Gravity Help ---urgent

[ConfusedKid]

Hi,

 

Im trying to answer a question that I have no idea about.

Its quite urgent and I was wondering if anybody could help me out, the question I am trying to solve is...

 

If two objects ( the same weight, but different mass) fall in water, which is most likely to sink in fastest. The one with the higher mass or the one with the least mass???

 

I would really appreciate it if you could post something back with an answer or help!!!!

 

Thanks =]

 

 

Aimee

[Country Boy]

The same weight but different mass?? How are you going to arrange that?

 

WEIGHT is defined as "force of gravity acting on an object" and is proportional to mass- you cannot have two objects with the same weight but different mass.

 

Did you mean "same weight but different VOLUME". In that case, the object with less volume would sink faster. Since the two objects have the same weight, the gravitational force downward would be the same for both objects. However, the upward bouyant force is proportional to volume (it is the density of water times the volume) so the object with greater volume will have a larger upward force subtracted from the downward, gravitational, force.

[ajb]

Same mass different density, is probabily what you mean. (I guess)

[ConfusedKid]

Thank you for your help, it made perfect sense to me, but I have another question based on your answers.

As you stated, if you have two objects that, lets say weigh the same weight e.g, 2oz, but one is twice as much volume than the other. One is in a sense going to want to 'float' more than the other due to the upward force from the displacement of the water, this would be the one with twice as much volume,bearing this in mind, if you were to pull both objects through the water at the same rate, which one is going to be harder to pull? Would it be easier to pull the object that has twice as much volume,as there is more upward force on it?, Or would it be harder to pull through the water as there is more resistance? Or would they just be equal?

 

I Really appreciate your help on this and your fast reply. Thank you.

[Klaynos]

The larger the surface area then the larger the resisting force. So assuming they are both symetric shapes then the one with the larger volume would be harder to move through the water...

[Country Boy]

Notice that "pulling through the water" is a very different matter from whether something will float or will sink faster than something else. As I said before, the gravitational force is the weight and so will be the same for any two objects that weigh the same will have the same downward force. However, the UPWARD force of bouyancy is equal to the weight of water of the same volume and so is proportional to the volume. The force pulling downward on an object is the difference between the two (if the second is larger, the object floats).

However, pulling an object through water has nothing to do with gravity. The force necessary to pull something at constant speed is the "drag" on the object. That tends to be (approximately) proportional to the speed times the cross section area perpendicular to the direction of motion: which for a sphere is pi times r^2. (I feel sure it is cross sectional area, not surface area but I may be wrong.)

[someguy]

in order for two objects to have the same weight and different mass they would need to be located on different planets or different celestial objects. like you could have two objects of different mass one on the moon and one on the earth and they could weigh the same. they could also be the same size. but your body of water on the moon would weigh less than your body of water on the earth. therefore your object on the moon would weigh more in comparison to the water. so it would be more likely to sink. but the rate at which it would fall would be determined by the acceleration the moon would put on it minus the drag. I'm not really sure about what the effects of drag would be for water on the moon as compared to on earth but water pressure would be greater on earth so i would guess that drag would be less significant on the moon, plus the larger mass of the object on the moon would require a stronger force to slow it down than the one on earth. you would need to calculate what the difference in drag would be (taking into account the new drag of water on the moon and the new mass of the object) and compare that to the difference in acceleration of the moon is and then see what you get. off the top of my head i'm not really sure. but if i had to guess i think i would put my money on the object on earth still falling faster, assuming it falls at all, so probably actually there is a zone of possibilities where the object on earth's buoyancy would slow it down to slower than the object on the moon, once it is massive enough to sink the answer may be different. i'm not sure, i think i'm just confusing myself the more and more i think about it.

[KSD]

(the same weight, but different mass) if one with less mass weighs the same as the one with more that means it would be bigger, therefore it would create more friction so it would reach the bottom last. BUT that would be impossable at the mo

[Davidwood15]

What do you mean same weight but different mass? As said before, mass and weight are proportional to each other therefore would be the same..

[tomgwyther]

Confusedkid: Could you not try the experiment yourself with a bucket of water and two objects of differing volumes; with equal mass?

 

[As mentioned before, weight is a mesure of mass under the influence of gravity]

[someguy]

if mass is linked to weight as a function of gravity then the experiment requires separate sources of gravity. if you use the earth and the moon for example, you can have two objects of different mass and equal weight and equal volume.

[J.C.MacSwell]

The one with less mass will generally sink fastest. Assuming they have the same weight in air, the one with the greater mass must be significantly larger in volume, so that it would only weigh the same due to the greater buoyant force of the air. In water this effect would be much greater, so all other factors being equal the one with the greater mass, and lower density, would sink more slowly.

[Country Boy]

Hey, guys, in the first couple of posts it became clear that he meant "volume" or "density', NOT mass.