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



I suspect that is why figure skaters don't suffer massive injuries when they fall at high speeds. The glancing component on the slippery ice causes no harm, while the vertical motion is no different than falling while standing on the ice - it hurts but does not break young bones.

Bob at PC

________________________________

From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of Steve Highland
Sent: Mon 8/25/2008 11:42 AM
To: Forum for Physics Educators
Subject: [Phys-l] One of those Yahoo Answers "URGENT" Physics problems (with a twist)



I¹ve been horsing around on Yahoo Answers a little bit today and I came
across this question;
------------
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 OEgentlest¹ 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 OEgently¹ 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
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