A physics problem.

[91TTZ]

Originally posted by: opticalmace
Turkz: The authors are correct, because this is based on observation. Certainly he knows that he is inside a steel ball, and knows he is on Earth, and is going down a waterfall. With that information, and his previous knowledge of gravitation on Earth, he knows his velocity will increase until he hits the water.

However, based on observation from his perspective only during the freefall, it is impossible to determine increasing velocity. What the authors are saying is that if he were magically born into a steel ball in this situation, he would be unable to determine if the ball had no velocity, in space, or if it were under constant acceleration, towards another body. He is weightless in both cases..

I think it's unrealistic to read a word problem like this and then ignore information that they gave you. I completely agree with this part of your post: "Certainly he knows that he is inside a steel ball, and knows he is on Earth, and is going down a waterfall. With that information, and his previous knowledge of gravitation on Earth, he knows his velocity will increase until he hits the water."

I think that right there solves the problem. To take information out of the problem, such as the part about being magically born into a steel ball and not knowing where he is, makes the problem unrealistic.

 

[91TTZ]

Originally posted by: Chaotic42

Originally posted by: Turkz
once he is in free fall, he is unaware of his increasing speed because the acceleration during the fall was constant.

It's been a while since my last physics class, but that seems to be the key there. Once he is in free fall, his acceleration is constant. You only feel jerk, you don't feel and change during constant acceleration. That is, once his free body diagram only has a down arrow of magnitude g, he will not feel an increasing "falling feeling", even if he accelerates to 20Km/s.

And since he feels the jerk and feels the g's, he knows that he's in freefall. The occupant's sense of equilibrium and sense of time is enough to solve this problem.

If you took a steel ball, equipped with only an accelerometer and a clock, and started recording the moment you dropped that ball, you could tell what the ball's speed is. That's because the other factors are known. It knows that if you accelerate at x g's for a duration of y, you must be going a certain speed.

 

[DrPizza]

For what it's worth, a human body does not feel velocity. (Or, to put it another way, are you aware that you're moving around the milky way at 10's of thousands of miles per hour? And, that you're on the surface of the earth which is spinning around?) Also, I could put you in a car with a blindfold on and headphones to mute the engine noise - in such a situation, you would not be able to tell the difference between moving at a constant velocity uphill (after traveling on level ground) and accelerating on level ground. All you would be able to do is detect that a change occurs.

edit: look at motion simulators in those theaters that have the chairs that tip back and forth while the image moves on the screen to simulate you going up or down hill, left and right. You'd swear you were actually moving and accelerating.

 

[rikadik]

Surely once he's in freefall, the only force acting on him is gravity, which is constant, so the author is correct?

 

[BrownTown]

Sometimes when I read this forum i get a little depressed about how so many well educated people are still able to get simple problems like this wrong. After the man falls over the cliff and until he hits the water the force acting on him is the same. There is no sensory input available to such a man to indicate his absolute speed. It is the FORCE acting on the blood in your veins, or the fluid in your inner ear which would allow you to sense a change (like feeling the blood rush to your head when you go over the cliff). It is a change in the force (IE: a jerk, not an acceleration) that would be noticible.

 

[opticalmace]

Originally posted by: 91TTZ

Originally posted by: opticalmace
Turkz: The authors are correct, because this is based on observation. Certainly he knows that he is inside a steel ball, and knows he is on Earth, and is going down a waterfall. With that information, and his previous knowledge of gravitation on Earth, he knows his velocity will increase until he hits the water.

However, based on observation from his perspective only during the freefall, it is impossible to determine increasing velocity. What the authors are saying is that if he were magically born into a steel ball in this situation, he would be unable to determine if the ball had no velocity, in space, or if it were under constant acceleration, towards another body. He is weightless in both cases..

I think it's unrealistic to read a word problem like this and then ignore information that they gave you. I completely agree with this part of your post: "Certainly he knows that he is inside a steel ball, and knows he is on Earth, and is going down a waterfall. With that information, and his previous knowledge of gravitation on Earth, he knows his velocity will increase until he hits the water."

