Re: Work - impulse


Peter wrote:
Ah, so your little thought experiment has changed somewhat -- that's
fine.
What you have initially is a puck with forward linear momentum, and a
lever with backwards linear momentum, prior to the collision. This
results from accelerating the puck, which imparts a backwards velocity
to the spaceship and, because the lever's axle is tied to the
spaceship, to the lever as well.
When you first posed this problem, you had posed it such that the lever
was at rest prior to the collision.

No change at all. Maybe I didn't explain it completely. Initially,
everything is at rest; then, the person accelerates the puck forward,
and the spaceship accelerates briefly backward: the total linear and
angular momentum of the whole system is zero at all times. The lever
and its pivot are on a table, which is secured to the spaceship, as it
is the person. The linear momentum of the lever, with respect to the
spaceship is zero at all times.

Peter

That's fine. However, we don't typically view things from a reference
frame that accelerates. Specifically, it is not good to try to measure
everything from a reference frame that is welded to the spaceship,
especially if the spaceship accelerates at some point in the process.

What would be better would be to choose a reference frame where
everything is at rest initially before the puck is launched. In this
reference frame, after the puck's launch, then the puck AND the
spaceship AND the lever are all moving. Now in this reference frame we
can then ask what happens when the puck strikes the lever.

PD

.

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