs must be right and the universe is wrong.

And oh yes, that you're a "student" who is here to learn.
But also that all of us are incompetent, the universe is
wrong, and your predictions of the effects of forces are
right and we're all wrong.

I'll answer your question about the collision when I understand
what the question is.

- Randy

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