A question about humans and acceleration

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  • #1
Sundown444
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I have been wondering, and curious as well, and thus I would like to ask: Generally, are acceleration as in reaching top speed or zero speed from top speed and acceleration in terms of changing direction equally easy for human beings at average and normally, or is changing speed easier than changing direction or vice versa?

Don't get me wrong, I know it is a possibility that humans can achieve both easily; wait, scratch that! I mean more of a certainty than a possibility, maybe, but even then, I have always thought there was a difference, with acceleration being an increase in speed over time, with speed being distance covered over time, and turning; as in centripetal acceleration, as I am describing change in direction here; which is, velocity or speed squared divided by radius. That has made me think there is a difference in the past. Of course, I could be wrong, I know.

But the question I want to ask is mainly this: Which, in terms of acceleration do humans find to be easier to do than the other? Is it change in speed, change in direction, or are both equally easy? Since this is a human doing the acceleration, I would say we are talking about a human changing speed or direction as they are running here. I am asking because I just want to make sure.
 

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  • #2
kuruman
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Why are you asking? Do the experiment on yourself and see which is easier for you. Of course, humans differ in their motor skills so what's easy for one person might not be as easy for someone else. Watch pass receivers in American football and the defensive players who try to cover them as they change speed and direction. Some are good, some are not so good and the ones who are good make the big bucks.
 
  • #3
DaveC426913
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Change in direction is change in speed.

If you're heading West, and suddenly you're heading North. it's because you've accelerated Northward while also accelerating Eastward.

(We might want to start talking about velocity - which is speed with direction.)
 
  • #4
Sundown444
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Why are you asking? Do the experiment on yourself and see which is easier for you. Of course, humans differ in their motor skills so what's easy for one person might not be as easy for someone else. Watch pass receivers in American football and the defensive players who try to cover them as they change speed and direction. Some are good, some are not so good and the ones who are good make the big bucks.

Like I said, as I have edited into my last post, I just wanted to make sure. I could do the experiment myself, but I just wanted an answer because I thought it would be better to get some, say, expert help. I hope this isn't doing anything bad.
 
  • #5
kuruman
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I would say that humans find it easier to do that which requires the least force. If you need more force to move faster in the forward direction than to change direction by running around a circle, then it's easier to change direction than change speed. However if the radius of the circle is too small, then changing your forward speed will be easier. So how much force you need (and can) apply in a given direction is the determining factor. I would say that your question does not have a binary answer.
 
  • #6
Sundown444
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I would say that humans find it easier to do that which requires the least force. If you need more force to move faster in the forward direction than to change direction by running around a circle, then it's easier to change direction than change speed. However if the radius of the circle is too small, then changing your forward speed will be easier. So how much force you need (and can) apply in a given direction is the determining factor. I would say that your question does not have a binary answer.

That is fine, and in fact; that may be the kind of answered I was looking.

Still, I must say as another reason why I asked; while changing speed and changing direction are part of the same thing, the human body, as I know, is not built to handle everything, not equally, anyway. With how the human body is built and what it can and can't do, I hope there was no harm in asking out of curiosity .
 
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kuruman
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No harm at all. At PF we appreciate curiosity.
 
  • #8
Sundown444
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No harm at all. At PF we appreciate curiosity.

Ah, good. One more question, though: About running around in a non-tight circle and running forward, and a tighter circle scenario, too, is that with the same force for each or a different force for each? Either way, I think whatever the answer is will be good enough for me.
 
  • #9
kuruman
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When you run around a circle you need a component of some force in a direction towards the center of the circle, the so-called centripetal component. On a flat track, friction does the trick and cleats enhance friction. On a skating rink there is hardly any friction. To turn, skaters lean into the turn and the horizontal component of the force exerted by the ice on their skates becomes the centripetal component. To accelerate forward, skaters push against the ice at an angle relative to the forward direction so that the the force exerted by the ice on the skates has a forward component. This is like a sailboat tacking into the wind.
 
