Quick question on work and energy

  • #1
I thought of this recently, and want to know if there's something I'm missing.

So, say a very fragile object decelerates to a stop from a given velocity. For example, a free-falling object hitting the ground.

But, all the kinetic energy is absorbed through use of some clever mechanics (assume magic for this part).

If no energy was transmitted to the object, and (therefore) no work was done on it, would the fragile object still be harmed?
 

Answers and Replies

  • #2
russ_watters
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Welcome to PF!

If a fairy could play baseball, what would her batting average be? I'm sorry, but if the scenario is "magic", there is no possible useful answer that science can provide. In Star Trek, they explain away the effects of absurdly high acclerations that would turn people into thin puddles of goo with the words "inertial dampers".

Can you rephrase it into something a little more physically realistic? Maybe a shock absorber or an airbag to stop the fall...?
 
  • #3
Sure, I guess. It has absurdly efficient shock-absorbing gear that absorbs all the energy.
Honestly, this is more of a conceptual question, so I didn't feel like an explanation was necessary.
 
  • #4
russ_watters
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Well, an airbag is very good at evenly absorbing a lot of energy from a falling object without damaging it.
 
  • #5
Dale
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If no energy was transmitted to the object, and (therefore) no work was done on it, would the fragile object still be harmed?
Energy per set isn't what harms fragile objects. Stress is what harms them.
 
  • #6
A.T.
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all the kinetic energy is absorbed
If all the object's KE is absorbed by something else than the object, then the object obviously won't get damaged.

and (therefore) no work was done on it,
Negative work is still being done on the object, to extract energy from it.
 
  • #7
Extract energy from it? I don't quite understand...
 
  • #8
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An egg, falling at terminal velocity, has kinetic energy. If it hits the ground where does the energy go? If it hit an air bag and didn't break?
 
  • #9
Dale
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Extract energy from it? I don't quite understand...
When it is in motion, it has KE > 0. When it is stopped, it has KE = 0. To go from one to the other requires energy to be removed.
 
  • #10
A.T.
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Extract energy from it? I don't quite understand...
The object has initially some KE. After the object stopped it has no KE. The KE must have been removed by doing negative work on the object.

The only question here is where the extracted energy goes. If it goes into deformation energy of the object, it will break. If can be be transferred away from the object it will survive.

 
  • #11
Ah. So then, could this be extended to a human as well? Say, if an airbag was surreptitiously strapped to someone's feet and they were shoved off a cliff (for science, of course), and all their kinetic energy was absorbed, they'd be fine?
 
  • #12
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The air bag is much larger than a person in order to gently slow them down, so it is usually the large target on the ground with a smaller "sweet spot" in the middle where it is most effective.
 
  • #13
A.T.
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Say, if an airbag was surreptitiously strapped to someone's feet and they were shoved off a cliff (for science, of course), and all their kinetic energy was absorbed, they'd be fine?
Depends on the size of the airbag. In reality you will never absorb all the KE, but you can reduce the amount that goes into the body. A good way to protect a human from impact damage is to put him into a capsule filled with water.
 
  • #14
CWatters
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Ah. So then, could this be extended to a human as well? Say, if an airbag was surreptitiously strapped to someone's feet and they were shoved off a cliff (for science, of course), and all their kinetic energy was absorbed, they'd be fine?

Not necessarily... As AT said... it depends on the size of the airbag.

If you used a small spherical airbag say 2ft in diameter the stopping distance would have to be less than 2ft. In other words after jumping off they would get faster and faster until the bottom of the airbag hit the ground. At that point they would going fast and only be 2ft off the ground. Then they would have to decelerate from a high speed to zero in less than the remaining 2 feet. The rate of deceleration ("g force" if you prefer) would have to be very high - like a car stopping from perhaps 100mph in less that 2 feet! Their internal organs might squish against the inside of there chest!

If you use a much larger air bag the stopping distance could be longer and the g forces lower. They are more likely to survive.

Stunt men sometimes leap off buildings onto air bags placed on the ground. These are usually quite deep so that the "stopping distance" isn't too short. Even so they still experience quite high g forces which is why they try and land flat on their back...

 
  • #15
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That video is 3 minutes of exponentially increasing adrenaline.
 
