Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jartza on 27/10/2010 23:43:07

When I am climbing a ladder in a gravity field, my chemical energy is turning into potential energy.
When I am climbing a ladder in a accelerating rocket, what energy is my chemical energy turning into?

Kinetic energy.
If the rocket stopped accelerating and you kept climbing, you'd accelerate ahead of the rocket. The problem is that the rocket is burning fuel and gaining kinetic energy from that burn at a faster rate than you can due to your climbing, so it catches up with you every time you push off the ladder.
Disclaimer: You could complicate matters by bringing up general relativity. General relativity would have a much more complicated answer, since energy of a gravitational field is a murkier concept...

Oh yes kinetic energy.
But what if two identical rockets are accelerating and one astronaut in one rocket is climbing a ladder, while another astronaut in another rocket is staying at the floor. Then the astronaut stops climbing and the rockets stop accelerating. Now everything is moving at the same speed in the two rockets. Where did the climbing energy go?

one astronaut in one rocket is climbing a ladder,
How about the climbing astronaut radiates more heat?

one astronaut in one rocket is climbing a ladder,
How about the climbing astronaut radiates more heat?
Yes but he also does the climbing work, in addition to the heating work.

Oh yes the kinetic energy difference between climbing astronaut and the other astronaut increases all the time. When the climber stops quite large kinetic energy is transferred from the astronaut to the rocket.

Where did the climbing energy go?
I believe the energy went back to it's former potential state. Are your rockets in a frictionless, gravity free state? Or are they climbing out of orbit?

With two identical rockets and the astronaut climbing in one and not in the other, I think that when they run out of fuel their centers of gravity will be at the same height, while the one with the climber will have its center of gravity moved toward the nose cone a little.
In terms of understanding this sort of puzzle, it might be instructive to consider where the energy goes if two identical rockets are positioned nose to nose (no gravity field or on two rail cars) and turned on. They wouldn't move, so where did the energy go? I think Bill S nailed it.

Excuse me if this is a stupid question, but...
If one's chemical energy is being converted into kinetic energy, and that's helping you move up the ladder in the rocket that you're moving at the same speed as, is your kinetic energy (which is coming from chemical energy, being converted to body heat, as bill s. said) really going to change the center of gravity of a rocket that ways tons and tons more than you?

Vereava:
As I see it, you should consider to what extent the climbing astronaut actually moved up the ladder and the amount the climb pushed the rocket down. Keep in mind that the effect of the relative weights of the astronaut and the rocket is a ratio, so the change in the center of gravity would be small.
Steve

The muscles of the climbing astronaut do NOT cause the astronaut to gain more speed.
BUT the muscles of the climbing astronaut cause the astronaut to gain more SPEED ENERGY. (also known as kinetic energy)
Here is a table, on the left is the speed of the rocket, and on the right is the speed of the climber, all speeds are relative to inertial observer, an observer that is not in a rocket:
10 11
20 21
30 31
40 41
Now here are kinetic energies corresponding to those speeds: (the astronaut has mass 2 kg, and so does the rocket)
100 121
400 441
900 961
1600 1681
Here is the difference table, where we can see that the climber is gaining extra kinetic energy at constant rate:
21
41
61
81

You are forgetting that with every action, there is an equal and opposite reaction. Every step on the ladder you take is stealing kinetic energy from the rocket and giving it to you. It might be easier to visualize this if you imagine standing on the nose of your rocket and jumping. The rocket would slow down and you would speed up. Climbing the ladder would have the same effect, though the KE of the rocket as a whole system (you included) would remain the same. Thus it would require the same amount of energy to stop both rockets no matter what you are doing inside.
Going back to the ideas of some earlier posts: Couldn't a photon with even a miniscule mass explain dark energy? Wouldn't it cause the red shifting that we see from all distant objects that we use as the fundamental measurement of the relative speed of said object?

