Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: gerardseal on 16/12/2021 15:14:57
-
I have been trying for a very long time to task this problem, but it does not work for me.
Can anyone help me?
You are on the roof of the physics building, 46.0 m above the ground. Your physics professor, who is 1.80 tall, is walking along side the building at a constant speed of 1.20 m/s. If you wish to drop an egg on your professor's head, how far from the building should the professor be when you release the egg? Assume that the egg is in free fall.Take the free fall acceleration to be 9.80 m/s2.
Thank you all very much for your answers! I also found on <link removed> a detailed description of the solution to this problem. I needed accurate calculations.
-
I have been trying for a very long time to task this problem, but it does not work for me.
Can anyone help me?
It is traditional on homework problems for the OP to show some effort, so that help can show the point at which the understanding is incorrect.
Nevertheless, the problem as worded is trivial.
You are on the roof of the physics building . . . Your physics professor . . . is walking along side the building at a constant speed . . . If you wish to drop an egg on your professor's head, how far from the building should the professor be when you release the egg?
That's easy, without any additional information, he needs to be the distance from the edge from which the egg is dropped, which is about an arms length if you hold the egg out. Since he's walking along side the building, his distance from it never changes.
As for the acceleration, friction, height of building or professor, none of these factors matter. They only affect when you should release the egg, not the distance from the building the prof maintains as he walks along side it.
-
And the drop time depends on the freshness and species of the egg. With a 44.2 m drop a stale hummingbird egg will reach a constant terminal speed but a fresh ostrich egg will probably still be accelerating when it hits him.
-
And the drop time depends on the freshness and species of the egg. With a 44.2 m drop a stale hummingbird egg will reach a constant terminal speed but a fresh ostrich egg will probably still be accelerating when it hits him.
Or you can read the question.
Assume that the egg is in free fall.
.
But the bit of the question you really need to read is this.
walking along side the building
As Halc pointed out.
-
What is the wind resistance upon the egg? This is usually neglected but should not be. If you just throw the egg hard you probably stand a better chance, aim head on as he is walking toward your position as a face shot is always better. Aiming head on will also stop any errors in timing, he will be in the position for far longer
-
What is the wind resistance upon the egg?
The wind resistance on the egg is a thing you have been told to ignore.
Assume that the egg is in free fall
-
Or you can read the question.
Quote from: gerardseal on Today at 15:14:57
Assume that the egg is in free fall.
.
If that were the case then you and the professor would suffocate.
-
But the bit of the question you really need to read is this.
Quote from: gerardseal on Today at 15:14:57
walking along side the building
As Halc pointed out.
and answered it before I had read the question.
-
Or you can read the question.
Quote from: gerardseal on Today at 15:14:57
Assume that the egg is in free fall.
.
If that were the case then you and the professor would suffocate.
I'm sure that both of us have sat in an exam or schoolroom and made calculations assuming free fall conditions (Knowing that if we tried to "show off" and include it, we would get the wrong answer) without asphyxia.
Why would it be different now?
-
walking along side the building
The answer could be quantized if this statement is replaced with walking toward the building (assuming there's an open door to go through).
-
Thanks! But I'm not sure if I can write an answer in this form. Can you do this more mathematically?
-
Thanks! But I'm not sure if I can write an answer in this form. Can you do this more mathematically?
You are given some information that enables you to find the distance that the egg falls, and some information that connects the distance the professor walks with the time that the egg takes to fall. What can you do with all that? Have you learnt any equations with these variables in?
-
The answer could be quantized if this statement is replaced with walking toward the building (assuming there's an open door to go through).
Huh?
-
The answer could be quantized if this statement is replaced with walking toward the building (assuming there's an open door to go through).
Huh?
I don't think "quantized" is the right word.
But it could be made into a more sensible maths problem by making that assumption.
-
I'm sure that both of us have sat in an exam or schoolroom and made calculations assuming free fall conditions (Knowing that if we tried to "show off" and include it, we would get the wrong answer) without asphyxia.
Why would it be different now?
Because Torricelli performed his experiments before I was born, and airplanes fly. Youngsters may have seen experiments in free fall where the ambient air was also in free fall, in planes and spacecraft, and some pensioners will recall seeing a hammer and feather in free fall towards the moon. Admittedly those who took their exams after Aristotle and before Torricelli would have a different idea of free fall, but those whose education predated Galileo would have wanted to know the mass of the egg.
-
Thanks! But I'm not sure if I can write an answer in this form. Can you do this more mathematically?
The equations of linear motion are
v = u + at
s = ut + ½at2
v2 = u2 + 2as
where v = final velocity
u = initial velocity
a = acceleration
s = distance
t = time
So you can apply them to the egg and the professor. (Hint: you only need one of the equations)
But Halc's reply remains the truth.
-
I'm sure that both of us have sat in an exam or schoolroom and made calculations assuming free fall conditions (Knowing that if we tried to "show off" and include it, we would get the wrong answer) without asphyxia.
Why would it be different now?
. Youngsters may have seen experiments in free fall where the ambient air was also in free fall, in planes and spacecraft, and some pensioners will recall seeing a hammer and feather in free fall towards the moon.
I imagine if you dropped a feather at the edge of a tall build g it would immediatley rise due to the upward air effects.
-
Thus saving all the pointless effort that birds put into flying. But how do they come down to feed?
-
Thus saving all the pointless effort that birds put into flying. But how do they come down to feed?
Powerdive.
-
I'm sure that both of us have sat in an exam or schoolroom and made calculations assuming free fall conditions (Knowing that if we tried to "show off" and include it, we would get the wrong answer) without asphyxia.