I think that right there solves the problem. To take information out of the problem, such as the part about being magically born into a steel ball and not knowing where he is, makes the problem unrealistic.

Thing is, when one gets into what is 'realistic' and what is not, one gets into a game of assumptions. What may work for this example won't work in others. The goal of physics is to explain all phenomena (as far as I know), and so it makes sense to point out that during his freefall he is lacking sensory input, and thus cannot directly verify his increasing velocity--which is what I think they have done.

That said, the authors worded their response poorly for this problem. Obviously someone getting into a ball knows what's going to happen, just as a skydiver does. They should have asked if the man could absolutely, scientifically verify what was happening during his freefall.

 

[PowerEngineer]

Originally posted by: BrownTown
Sometimes when I read this forum i get a little depressed about how so many well educated people are still able to get simple problems like this wrong. After the man falls over the cliff and until he hits the water the force acting on him is the same. There is no sensory input available to such a man to indicate his absolute speed. It is the FORCE acting on the blood in your veins, or the fluid in your inner ear which would allow you to sense a change (like feeling the blood rush to your head when you go over the cliff). It is a change in the force (IE: a jerk, not an acceleration) that would be noticible.

Well put. It's the change in acceleration that the body can sense, not acceleration (or velocity) itself.

 

[DrPizza]

Originally posted by: PowerEngineer

Originally posted by: BrownTown
Sometimes when I read this forum i get a little depressed about how so many well educated people are still able to get simple problems like this wrong. After the man falls over the cliff and until he hits the water the force acting on him is the same. There is no sensory input available to such a man to indicate his absolute speed. It is the FORCE acting on the blood in your veins, or the fluid in your inner ear which would allow you to sense a change (like feeling the blood rush to your head when you go over the cliff). It is a change in the force (IE: a jerk, not an acceleration) that would be noticible.

Well put. It's the change in acceleration that the body can sense, not acceleration (or velocity) itself.

And, even then, the body can be fooled to think it feels a change in acceleration. (virtual reality theaters with seats that pivot and recline)

 

[3NF]

Originally posted by: DrPizza

Originally posted by: PowerEngineer

Originally posted by: BrownTown
Sometimes when I read this forum i get a little depressed about how so many well educated people are still able to get simple problems like this wrong. After the man falls over the cliff and until he hits the water the force acting on him is the same. There is no sensory input available to such a man to indicate his absolute speed. It is the FORCE acting on the blood in your veins, or the fluid in your inner ear which would allow you to sense a change (like feeling the blood rush to your head when you go over the cliff). It is a change in the force (IE: a jerk, not an acceleration) that would be noticible.

Well put. It's the change in acceleration that the body can sense, not acceleration (or velocity) itself.

And, even then, the body can be fooled to think it feels a change in acceleration. (virtual reality theaters with seats that pivot and recline)

But if the seats are pivoting and reclining, is there not a change in acceleration?

Let's make the problem more interesting and include air resistance, and don't treat the person as a point object

 

[dighn]

Originally posted by: PowerEngineer

Originally posted by: BrownTown
Sometimes when I read this forum i get a little depressed about how so many well educated people are still able to get simple problems like this wrong. After the man falls over the cliff and until he hits the water the force acting on him is the same. There is no sensory input available to such a man to indicate his absolute speed. It is the FORCE acting on the blood in your veins, or the fluid in your inner ear which would allow you to sense a change (like feeling the blood rush to your head when you go over the cliff). It is a change in the force (IE: a jerk, not an acceleration) that would be noticible.

Well put. It's the change in acceleration that the body can sense, not acceleration (or velocity) itself.

surely you can sense acceleration through a variety of means. a change in the way you weigh is a telling sign that your acceleration is modified, and the sense of orientation can tell you in which direction you are accelerating. it seems that the author is referring to very instinctual sensations of movement, but really "aware" can be interpreted to mean more.