You are forgetting that with every action, there is an equal and opposite reaction.
Who? Me? Am I really forgetting that? [;D]
By the way there is no reaction involved climbing up a ladder. [;D]
A 70 kg person standing on a ladder weighs 700N.
A 70 kg person climbing up a ladder weighs 700N.
See? [;D]
Welcome to the forum [:)]

Jartza. You say"there is no reaction involved climbing up a ladder." This is untrue and you can do the experiment yourself. Put a block of wood on a scale and stand on the scale next to it. Notice the weight registered. Step up onto the block and the measured weight will go up while you are pushing down on the block to raise yourself. Equal and opposite reaction. Steve

Jartza. You say"there is no reaction involved climbing up a ladder." This is untrue and you can do the experiment yourself. Put a block of wood on a scale and stand on the scale next to it. Notice the weight registered. Step up onto the block and the measured weight will go up while you are pushing down on the block to raise yourself. Equal and opposite reaction. Steve
Yes, but a guy climbing up a ladder in an accelerating rocket is just continuously shifting his weight from a step to another step, increase of weight on one step is canceled out by decrease of weight on another step.
Climber is not stealing energy from rocket.
It's more like climber is helping the rocket to accelerate, because in the end
climber's chemical energy has been turned into rocket's kinetic energy, when the climber has bumped on the ceiling, or otherwise stopped climbing.

Jartza:
You are focused on the total mass of the rocket, including the climber. Just think it through and include the energy of the climbers leg thrusting down against the ladder, and thereby the rocket, to move himself up. Action and reaction. Each step acts against the rocket thrust by temporarily adding the force of the leg thrust to the mass of the climber.
Another way to look at this is to alter the forces involved. Suppose that the rocket was very small and could only lift a platform, a ladder, and a very large and strong ladder climbing expert that is more than half the overall mass of the whole contraption. As the rocket lifts slowly to one meter the climber races up the ladder thrusting it back to the ground.
Steve

Yes, but "OUR" climber uses his leg to push the ladder with a force that is the same as the climbers "weight".

Yes, but "OUR" climber uses his leg to push the ladder with a force that is the same as the climbers "weight".
I don't think that is correct. If his leg only exerted sufficient force to support his weight, he would not be able to climb the ladder. He has to supply additional force to accelerate his mass.

Has anyone considered that it might be a little bit difficult to climb up a ladder while being subjected to 36g? The original question did state that the rocket was accelerating in a gravity field. I am assuming however that this accelerating rocket is attempting to reach escape velocity of 11.2km/second. [;D]

Has anyone considered that it might be a little bit difficult to climb up a ladder while being subjected to 36g? The original question did state that the rocket was accelerating in a gravity field. I am assuming however that this accelerating rocket is attempting to reach escape velocity of 11.2km/second. [;D]
Knowing the MO of the OP, I would say he is not interested in escape velocity w.r.t. this question.
It seems hard enough to explain the simplest concepts of freespace acceleration to the OP without the need to bring gravity or escape velocities into it.

Jartza  force = mass x acceleration.
For the climber to go from one step to the other he must accelerate upwards, and this requires a force. This force is imparted downwards onto the step, and is in addition to the force of weight that is also imparted onto the step.
As the climber transfers his weight to the higher step the force imparted on it will only be his weight, as the kinetic energy gained by exerting a force downwards onto the previous step will have been transformed into potential energy, and there is no other force anywhere, until he steps up again.
Yes, but "OUR" climber uses his leg to push the ladder with a force that is the same as the climbers "weight".
No not at all. The man's bone's exert a force downwards equal to the man's weight, and the ladder exerts an equal and opposite force upwards.
Only when the climber moves do the muscles exert force, and this is in addition to the weight.
OK, this is slightly simplified as our bones do not lock out fully and some muscle is need to maintain posture, but the principle is the same. To move more force must be put into the system.
Without a net force there is no acceleration. Newton's laws of physics. For the man to accelerate upwards the force on the step of the ladder must be larger than the weight of the man. The ONLY way this is possible is that this extra force comes from his muscles.
That is indeed the reason why we have muscles. [;)]
Yes, but a guy climbing up a ladder in an accelerating rocket is just continuously shifting his weight from a step to another step, increase of weight on one step is canceled out by decrease of weight on another step.
Wrong. I can climb up a ladder, and have one foot on one step and one foot on another. I can continually shift my weight from one to the other by changing my posture slightly without going anywhere. Just as I can shift my weight from side to side whilst standing with my legs together without going anywhere.
If there were no other forces apart from decreasing on one step cancelled out by another then the man cannot be moving. If he is, then there are. The force on the step will be larger than his weight so long as he is pushing on it to lift himself up.
Climber is not stealing energy from rocket.
It's more like climber is helping the rocket to accelerate, because in the end
climber's chemical energy has been turned into rocket's kinetic energy, when the climber has bumped on the ceiling, or otherwise stopped climbing.
No. If the climber goes from the bottom of the rocket to the top then he must have travelled faster than if if he had stayed at the bottom.
The total kintetic and potential energy of the manrocket system is the same regardless of what the man does as long as he holds on. But if the man climbs up the rocket he gains more potential energy than he would otherwise, and the rocket gains less potential energy. The man will be 20 metres higher than if he didn't climb, the rocket will be a few centimetres lower.
The same applies in a boat. If the boat is accelerating, but you walk forward in the direction of travel the boat does not accelerate as much. This is because you are exerting a backwards force on the boat, to which it must equally respond. So the some of the boat's engine power goes into accelerating you and not the boat. But you give that power back as soon as you stop walking (unless you fall off the front of the boat).

