Time of descent decreases then increases as mass increases

[FlySkyHigh99]

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Seriously need help on this. Take this 3 masses as example, 0.15kg, 0.25kg and 0.35kg.
For instance,
When the mass is 0.15kg, the time of descent is 0.4 seconds,
When the mass is 0.25kg, the time of descent is 0.3 seconds,
Lastly when the mass is 0.35kg, the time of descent is 0.35 seconds.
Can anyone explain to me how this phenomenon happen? Please really need help in this.
P/S : Air resistance is present and it is not a free fall

 

[premed1001]

are you talking about a free falling object? those are independent of the mass

 

[FlySkyHigh99]

are you talking about a free falling object? those are independent of the mass

Hi, it's not free fall. Everything is present

 

[premed1001]

Hi, it's not free fall. Everything is present

explain?

 

[FlySkyHigh99]

explain?

Yes. How can the time of descent decreases first, and then increases as the mass increases.

 

[premed1001]

Yes. How can the time of descent decreases first, and then increases as the mass increases.

can you explain the original problem? i don't understand it

 

[DrknoSDN]

Seriously need help on this. Take this 3 masses as example, 0.15kg, 0.25kg and 0.35kg.
For instance,
When the mass is 0.15kg, the time of descent is 0.4 seconds,
When the mass is 0.25kg, the time of descent is 0.3 seconds,
Lastly when the mass is 0.35kg, the time of descent is 0.35 seconds.
Can anyone explain to me how this phenomenon happen? Please really need help in this.
P/S : Air resistance is present and it is not a free fall

Air resistance is the only important variable here. A less massive object is more effected by the upward force produced by the air resistance. That's why dust can float. It's Mass and surface area relative to the air resistance causes it to not fall.

In this question the less massive object (assuming they are the same exactly shape), will have a similar force of air resistance applied to a less massive object.
So as you increase the mass of the object air resistance because "less important" in determining acceleration.

Remember in this example: Acceleration = MG - Force(air)
As MG > infinity, Force(Air) becomes insignificant.

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[edit]
One reason that a more massive object could fall slower than a less massive object is shape.
First thing that comes to mind is a parachute. As the mass of the parachute began to increase (larger chute) the force of air resistance would increase at a greater rate...

It seems like you would need more information if that was the case. But it is a strong possibility.

 

[DrknoSDN]

dupe

 

[premed1001]

Air resistance is the only important variable here. A less massive object is more effected by the upward force produced by the air resistance. That's why dust can float. It's Mass and surface area relative to the air resistance causes it to not fall.

In this question the less massive object (assuming they are the same exactly shape), will have a similar force of air resistance applied to a less massive object.
So as you increase the mass of the object air resistance because "less important" in determining acceleration.

Remember in this example: Acceleration = MG - Force(air)
As MG > infinity, Force(Air) becomes insignificant.

how is air resistance dependant on mass?
i thought it was dependent on the surface area and velocity of the object

 

[DrknoSDN]

I didn't say air resistance is dependent on mass. I said a less massive object is more influenced because of the air resistance.

If 2 objects both experience the same force of air resistance because of their surface area and velocity... The less massive one will "feel" the force more while the more massive one "ignores" the force.

Not sure if that explains, but use the equation i provided to think about it..
Acceleration = MG - Force(Air)

If air resistance is the same for all objects, it will be a larger part of the equation if Mass is small.

 

[premed1001]

I didn't say air resistance is dependent on mass. I said a less massive object is more influenced because of the air resistance.

If 2 objects both experience the same force of air resistance because of their surface area and velocity... The less massive one will "feel" the force more while the more massive one "ignores" the force.

Not sure if that explains, but use the equation i provided to think about it..
Acceleration = MG - Force(Air)

If air resistance is the same for all objects, it will be a larger part of the equation if Mass is small.

makes sense thanks! but then why is the second largest mass quicker then the lighter mass?

 

[DrknoSDN]

makes sense thanks! but then why is the second largest mass quicker then the lighter mass?

Can't be certain without looking at the system in question but perhaps if we use my parachute visualization... Maybe the smallest and medium parachutes were too small to be significantly helpful so mass was a bigger factor than surface area.

The 0.25kg mass falls faster because it's heavier so "ignores" air resistance more than the 0.15kg mass..
By the time you increase mass to 0.35kg (much larger surface area), the mass is less of a factor than the air resistance.

A Frisbee scenario works well also. An extremely small frisbee would fall to the ground. A slightly larger one would fall faster because it's heavier and still can't "catch air", but once it gets big enough, the shape is a bigger factor than the mass..

I'm also not sure if there is a right or wrong answer because of the OP saying:

Can anyone explain to me how this phenomenon happen?

Seems like Fly is just looking for hypotheticals.

Bottom line when air resistance is a factor, there could be many answers to this question.

 

[FlySkyHigh99]

Can't be certain without looking at the system in question but perhaps if we use my parachute visualization... Maybe the smallest and medium parachutes were too small to be significantly helpful so mass was a bigger factor than surface area.

The 0.25kg mass falls faster because it's heavier so "ignores" air resistance more than the 0.15kg mass..
By the time you increase mass to 0.35kg (much larger surface area), the mass is less of a factor than the air resistance.

A Frisbee scenario works well also. An extremely small frisbee would fall to the ground. A slightly larger one would fall faster because it's heavier and still can't "catch air", but once it gets big enough, the shape is a bigger factor than the mass..

I'm also not sure if there is a right or wrong answer because of the OP saying:

Seems like Fly is just looking for hypotheticals.

Bottom line when air resistance is a factor, there could be many answers to this question.

Hi Drkno, thanks for the effort of answering my questions. Appreciate it. Well actually all these data are calculated. Theoretically. In my question, there is nothing stated regarding the shape of the object. It's just a simple ball. Being thrown up to the air and then fall back to the ground. If shape is constant here, what could be the explanation for this to happen? I'll give you the more accurate data (calculated)
0.05kg --> 0.4301 seconds
0.15 --> 0.4209 s
0.25 --> 0.4307
0.35 --> 0.4398
0.45 --> 0.4472
In my question, only air resistance and gravitational force is mentioned. Assuming the shape is constant, how could this happen?

 

[DrknoSDN]

Hi Drkno, thanks for the effort of answering my questions. Appreciate it. Well actually all these data are calculated. Theoretically. In my question, there is nothing stated regarding the shape of the object. It's just a simple ball. Being thrown up to the air and then fall back to the ground. If shape is constant here, what could be the explanation for this to happen? I'll give you the more accurate data (calculated)
0.05kg --> 0.4301 seconds
0.15 --> 0.4209 s
0.25 --> 0.4307
0.35 --> 0.4398
0.45 --> 0.4472
In my question, only air resistance and gravitational force is mentioned. Assuming the shape is constant, how could this happen?

At first glance the points all seem close enough to each other that the disparity would not be statistically significant.
Your talking hundredth's of a second... Also the results are 'calculated' so it probably has nothing to do with air resistance or shape unless those factors are part of the equation.

How are you calculating the results? What equations are you using? If you repeat the same trial multiple times does it give the exact same result or is there a standard deviation? Is it a simulation?
I would suggest you perform a statistical analysis to see if the variation in time is even relevant.

Also could this be rounding error? You only have 2 significant figures of mass but 4 of time. If you are 'calculating' the results you can't forget sig figs.