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gwalchmai
12-23-2012, 11:31
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?

hogfish
12-23-2012, 11:37
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?

More. :)

sombunya
12-23-2012, 11:40
I would agree. The energy expended by the person jumping has to go somewhere and the elevator absorbs it. Small amount, but there nonetheless.

FullClip
12-23-2012, 11:40
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):supergrin:

Jonesee
12-23-2012, 11:46
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.

dango
12-23-2012, 11:48
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...........:supergrin:

hogfish
12-23-2012, 11:50
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.

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

Cybercowboy
12-23-2012, 12:00
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.

ysr_racer
12-23-2012, 12:04
Treadmill?

Harper
12-23-2012, 12:05
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.

Jonesee
12-23-2012, 12:06
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.

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.

Cybercowboy
12-23-2012, 12:13
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.

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.

DanaT
12-23-2012, 12:16
There is no net change in the amount of work the elevator does.

This is your correct answer. Dont get work and impulse confused.

Jonesee
12-23-2012, 12:17
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.


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.

Cybercowboy
12-23-2012, 12:22
This is your correct answer. Dont get work and impulse confused.

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.

Cybercowboy
12-23-2012, 12:22
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.

Did you take physics in college?

Harper
12-23-2012, 12:24
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.

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

Jonesee
12-23-2012, 12:24
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!!!!

Jonesee
12-23-2012, 12:28
So you ride down pinned to the ceiling of the elevator? That doesn't sound safe.


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

Harper
12-23-2012, 12:28
science > common sense

Harper
12-23-2012, 12:30
LOL, no it was just a fraction of a second.

If the elevator descends faster than a free fall your feet will leave the floor whether you intend to or not.

Jonesee
12-23-2012, 12:30
Did you take physics in college?


No, I only took calc, quant, stats and all advanced econ. Physics was too tough for me.

But I know real life when it happens to me. Go try it and report back.

Grumpy Gardener
12-23-2012, 12:30
Conservation of energy. You're in and part of the 'system' (the elevator). Being part of that system, you cannot affect the overall conservation of energy of that system.
gg.

Jonesee
12-23-2012, 12:33
If the elevator descends faster than a free fall your feet will leave the floor whether you intend to or not.


Only when you jump.

Gang I'm not going to argue. I know what happens. Some of you need to leave your keyboards and try it.


Have fun when you do.

Cybercowboy
12-23-2012, 12:34
No, I only took calc, quant, stats and all advanced econ. Physics was too tough for me.

But I know real life when it happens to me. Go try it and report back.

Well, these sorts of problems were covered in about the first four weeks of calculus-level physics classes. I had a bunch of them when I worked on my Electrical Engineering degree. Physics I and II, Statics, Dynamics, Thermodynamics, Quantum Physics/ Relativity, then about a bazillion electrical and signal theory classes.

Trust me, you're not circumventing the simple rules of conservation of energy by jumping.

JoeInKS
12-23-2012, 12:39
The elevator and counterweight assembly are in place in order to minimize the work that the motor needs to do in moving the system. Additional weight added in the form of load adds to the work the motor needs to do.

Spring assemblies and natural cable stretch under tension typically take into account normal momentary loading changes.

If you jump up the net affect would be minimal. The action causes a reaction in the elevator spring / compensator that would be again realized on impact. If you look at the effect of friction, you've made the elevator less efficient overall because you have brought more forces into the equation because more of the overall system is impacted.

All fun stuff. Look at Lagrangian Dynamics / Mechanics if you ever get REALLY bored.

Harper
12-23-2012, 12:48
Only when you jump.



No, if the elevator descends faster than a free fall your feet will leave the floor whether you intend to or not.

Here's a real life example and an explanation of the exact elevator situation you're talking about.

http://youtu.be/twSU5qnYDZ8

certifiedfunds
12-23-2012, 13:21
W=Fd

F=ma

W=mad

yes

gwalchmai
12-23-2012, 13:26
What if you delay your jump until the elevator reaches its maximum speed?

Cybercowboy
12-23-2012, 13:45
What if you delay your jump until the elevator reaches its maximum speed?

The elevator could move 1" per year or 5000 MPH. You could jump 1 time or 1000 times. The work done by you on the elevator (as a complete system) would still be zero.

certifiedfunds
12-23-2012, 14:18
What if you delay your jump until the elevator reaches its maximum speed?