  • #10
Sundown444
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When you run around a circle you need a component of some force in a direction towards the center of the circle, the so-called centripetal component. On a flat track, friction does the trick and cleats enhance friction. On a skating rink there is hardly any friction. To turn, skaters lean into the turn and the horizontal component of the force exerted by the ice on their skates becomes the centripetal component. To accelerate forward, skaters push against the ice at an angle relative to the forward direction so that the the force exerted by the ice on the skates has a forward component. This is like a sailboat tacking into the wind.

Alright, I think I get it. Thank you.
 
  • #11
PeroK
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Summary:: It's a question about what humans are capable of, acceleration wise.

I have been wondering, and curious as well, and thus I would like to ask: Generally, are acceleration as in reaching top speed or zero speed from top speed and acceleration in terms of changing direction equally easy for human beings at average and normally, or is changing speed easier than changing direction or vice versa?

Don't get me wrong, I know it is a possibility that humans can achieve both easily; wait, scratch that! I mean more of a certainty than a possibility, maybe, but even then, I have always thought there was a difference, with acceleration being an increase in speed over time, with speed being distance covered over time, and turning; as in centripetal acceleration, as I am describing change in direction here; which is, velocity or speed squared divided by radius. That has made me think there is a difference in the past. Of course, I could be wrong, I know.

But the question I want to ask is mainly this: Which, in terms of acceleration do humans find to be easier to do than the other? Is it change in speed, change in direction, or are both equally easy? Since this is a human doing the acceleration, I would say we are talking about a human changing speed or direction as they are running here. I am asking because I just want to make sure.
It's easier to stop than to change direction. You just fall onto the ground and you'll stop quickly.
 
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  • #12
sophiecentaur
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The value of acceleration that's achievable and its effect on you will depend on circumstances.

Run into a tree and your (negative ) acceleration can be very high but lasting for a very short time. Foot flat on the accelerator pedal can give you moderate acceleration but only last until you are near the car's top speed (less than a minute) and it will be somewhat less than g. Sitting in a centrifuge (as with astronauts etc.) can give you several g for as long as the machine is running. The accelerations in all cases are actually the same thing - just different values and directions.
 
  • #13
Lnewqban
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Still, I must say as another reason why I asked; while changing speed and changing direction are part of the same thing, the human body, as I know, is not built to handle everything, not equally, anyway.
Please, see:
https://en.m.wikipedia.org/wiki/G-force#Human_tolerance

1728px-Human_linear_acceleration_tolerance.svg.png
 
  • #14
berkeman
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Summary:: It's a question about what humans are capable of, acceleration wise.

I have been wondering, and curious as well, and thus I would like to ask: Generally, are acceleration as in reaching top speed or zero speed from top speed and acceleration in terms of changing direction equally easy for human beings at average and normally, or is changing speed easier than changing direction or vice versa?
I was about to post the same as @Lnewqban but he beat me to it (as usual). :smile:

Beyond simple comfort issues, the real limits to human tolerance of acceleration in different directions is based on our cardiovascular (CV) system. If the acceleration is too high in the wrong direction with respect to our body position, the blood pressure (BP) will drop in our head leading to GLOC and eventually death (one way or another).

So for situations that are important for humans to be able to survive high-g situations for short periods (fighter aircraft pilot/RIO, astronaut launches, etc.), the vehicle ergonomics and accessories (like g-suits) are designed to help mitigate the CV problems and maintain reasonable BP delivered to the brain.
 
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Lnewqban
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I had to look this one up: :smile:

GLOCGravity-Induced Loss of Consciousness (also seen as G-LOC)
 
  • #17
hutchphd
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One of my favorite crazy people Dr John Stapp:

 
  • #18
A.T.
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Since this is a human doing the acceleration, I would say we are talking about a human changing speed or direction as they are running here.
Regarding the achievable accelerations by own power:
You could compare the max. straight line accelerations achieved by top sprinters to the max. centripetal accelerations observed during cutting maneuvers in professional ball sports.

Regarding the tolerance by the human body:
Cutting maneuvers have more risk of injury at the leg. But neither type of acceleration by own muscle power reaches the tolerance levels for loss of consciousness, which depend on the pose and orientation of the body relative to the proper acceleration. But the tolerance does not depend on whether you are accelerating in a straight line or along a circle.
 

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