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Why is this so?

ScienceBorn

Sep 26, 2013
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It is said that "ignoring air resistance, two object of different masses get to the ground at the same time" MY QUESTION-why is this so?, despite the different in mass of the two objects, they both reach the ground at equal time interval WHY? I know my question does not relate much with electronics but it is very important i know.I NEED IDEAS PLEASE. Anybody?
 

Laplace

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Simple physics. Consider the equation for motion -- Force = Mass x Acceleration; and the equation for the weight of an object -- Weight = Mass x Gravity. When an object is falling, and ignoring air resistance, the only force acting on the object is its weight. So substitute weight for the force in the equation for motion. Mass x Gravity = Mass x Acceleration --> Gravity = Acceleration!

So regardless of the mass of an object, it falls to the earth with the same acceleration as any other object (ignoring air resistance). Objects traveling the same distance at the same acceleration will cover the distance in the same amount of time. The equations of motion apply.
 

davenn

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Moved this to general chat ... better here than homework

Dave
 

ScienceBorn

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Simple physics. Consider the equation for motion -- Force = Mass x Acceleration; and the equation for the weight of an object -- Weight = Mass x Gravity. When an object is falling, and ignoring air resistance, the only force acting on the object is its weight. So substitute weight for the force in the equation for motion. Mass x Gravity = Mass x Acceleration --> Gravity = Acceleration!

So regardless of the mass of an object, it falls to the earth with the same acceleration as any other object (ignoring air resistance). Objects traveling the same distance at the same acceleration will cover the distance in the same amount of time. The equations of motion apply.

Cool!!!. Nice explanation, you have said it all Laplace.one more question for you: does the reflection of light on the two object of different masses also have something to do with the objects getting to the ground at the same time?
 

davenn

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Cool!!!. Nice explanation, you have said it all Laplace.one more question for you: does the reflection of light on the two object of different masses also have something to do with the objects getting to the ground at the same time?

no it doesnt, why would you think so ?

its all due to the gravitational attraction

Dave
 

ScienceBorn

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no it doesnt, why would you think so ?

its all due to the gravitational attraction

Dave

I know the question sounds stupid.The reason why i asked the question was because i noticed drops of water from the surface of my home(at night using touch light because everywhere was a little bit dark). I asked myself this question "will reflection of light through the small water ball on the surface as it drops to the ground and letting the small water ball get to the ground without reflecting it with light, will they get to the ground at the same time"?.Since it was only a small water ball that was dropping from the surface i decided to reflect light through it using my touch light and a stopwatch, when the small water ball dropped to the ground i recorded the time taken for it to do so, then removed the touch light to stop the reflection, then i did the same thing in the dark. I found out that the time taken for a small water ball to get to the ground when reflected with my touch is less than the time taken for the same water ball to get to the ground when in the dark.I used a bowl to know when the water drops.WHAT DO YOU GUYS THINK?
 
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batkin

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I think it was an optical illusion. Light won't have any effect on the speed at which an object falls. The only thing that makes a difference is wind resistance. The more an object catches the wind as it falls in relation to its mass the slower it will fall.

Here is a video of a feather and a ball being dropped. The feather falls slower because it catches more wind. Then the container has all of its air pumped out (no wind) and they are dropped again. Both objects fall at the same speed.
 

(*steve*)

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despite the different in mass of the two objects, they both reach the ground at equal time interval

If the masses are large enough you would see a difference.

The masses would need to be so large that the gravitational fields of the objects (or at least one of them) would be a noticeable fraction of that of earth.

As an example, you could calculate the period of orbit of a small object at the earth-moon distance and compare it to that of the moon.

However, the chance of actually dropping a moon size object is fairly slim.

I think it was an optical illusion.

Agreed.

Light won't have any effect on the speed at which an object falls.

But yes it will. Light exerts a small force on the object which could affect its motion. In a practical sense, the amount of light required would be extreme, and an experiment may have to be carried out in a vacuum to notice any difference.
 

ScienceBorn

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I think it was an optical illusion. The only thing that makes a difference is wind resistance.


Yeah it should be an optical illusion or probably my timing.NOTE: I was conscious of the air resistance, the video link
was invaild. THANKS FOR YOUR EFFORT.
 

ScienceBorn

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The masses would need to be so large that the gravitational fields of the objects (or at least one of them) would be a noticeable fraction of that of earth.


However, the chance of actually dropping a moon size object is fairly slim.