It still pushes back with the same force you push down to jump

Grumpy Gardener
12-23-2012, 14:18
In other words...you can't push your car while you're inside the car. :wavey:

gg

JoeInKS
12-23-2012, 14:24
In other words...you can't push your car while you're inside the car. :wavey:

gg

No but you can move your body weight quickly and have a net effect on the car while it compensates for the change.

NOLA_glock
12-23-2012, 14:54
Free-body diagram or GTFO.

NOLA_glock
12-23-2012, 15:12
Nothing to add, here.

Position(t)

Velocity(t)=Position'(t)

Acceleration(t)=Velocity'(t)

Jerk(t)=Acceleration'(t)

Snap(t)=Jerk'(t)

Crackle(t)=Snap'(t)

Pop(t)=Crackle'(t)=Position'''''(t)

Is there a name for the 7th derivative of Position(t)? :rofl:

ETA: Google to the rescue!

Position'''''''(t)= Lock(t)

Position''''''''(t)=Drop(t)

Who names these? :rofl:

Harper
12-23-2012, 21:27
Nothing to add, here.

Position(t)

Velocity(t)=Position'(t)

Acceleration(t)=Velocity'(t)

Jerk(t)=Acceleration'(t)

Snap(t)=Jerk'(t)

Crackle(t)=Snap'(t)

Pop(t)=Crackle'(t)=Position'''''(t)

Is there a name for the 7th derivative of Position(t)? :rofl:

ETA: Google to the rescue!

Position'''''''(t)= Lock(t)

Position''''''''(t)=Drop(t)

Who names these? :rofl:

http://scoop.diamondgalleries.com/Image/NewsImage/4/70195/164028/2

NOLA_glock
12-23-2012, 23:08
http://scoop.diamondgalleries.com/Image/NewsImage/4/70195/164028/2

:rofl:

Huey - Pop, Lock & Drop It - YouTube

^^^^^Physicists. (σلσ)

goldenlight
12-23-2012, 23:53
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?

The elevator does the same amount of work, because the total mass of the elevator remains constant.

You can't change the laws of physics, just by jumping up in the air for a moment.

Louisville Glocker
12-24-2012, 00:40
With perfect efficiency, the work done simply equals the change in potential energy, so it doesn't matter what you do in the elevator.

But I suspect, in reality, a small amount of energy will be lost during the jump and impact, simply due to the imperfections in the cable linkage, and the elevator will have to do a tiny bit more work. Negligible.

I teach physics (PhD), but don't work on elevators....

And yeah there have been some errors by previous posters in this thread, but I won't call you out.

p.s. My favorite elevator problem involves a monkey hanging from a chandelier in an elevator.

NorthCarolinaLiberty
12-24-2012, 01:02
I usually take the stairs.

686Owner
12-24-2012, 01:45
delete

686Owner
12-24-2012, 01:47
delete

airmotive
12-24-2012, 06:09
Oooo...Free Lunch in an elevator.

TINSTAAFL!
(There Is No Such Thing As A Free Lunch)

gwalchmai
12-24-2012, 06:48
What if you have a bunch of helium balloons tied to you?

686Owner
12-24-2012, 06:56
What if you have a bunch of helium balloons tied to you?

You add their mass to the elevator.

hogfish
12-24-2012, 11:12
With perfect efficiency, the work done simply equals the change in potential energy, so it doesn't matter what you do in the elevator.

But I suspect, in reality, a small amount of energy will be lost during the jump and impact, simply due to the imperfections in the cable linkage, and the elevator will have to do a tiny bit more work. Negligible.

I teach physics (PhD), but don't work on elevators....

And yeah there have been some errors by previous posters in this thread, but I won't call you out.

p.s. My favorite elevator problem involves a monkey hanging from a chandelier in an elevator.

As for the monkey in the elevator, the answer is suicide, but I don't know who put the chandelier in the elevator. :supergrin:

hogfish
12-24-2012, 11:16
You add their mass to the elevator.

They contain helium. Surely that must account for something, no?

hockeyrcks9901
12-24-2012, 21:31
In an ideal world, nothing changes. You've imparted a force on the elevator when you jumped (to move up) and the elevator imparted a force on you when you landed (to stop your downward movement).

Work = force * displacement

They contain helium. Surely that must account for something, no?

The mass would decrease. The value of that decrease is equal to the mass of the air displaced by the balloons minus the mass of the balloons with helium.