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Re: [Phys-l] One of those Yahoo Answers "URGENT" Physics problems (with a twist)



For Curtis's Physics-like model, in a vacuum as it were,
he finds the horizontal velocity is unchanging, so impact at the peak of
the trajectory is indicated.
For Steve Highland's more realistic model accounting for air resistance,
the more time loitering in a tall trajectory, the lower would be
the horizontal velocity.

But we are told, Jack wants only a horizontal velocity, so the point is moot.
Perhaps we could ask: for realistic conditions of air resistance, is it possible
to specify a vertical velocity at impact ( = 0), any horizontal velocity,
a vertical offset, and a horizontal offset for the peak of the trajectory.
Or could this problem be over-constrained?

Brian W

At 11:24 AM 8/25/2008, Curtis Osterhoudt, you wrote:
I think Don's question is a good one. Assuming the typical things---no air
resistance, parabolic arc, smooth surfaces, all that---the horizontal
component of the pebble shouldn't change at all, throughout the trajectory.

///

------------
Jack is chucking pebbles gently up to Sarah's window, and he wants the
pebbles to hit the window with only a horizontal component of velocity. He
is standing at the edge of a rose garden 8.00 meters below her window and
11.0 m from the base of the wall. How fast are the pebbles going when they
hit her window?
------------

Of course this is an easy problem for a non-beginner. What interests me
is
the assumption that throwing the pebble so it hits the window horizontally
is the ?gentlest¹ throw. I don¹t think so. I would think lobbing the
pebble higher than the window so it came down and struck the pane at a
glancing angle would actually be a gentler impact. But I'm not 100% sure.
It would hit with a lesser horizontal velocity and deliver less of an
impulse to the window, but is that the only thing that matters? I doubt
it.
The duration of contact with the window must play a role, and that time
must
get shorter with higher vertical speeds, right? That would seem to
increase
the force and be bad for the window.

What does govern how ?gently¹ the pebble hits the window? Is my instinct
that a glancing blow at higher speed is gentler than a head-on impact at a
slower speed right or wrong?

Steve Highland


Brian Whatcott Altus OK Eureka!