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Physics Question pertaining to Velocity and Gravity

 
 
Reply Tue 9 Sep, 2014 06:46 pm
Two identical eggs of mass 0.1 kg are dropped from rest at height h. One is dropped on Earth and the other on the surface of the moon. Egg dropped on earth experiences a force due to air resistance (F=-kv) where v is velocity of egg and k is constant of proportinality equal to Ns/m. No forces of resistance on the moon.

Gravity on earth = 9.81 m/s^2 Gravity on moon = 1.62 m/s^2

What is terminal velocity of the egg falling through Earth’s atmosphere?
From what height will the eggs hit the ground at the same speed?
Should the value of k at a high altitude be >, <, or = to k at sea level and why?
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Type: Question • Score: 5 • Views: 1,676 • Replies: 19
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View best answer, chosen by jasonmoon
One Eyed Mind
 
  -4  
Reply Tue 9 Sep, 2014 06:49 pm
Wait, how does an egg drop on the surface of a moon? Doesn't it just float down?
jasonmoon
 
  1  
Reply Tue 9 Sep, 2014 07:45 pm
@One Eyed Mind,
No, there is still gravity acting on the moon, just at a weaker rate.
Re read the question plz
One Eyed Mind
 
  -1  
Reply Tue 9 Sep, 2014 07:47 pm
@jasonmoon,
No, I know that.

What I'm saying is, when something falls slowly, it's not being dropped down; it's floating down.
engineer
  Selected Answer
 
  2  
Reply Tue 9 Sep, 2014 08:10 pm
@jasonmoon,
1) Terminal velocity is where the egg no longer feels acceleration. That is the point that the air resistance matches the gravity. Force = mass x acceleration so:

9.81 m/s^2 x 0.1 kg = k v
v = 0.981/k

2) If you don't want to use differential calculus on this one, assume the earth egg is at terminal velocity and find out how long the moon egg has to go to reach the same velocity. The moon egg velocity = a t = 1.62 t. Set that equal to the terminal velocity above and you get t = 0.981/1.62/k = .605/k seconds.

The formula for distance traveled is H = 1/2 a t^2 = (0.5) (1.62) (.605/k)^2 =.296/k^2

3) The thicker the air, the greater the drag due to air resistance so you would expect k to be higher at sea level than at high elevations where the air is thin.
engineer
 
  2  
Reply Tue 9 Sep, 2014 08:12 pm
@One Eyed Mind,
One Eyed Mind wrote:

What I'm saying is, when something falls slowly, it's not being dropped down; it's floating down.

"Floating" means buoyed on something like air or water. It is not floating on the moon, just falling slowly at first then speeding up just like on Earth.
One Eyed Mind
 
  0  
Reply Tue 9 Sep, 2014 08:15 pm
@engineer,
"Floating down" is not the same as "floating". So for an example, someone let's go of a feather, it "floats down"; it does not "drop down".

This is a drop down:

|
|
|
______________________

This is floating down:

/

\

/

______________________
engineer
 
  2  
Reply Tue 9 Sep, 2014 08:15 pm
@One Eyed Mind,
On the moon, the feather will drop down since there is no air to float upon. Your icons describe if perfectly. On the moon, a feather will |, not /
One Eyed Mind
 
  0  
Reply Tue 9 Sep, 2014 08:27 pm
@engineer,
I know, but the example is not about whether an object sways back and forth towards the ground slowly, but rather the terminology for "drops down" is not used for objects that slowly falls to the ground, they say "float down". So by terminology, you would say "the egg floats down", not "the egg drops down". When you swim, you "float" on the surface and as you swim, because "floating" is not "falling".
0 Replies
 
engineer
 
  1  
Reply Tue 9 Sep, 2014 08:27 pm
@One Eyed Mind,
After 10 seconds on the moon, the feather, ball, whatever is dropping at 81m/s which is a pretty good clip. I don't think you would say anything dropping that fast was "floating". Also, "floating" really does refer to something being acted upon by a buoyant force, usually air or water.

Feather vs a Ball Bearing in air and in vacuum.

https://www.youtube.com/watch?v=_XJcZ-KoL9o
One Eyed Mind
 
  -1  
Reply Tue 9 Sep, 2014 08:31 pm
@engineer,
There's a reason why we use "drop down" "float down" and "sink down".

Things drop down because of "gravity". The moon has 1/6th of this planet's gravity, therefore... It cannot constitute the term "drop down", as nothing is causing objects to "drop down", but instead "float down".

Why is this so difficult to understand? This isn't semantics. This is their school making an error in terminology. Astronauts don't drop down when descending from the apex of their jump - they "float down".
engineer
 
  1  
Reply Tue 9 Sep, 2014 08:34 pm
@One Eyed Mind,
One Eyed Mind wrote:

Why is this so difficult to understand? This isn't semantics. This is their school making an error in terminology.

You might want to look up the term if this is all about terminology. https://www.google.com/webhp?sourceid=chrome-instant&amp;ion=1&amp;espv=2&amp;ie=UTF-8#q=float%20definition
One Eyed Mind
 
  -1  
Reply Tue 9 Sep, 2014 08:46 pm
@engineer,
By your logic, if I was walking through a canyon and a boulder fell on me, you'd see on the news: "today a man was killed by a rock floating down an unsettled cliff".

By your logic, if someone was walking through a city and a bag fell on someone's head, suffocating them, you'd see on the news: "today a man was killed by a bag dropping down from the sky".

By the way, "drop down" is word play based on raindrops and how they fall down from the sky.

Do snowflakes fall down or float down?
maxdancona
 
  1  
Reply Tue 9 Sep, 2014 09:15 pm
@One Eyed Mind,
You don't have a clue One Eyed Mind.

Engineer is correct. The word "float" implies a buoyant force. You don't get a buoyant force without an atmosphere... meaning on the moon, nothing floats. They just drop.


One Eyed Mind
 
  -1  
Reply Tue 9 Sep, 2014 09:16 pm
@maxdancona,
Saying he is correct is very specific.

Does the snowflake drop down or float down?
maxdancona
 
  1  
Reply Tue 9 Sep, 2014 09:18 pm
@One Eyed Mind,

One Eyed Mind
 
  0  
Reply Tue 9 Sep, 2014 09:25 pm
@maxdancona,
Alright, alright.

I stand corrected. Objects do drop down on the moon, whether it be a hammer or a feather, therefore an egg can drop down as well.

BUT, if the egg breaks, is it still an egg? "Next question" by OEM, coming to theaters near you.
0 Replies
 
jasonmoon
 
  1  
Reply Tue 9 Sep, 2014 10:46 pm
@engineer,
This seems really well thought out, but I don't understand the employment of k - am I supposed to plug in numbers for k?
jasonmoon
 
  1  
Reply Tue 9 Sep, 2014 10:48 pm
@engineer,
Sorry for the spam, but I see that the formula is F= -kv
again, am I supposed to plug something in for k?
0 Replies
 
engineer
 
  1  
Reply Wed 10 Sep, 2014 05:04 am
@jasonmoon,
jasonmoon wrote:

This seems really well thought out, but I don't understand the employment of k - am I supposed to plug in numbers for k?

The problem never gave you the value for K and part three tells you that K can be variable, so you just have to leave it as an unknown value.
0 Replies
 
 

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