All of you guys,
How come climbing energy is height times WEIGHT?
Anyone want to correct me on these statements:
A 70 kg person standing on a ladder weighs 700N.
A 70 kg person climbing up a ladder weighs 700N.
What are the real numbers?

Jartza:
You are very confused. Newton is a unit of force. A ~70kg astronaut standing on a ladder in a rocket before lift off exerts 700N of force downward on the ladder. This is because his mass is being accelerated downward by the earth's 1g. If the rocket takes off and accelerates upward at 1g, thereby increasing the total acceleration downward for the rocket and its contents to 2g, the force on the ladder exerted by the astronaut increases to 1400N. If he then takes a step upward on the ladder with the acceleration of 1/7g the force he exerts on the ladder increases to 1500N while he is pushing down.
Steve

Jartza:
You are very confused. Newton is a unit of force. A ~70kg astronaut standing on a ladder in a rocket before lift off exerts 700N of force downward on the ladder. This is because his mass is being accelerated downward by the earth's 1g. If the rocket takes off and accelerates upward at 1g, thereby increasing the total acceleration downward for the rocket and its contents to 2g, the force on the ladder exerted by the astronaut increases to 1400N. If he then takes a step upward on the ladder with the acceleration of 1/7g the force he exerts on the ladder increases to 1500N while he is pushing down.
Steve
Well of course.
Oh yes, I guess I should tell you that unit of weight is Newton.

Let's start again from the beginning.
There's an accelerating rocket in outer space, in the rocket there's Bob "climbing" a
ladder "upwards". Where does Bob's energy go?
Answer: The energy turns into kinetic energy of Bob.
Question 2: Bob stops climbing, where is Bob's energy now?
Obvious answer: Bob's kinetic energy is evenly spread into Bobrocket system.
Advanced considerations: Bob starts climbing, climbs, and stops climbing. This has zero
effect on Bobrocket system, probably. Where does Bob's energy go in this case?
Answer: The energy must be in the exhaust gases.

Jartza:
I, for one, am not going to keep chasing your changing questions. In twenty some posts several people have tried to help you understand where you are in error. However, you don't admit your errors but instead continue on your circle of questions. You stated the 700N man bit two times and have been corrected. Do you now admit that the force a man exerts on a ladder increases when he climbs?
Steve

A good point Steve. I feel thread locking time is fast approaching!

Seconded.

Jartza:
I, for one, am not going to keep chasing your changing questions. In twenty some posts several people have tried to help you understand where you are in error. However, you don't admit your errors but instead continue on your circle of questions. You stated the 700N man bit two times and have been corrected. Do you now admit that the force a man exerts on a ladder increases when he climbs?
Steve
So the ladder and the weight that a climber exerts on it ... Do you
want to discuss that?
My opinion is that when this ladder is standing on the surface of the
earth, the force is m*g , m is mass of the climber, g is gravitational acceleration.

Jartza:
Just answer the question Do you now admit that the force a man exerts on a ladder increases when he climbs?
Steve

Elfabyanos, can you explain to me why fly in an plane behaves like this:
The only way the fly can change the weight of the plane is by either altering the mass of itself, the air or the plane, or by altering the strength of gravity. The fly cannot do this, therefore it cannot affect the weight.
But guy on a ladder is a different thing altogether?

jartza  A repeated refusal to directly answer the majority questions put to you about why you can't accept the most basic (of schoolchild level) mechanics and a purposeful effort to not even attempt to learn for yourself  has lead to the decision to lock this thread.
If you feel that you have a valid argument for reopening this, then you can PM to explain why you are unable to do these things (and appear to expect others to work for you  work that you then reject).
I apologise for any other members who are inconvenienced by this, but enough is enough!