Why would it be different now?
Because Torricelli performed his experiments before I was born, and airplanes fly. Youngsters may have seen experiments in free fall where the ambient air was also in free fall, in planes and spacecraft, and some pensioners will recall seeing a hammer and feather in free fall towards the moon. Admittedly those who took their exams after Aristotle and before Torricelli would have a different idea of free fall, but those whose education predated Galileo would have wanted to know the mass of the egg.
My mum was a teacher and, when she retired she took to marking exams. That's not a very unusual story.
She talked about one of the more challenging aspects of the process- the issue of a candidate who failed to read the question but, while giving an answer to the wrong question, demonstrated that they had a good understanding of the topic that was being tested.
If my memory serves me, it's called "departure from rubric",
I wonder how often you called it into play (presumably unknowingly).
-
I'm sure that both of us have sat in an exam or schoolroom and made calculations assuming free fall conditions (Knowing that if we tried to "show off" and include it, we would get the wrong answer) without asphyxia.
Why would it be different now?
. Youngsters may have seen experiments in free fall where the ambient air was also in free fall, in planes and spacecraft, and some pensioners will recall seeing a hammer and feather in free fall towards the moon.
I imagine if you dropped a feather at the edge of a tall build g it would immediatley rise due to the upward air effects.
Like a stopped clock, even Petrochemicals is occasionally right, and I think this may be one of them.
-
If my memory serves me, it's called "departure from rubric",
I wonder how often you called it into play (presumably unknowingly).
Once, knowingly, during a viva. The examiner got very interested, the exam overran, and we ended up with a publication.
-
As for air blowing up the side of a tall building, what goes up must come down, so whether the feather rises or falls depends on the wind direction. Large birds and skilled glider pilots are very good at soaring on the windward side but the air downwind is best avoided: there's a turbulent "rotor" close to the structure and possibly a smooth downdraft a little further back. The smooth wave can often be detected at 5 times the height of the obstruction.
Anyway, if we drop an egg it will take about 3 seconds to reach the prof's head in vacuo (the ambient, not the prof's head). If we use a small egg, with a large surface/mass ratio, and real air, we might be looking at 5 seconds fall time, which could result in a considerable lateral displacement if there is any wind. The gentlest of breezes (1 m/s, 2 mph) would pretty well guarantee a miss first time. Which is why it takes 6 months to turn a competent marksman into a reliable sniper.
-
The answer could be quantized if this statement is replaced with walking toward the building (assuming there's an open door to go through).
Huh?
I don't think "quantized" is the right word.
But it could be made into a more sensible maths problem by making that assumption.
Perhaps the word "quantified" is more appropriate here.
-
Throw lots of eggs, observe how each one falls and correct for the extent by which you miss.
There's not much the prof can do about it until he gets into the building, or escapes beyond your reach.
That's why the military always like "the high ground".
Beware of professors with umbrellas or mobile phones.
-
Stories of long sniper kills mostly have a maximum of 3 shots - the target tends to get suspicious after the second one. To quote James Bond: "Once is happenstance. Twice is coincidence. Three times is enemy action."
So set yourself up with an anemometer and wind vane (physics) rather than a bucket of eggs (engineering). Add a stopwatch if you are using small eggs.
And don't presume that the professor is male - you can get into serious trouble for using the word "he" these days.
-
And don't presume that the professor is male - you can get into serious trouble for using the word "he" these days.
I didn't "assume" it; I deduced it from.
"Your physics professor, who is 1.80 tall".
I'm not saying that's a perfect proof, but it gave me enough evidence to call it on the balance of probabilities.
Stories of long sniper kills mostly...
... are irrelevant if you are effectively using a machine gun.
-
To quote James Bond: "Once is happenstance. Twice is coincidence. Three times is enemy action."
It was said to Bond, not by him.
-
If you are using a machine gun, you don't usually
observe how each one falls
-
If you are using a machine gun, you don't usually observe how each one falls
https://en.wikipedia.org/wiki/Tracer_ammunition
-
The equations that Alan quotes are a good approximation for bodies falling a few hundred meters but would only be accurate if the Earth was flat and of infinite extent.
As the distance from the centre of the Earth increases the force of gravity falls off otherwise the bodies would tend to acquire infinite energy !
These equations caused me considerably confusion when they were first foisted on me at school but I know better now
-
The equations that Alan quotes are a good approximation for bodies falling a few hundred meters but would only be accurate if the Earth was flat and of infinite extent.
As the distance from the centre of the Earth increases the force of gravity falls off otherwise the bodies would tend to acquire infinite energy !
These equations caused me considerably confusion when they were first foisted on me at school but I know better now
was Jules Verne your physics teacher?
-
https://en.wikipedia.org/wiki/Tracer_ammunition
So 1 in 5 eggs should have a magnesium shell. I'll have a chat with the chickens.
-
The equations that Alan quotes are a good approximation for bodies falling a few hundred meters but would only be accurate if the Earth was flat and of infinite extent.
No, they are absolute mathematical definitions, nothing to do with gravitation or any other aspect of practical physics. It happens that at 0 - 42m potential difference from the surface of the earth, a = g = constant to a very close approximation.
-
https://en.wikipedia.org/wiki/Tracer_ammunition
So 1 in 5 eggs should have a magnesium shell. I'll have a chat with the chickens.
Good luck with getting them to deliver the eggs at about the speed of sound.
-
Tracer shells are not manufactured by a process that would cause them to ignite in the factory.
You should visit a farm sometime: chickens don't actually squeeze pickled eggs out of their backsides, but you can control various aspects of the egg by breeding and feeding.