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# Physics question

Discussion in 'The Okie Corral' started by gwalchmai, Dec 23, 2012.

1. ### gwalchmaiLucky Member

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Assume you're in an elevator and go from the 1st floor to the second. If you jump one foot off the ground as soon as the elevator starts going up does the elevator do more or less work than if you don't?

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More.

3. ### sombunyaUse it up, throw it away

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I would agree. The energy expended by the person jumping has to go somewhere and the elevator absorbs it. Small amount, but there nonetheless.

4. ### FullClipNRA BenefactorCLM

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It's the same, as the force you exerted for a fraction of a second on the floor in jumping is offset by fraction of second you were in the air.

Actually, perhaps you created more work for the poor elevator, given the downward force on the floor you exerted on the initial jump, and then again by the force you create when you land on the floor (your mass and velocity vs just mass)

I'm way too far into the Aldaris cans to offer a formula, but will watch as the math guys start to chime in. (yes....beer...but I'm 7 hours ahead of EST)

5. ### Jonesee

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When I was younger and in college I was a bank runner for my bank and made runs to the Fed Res Bank in KC.

They have a very fast elevator. About a floor a second both up and down. On the way up if you jumped as it started moving you could barley get off the ground. On the way down if you jumped as it started moving you could hit your head on the grates in the ceiling.

I have no doubt the FRB had cameras in the elevator and I'm sure when I made my runs the guards watching the cameras were laughing at me everyday.

6. ### dango

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Blatant elevator abuse if you ax me ! Ban elevator jumpers or is it just a case of evil shoes....Hmmm?
Hell , just ban buildings more than one story , now that's physical...........

7. ### hogfishSeñor Member

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I see. You jumped too many times on your way down. Thanks.

8. ### CybercowboySupport the 2nd

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There is no net change in the amount of work the elevator does. Neglecting things like friction and other mechanical inefficiencies, the system of the elevator plus you moves from the 1st story to the 2nd story and the total work done is equal to the net change in the potential energy. PE = mgh (Potential Energy change = mass times the acceleration of gravity times the height difference.)

You can jump around all you want. Nothing changes that. Also note that an "ideal elevator" moves from floor to floor using zero energy because if we assume frictionless pulleys and idealized mechanics, the counterweight of the elevator car offsets any change in potential energy of the car itself. Car goes up, counterweight goes down. No net energy change. The motor running an elevator system simply has to overcome the friction of the pulleys and other mechanical inefficiencies to move the empty elevator from floor to floor.

When the elevator has people in it, the change in the potential energy of moving that mass over that distance is how much additional work the electric motor had to do versus running the same distance empty, no matter how much jumping around you did. For a 100 kilogram human getting moved 4 meters:

PE = 100 * 9.8 * 4 = 3920 Joules of energy.

9. ### ysr_racer

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10. ### Harper

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I believe it's the same. Work=force*distance
The elevator has to move a certain weight(force) over a certain distance. By jumping and landing in the same spot relative to the elevator you've done no work on the elevator. Now if the electric bill ends up being higher - I don't know.

11. ### Jonesee

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I dont know about physics. My background is in economics. But...

Get in a fast elevator and jump when it starts moving. Jump at the very moment it starts moving. Then report back here.

I know what you will learn.

The easiest way to explain it is:
When I jumped at the moment it begins to drop, it effectively drops out from under me a bit. That is why your head can hit the lighting grates. Try it and see.

I did this run for almost a year. I know exactly what happens when you do it.

Last edited: Dec 23, 2012
12. ### CybercowboySupport the 2nd

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When you jump you apply an impulse force equal to the change in the potential energy you attain with your briefly increased maximum jump height. The elevator moves "freely" as long as you are in the air, and then you land, again applying an impulse force to the floor of the elevator. The amount of work you did on the elevator floor due to your impulse forces is exactly counteracted by the reduced amount of work the elevator does while you are in the air.

Gravity: It's not just a good idea, it's the law.

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14. ### Jonesee

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So if the elevator is moving freely down while i am in the air it has dropped while I am airborn. Seriously, try it!

And it was moving at a floor a second. Clearly faster than a free fall from a dead stop. It is being powered down, I am not.

15. ### CybercowboySupport the 2nd

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I'm trying to keep things as simple as possible. However, an impulse force is a force applied over a duration of time and has units of Newton-seconds (metric system.) It is equivalent to a change in momentum.

16. ### CybercowboySupport the 2nd

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Did you take physics in college?

17. ### Harper

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So you ride down pinned to the ceiling of the elevator? That doesn't sound safe.

18. ### Jonesee

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Think about it, why else do you feel heavier as an elevator begins it ascent? and lighter when it begins its descent?

By jumping right then it has an impact.

Quick somenbody go ride a fast elevator and report back!!!!

19. ### Jonesee

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LOL, no it was just a fraction of a second.

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