I agree with you *steve*


Light exerts a small force on the object which could affect its motion. In a practical sense, the amount of light required would be extreme, and an experiment may have to be carried out in a vacuum to notice any difference.

yeah still agree with you, a mere light won't have any effect on the objects, light travels at a speed of 3 * 10 to the power of eight everything must concur to see a true effect.
 

Laplace

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You might want to look up information on the Crookes Radiometer to determine if there is any bearing on your question. Also, consider the following extract on the pressure of light:

http://www.dartmouth.edu/~pressureoflight/history/history2.html

During the period 1900-1903, Ernest Fox Nichols and Gordon Ferrie Hull carried out the first precise measurement of the radiation pressure of light on a macroscopic body in the Wilder Physical Laboratory, Dartmouth College. James Clerk Maxwell had predicted this effect in 1873 based upon calculated stresses in the electromagnetic field. It was also predicted by Adolpho Bartoli in 1876 based on a thermodynamics argument. All earlier attempts to experimentally observe this effect had been thwarted due to the disturbing action of the residual gases surrounding the body upon which the radiation fell, even in the best vacuums achievable at the time. The radiation pressure was predicted to be the energy density in the light beam, independent of the wavelength. The experiments of Nichols and Hull succeeded, where others had failed, by making a detailed empirical analysis of the ubiquitous gas heating and ballistic effects. The published papers [Nichols, E. F., and Hull, G. F., A preliminary communication on the pressure of heat and light radiation, Phys. Rev. 13, 307 (1901); The Pressure Due to Radiation. (Second Paper.), Phys. Rev. 17, 26 (1903)] reveal the incredible experimental acumen of Nichols and Hull. The final results agreed with Maxwell's theory to better than one percent. The tiny force involved in their torsion balance radiometer was of order 10-4 dyne.
 

davenn

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Laplace

it was been long held belief that its light pressure (photons) that were turning the vanes, and tho there has been lots of debate. It is these days generally believed to be a thermodynamic process

wiki has a good description and is usually the place that "The Physics Forum" refers people to

cheers
Dave
 

(*steve*)

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yeah still agree with you, a mere light won't have any effect on the objects, light travels at a speed of 3 * 10 to the power of eight everything must concur to see a true effect.

Photons are somewhat special. You can calculate their energy, but they have no mass. However if you absorb a photon, that energy must go somewhere, and that somewhere could be into kinetic energy.

This link will help explain it and the (tiny) magnitude of it.

It is true that the rotating vane thing is not activated by the pressure of the radiation as is evidenced by the fact that it won't work in a perfect vacuum. Most good explanations will tell you that if it did work, it would also rotate in the other direction (and the link Dave provided is a good one!)
 

ScienceBorn

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Scientists have long proved the existence of pressure in light radiation[that is, it is possible to obtain a different in time interval when small water balls are reflected with infrared radiation then timed and when timed directly without reflection of light]. Firstly, James Maxwell observed this in his field of study [electromagnetism], theorised it and predicted its existence. Lucky enough, he lived to see the outcome of his prediction through Crooke's radiometer invented by Crooke. The radiometer was later discovered to have little problem not obeying the fact that black bodies are good absorbers of heat, but that was not the aim of Crooke's experiment, he only wanted to prove the existence of pressure in light radiation which he did.GREAT LECTURE!! Thanks.
 

davenn

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Scientists have long proved the existence of pressure in light radiation[that is, it is possible to obtain a different in time interval when small water balls are reflected with infrared radiation then timed and when timed directly without reflection of light]. Firstly, James Maxwell observed this in his field of study [electromagnetism], theorised it and predicted its existence. Lucky enough, he lived to see the outcome of his prediction through Crooke's radiometer invented by Crooke. The radiometer was later discovered to have little problem not obeying the fact that black bodies are good absorbers of heat, but that was not the aim of Crooke's experiment, he only wanted to prove the existence of pressure in light radiation which he did.GREAT LECTURE!! Thanks.

DID you not read my link ??

Its NOT light pressure!

Dave
 

ScienceBorn

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From what i read, crooke's radiometer was used to measure the radiant energy of heat and light. But he discovered the vanes where moving and according to him he believed that what made the vanes moving contineously was the existence of "light radiation pressure". If am wrong please correct me.
 

(*steve*)

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ScienceBorn, that's what Crooke thought, yes. But he was wrong.
 
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