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Author Topic: Physics question...
maybe.logic
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Icon 1 posted March 31, 2006 04:47      Profile for maybe.logic     Send New Private Message       Edit/Delete Post   Reply With Quote 
A high jumper, falling at -4m/s lands on a foam pit and comes to rest, compressing the pit .40m. If the pit is able to exert an avg force of 1200N on the high jumper in breaking the fall, what is the athletes mass???

Lets see who can get it the quickest?
I will make them diffucult in time.

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TheMoMan
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Icon 1 posted March 31, 2006 05:24      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
maybe.logic_________________Exlax will make you you go. What is the densety of the foam, compressablity and rebound?

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Ugurcan

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Icon 1 posted March 31, 2006 08:24      Profile for Ugurcan   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Speed cannot be negative.

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Mac D
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Icon 1 posted March 31, 2006 08:37      Profile for Mac D     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Ugurcan:
Speed cannot be negative.

Maybe the foam is above the jumper. And (s)he is falling up. I saw that too and kind of wondered.

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TMBWITW,PB

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Icon 1 posted March 31, 2006 08:38      Profile for TMBWITW,PB     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Ugurcan:
Speed cannot be negative.

In my physics class we used negative speed all the time. It was to show that they were traveling in one direction instead of the opposite. It also helps when you have to calculate something with two objects traveling in opposite directions.

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Grummash

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Icon 1 posted March 31, 2006 10:30      Profile for Grummash     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Ugurcan:
Speed cannot be negative.

Come and work in my office - you'll find plenty of evidence of negative speed. [Big Grin]

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Mac D
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Icon 1 posted March 31, 2006 10:35      Profile for Mac D     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by TMBWITW,PB:
quote:
Originally posted by Ugurcan:
Speed cannot be negative.

In my physics class we used negative speed all the time. It was to show that they were traveling in one direction instead of the opposite. It also helps when you have to calculate something with two objects traveling in opposite directions.
Thats what I was trying to say. Because falling in most cases is down. So if the number is negitive it would imply that the person falling was going up.

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maybe.logic
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Icon 1 posted March 31, 2006 11:07      Profile for maybe.logic     Send New Private Message       Edit/Delete Post   Reply With Quote 
Vf^2 = Vi^2 + 2ax
Vf = 0(since object ends at rest)
Vi = -4 m/s
x = .4 m
0 = 16 + 2* (.4) * a
-16 = .8 * a
-20 m/s^2 = acceleration

F = Mass * acceleration
1200 N = mass * -20 m/s^2
mass = 60 kg

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littlefish
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Icon 1 posted March 31, 2006 11:32      Profile for littlefish   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Speed cannot be negative.
This is correct, as it is scalar, but velocity is a vector, and can be negative.
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maybe.logic
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Icon 1 posted March 31, 2006 11:33      Profile for maybe.logic     Send New Private Message       Edit/Delete Post   Reply With Quote 
and...
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The Famous Druid

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Icon 1 posted March 31, 2006 11:39      Profile for The Famous Druid     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Mac D:
quote:
Originally posted by TMBWITW,PB:
quote:
Originally posted by Ugurcan:
Speed cannot be negative.

In my physics class we used negative speed all the time. It was to show that they were traveling in one direction instead of the opposite. It also helps when you have to calculate something with two objects traveling in opposite directions.
Thats what I was trying to say. Because falling in most cases is down. So if the number is negitive it would imply that the person falling was going up.
Um, no.
Velocity can be negative, speed cannot.

Something about speed being a scalar quantity, while velocity is a vector (speed + direction). I disremember the details, it's 30 years since I did all that stuff.

[edit] Note to self: read all the way to the end of a thread before hitting the 'reply' button.

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ChildeRoland
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Icon 1 posted March 31, 2006 14:59      Profile for ChildeRoland     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by maybe.logic:
Vf^2 = Vi^2 + 2ax
Vf = 0(since object ends at rest)
Vi = -4 m/s
x = .4 m
0 = 16 + 2* (.4) * a
-16 = .8 * a
-20 m/s^2 = acceleration

F = Mass * acceleration
1200 N = mass * -20 m/s^2
mass = 60 kg

If the velocity is negative, and the acceleration is negative, won't the final velocity end up being even higher (more negative)?

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maybe.logic
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Icon 1 posted March 31, 2006 15:31      Profile for maybe.logic     Send New Private Message       Edit/Delete Post   Reply With Quote 
Clever Boy!!!! [Big Grin]
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The Famous Druid

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Icon 1 posted March 31, 2006 19:48      Profile for The Famous Druid     Send New Private Message       Edit/Delete Post   Reply With Quote 
Sorry Mr Logic, but if you're going to be a sarky smart-alec, it's important to make sure that you're right.

Your calculations above fail to make allowance for the force of gravity on the falling object.

A 60 kg mass would experience a net force of 1200 - (60*9.8=588) = 612 N, which would produce a net decelleration of 612/60 = 10.2 m/s^2.

Your high-jumper's mass is more like 40 kg.

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If you watch 'The History Of NASA' backwards, it's about a space agency that has no manned spaceflight capability, then does low-orbit flights, then lands on the Moon.

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