  • #16
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a capsule filled with water
Essentially you'd need as much water to travel through as if you fell and hit a pool but without the *smack* of transitioning from air to liquid? Wouldn't that be capable of a greater rebound back like a shockwave?
 
  • #17
A.T.
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Essentially you'd need as much water to travel through as if you fell and hit a pool
No. You would barley move through the fluid, if you adjust its density to match the density of the body. The capsule would have to be just big enough for the human to fit in.


 
  • #18
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That just doesn't sound right to me. Say you have centimeters of water in front of and all around you and you hit the ground and come to an almost complete stop (barely move as you put it) instantly while "protecting the human" who experiences very high g's in this sudden stop.
 
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  • #19
A.T.
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That just doesn't sound right to me.
Consider a plastic bag filled with a liquid floating in a capsule filled with the same liquid. Now the capsule undergoes high acceleration, but doesn't break or deform significantly. Why should the liquid in the bag move relative to the rest of the liquid?

The human body is obviously not just a bag of liquid, so high pressure gradients / shock waves propagating though it can cause internal damage too. But the body will not suddenly start moving fast though an accelerated liquid of same density to hit the capsule wall.
 
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  • #20
So, if rapid deceleration harms the body upon impact, is there any way to circumvent this? I had another thought where the person would be standing on some mechanical contraption that converted the downward direction of deceleration into a rotation - rather like an odd cross between a hamster wheel and a unicycle. This would then be submerged in some very viscous fluid. As the person landed, the impact of hitting the ground, while not decelerating them initially, would cause them to begin rotating about the center axis of this device, and the fluid would absorb energy and acceleration through friction. This would (hopefully) cause the human to not experience any rapid deceleration, and absorb all the kinetic energy as well - but is something like this even possible? I feel like if it was, it would've been done before...

As to the previous posts regarding the bag of liquid, could this be practically utilized as a sort of 'covering'? That is, could a thick shell be made where the body is submerged in liquid, and this would absorb all the acceleration?

Thanks for the responses so far, I'm learning a lot.
 
  • #21
russ_watters
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An olympic ski jump does that. Their run starts around 140m off the ground and the launch point 85m. They land on a slope 25m above the ground. If not for the slope, the 60m fall would probably kill them. Instead, the slope lightly catches them, then curves gently to covert the downward motion to horizontal motion. The key is having a gentle enough - and therefore wide enough - curve to handle the deceleration standing up.
 
  • #22
A.T.
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I had another thought where the person would be standing on some mechanical contraption that converted the downward direction of deceleration into a rotation - rather like an odd cross between a hamster wheel and a unicycle.
Yes, using a long curved (circular) braking path might be more place saving than a long linear breaking path. But it's the same principle: you use a long duration to slow down, thus reducing the force.

As to the previous posts regarding the bag of liquid, could this be practically utilized as a sort of 'covering'? That is, could a thick shell be made where the body is submerged in liquid, and this would absorb all the acceleration?
It doesn't absorb the acceleration. The body does undergo s huge acceleration, but the force is applied such that the acceleration is relatively uniform across the body, keeping the internal stresses relatively low. Short of science fiction stuff (like the "inertia dampers" from Star Trek), buoyant force seems like the best option for applying a huge acceleration to the body.
 
  • #23
Where is the energy transferred once the viscous liquid absorbs it?

Could transfer the energy as exhaust after using the rotational motion to spin a gyroscope powering a generator...

The energy needs to be transferred from the system with the human to the system without the human.
 
  • #24
sophiecentaur
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I am trying to come to terms with the idea of converting linear momentum to angular momentum and I can't quite see how it would help. Either way, it's a matter of dissipating Kinetic Energy. Would a rotation make this any easier?
You have to slow down all of your body (inside bits too) at nearly the same rate. A G suit will protect you from the effects of blood sloshing down to your feet and away from your brain as you pull out of a dive because it encases your body tightly restricting the ability to expand and keeping blood up in your head. A rigid fluid sac would achieve the same thing but it would not stop more dense organs from sinking down and damaging tissues.
It really would depend upon what sort of spec you actually needed and how large your protective 'egg' could be. The longer time you can spread the effects of the acceleration, the less force is involved. (The crumple zone in a car does it for relatively mild impacts.)
 
  • #25
A.T.
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Either way, it's a matter of dissipating Kinetic Energy. Would a rotation make this any easier?
If you have limted space, yes.
 

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