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Density(Science)

Mon 24 Sep, 2018 03:41 am
In school, we are taught that when a plasticine is mould into a boat shape, it can float on the water. We were taught that by moulding the plasticine, we can increase the volume of the plasticine without increasing its mass in order to reduce the density.
But , if I use a 4*4*4 cubic plasticine=64cm cube
If I mould it into 8*8*1 rectangle plasticine (as its height is 1cm)=64cm cube too!
It does not changes its volume and mass/weight but it can float on water?
If by moulding into a boat shape can increase the space between particles, then if i keep moulding it, make it even thinner, can i make it into liquid?
Its really confusing!!!
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Type: Question • Score: 0 • Views: 277 • Replies: 12

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rosborne979

2
Mon 24 Sep, 2018 04:32 am
@YRX,
The density of the material doesnâ€™t change. The boat shape just causes it to displace air, which makes it float.
engineer

2
Mon 24 Sep, 2018 11:23 am
@rosborne979,
Exactly. If you take the 4x4x4 cube and change it into a 14cm diameter sphere with 0.1cm thick walls, that ball would float.
YRX

0
Fri 28 Sep, 2018 07:22 am
@engineer,
but if you make it a sphere, its volume will include the air inside
as: 22/7 x 7 cube x 4/3 = 1437.3333
and the mass of 1 cm cube of plasticine is way more heavier than 1 cm cube of air, which directly reduces its density.
Furthermore, its already away from my topic. I am asking to mould it into a boat shape
YRX

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Fri 28 Sep, 2018 07:23 am
@rosborne979,
Can you furthur explain it ?
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engineer

2
Fri 28 Sep, 2018 07:25 am
@YRX,
YRX wrote:

but if you make it a sphere, its volume will include the air inside

Exactly, that is why a boat floats, because it includes the air inside. Your typical ocean freighter is made out of steel and aluminum. If you punch a hole in the side and let the water in/air out, it sinks. It's the air inside that makes the effective density lower and allows it to float.
YRX

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Fri 28 Sep, 2018 07:27 am
engineer

2
Fri 28 Sep, 2018 07:30 am
@YRX,
It doesn't matter the type of boat. When something is floating, its effective density is the weight of the object divided by the volume it displaces. By shaping the material into the shape of a boat, it displaces air making the effective density lower. The material density is unchanged, but the effective density includes the air.
YRX

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Fri 28 Sep, 2018 07:45 am
@engineer,
There is difference between an open boat and a closed boat as the air inside a closed boat is contained inside while the air "on" an open boat is always moving.
A ship just as titanic can float although it is heavy is because it can contain air inside it which reduces its density(ballast tank).
A boat made by wood has lower density because it has air in the wood. If the wooden boat is left in water for too long it still sinks.
As for this plasticine boat,
First, plasticine's particle is more concentrated( I suppose i would call that) which makes it heavier than wood and thus higher density than wood.
Secondly, the boat's upper part is not closed. How can it include the air's volume and density as the air is always moving?
Its OK to agree with you. But if you said so you are saying that the textbook had taught us wrongly?
engineer

3
Fri 28 Sep, 2018 08:00 am
@YRX,
YRX wrote:

There is difference between an open boat and a closed boat as the air inside a closed boat is contained inside while the air "on" an open boat is always moving.

No there is not. The only part of the air that counts is the air below the waterline. If you make a sphere, it will displace the exact amount of water as if you make an open boat. You could make a sphere with a hole in the top if you want, it will float the exact same way.
Quote:
If the wooden boat is left in water for too long it still sinks.

This is clearly wrong. As long as the boat doesn't have a leak in it, it will float.
Quote:
How can it include the air's volume and density as the air is always moving?

The air is not important, the displaced volume is. You you pumped all the air out of the boat, it would float just as well (actually slightly better because you have lowered the mass of the boat). The key thing is that the shape you use is pushing water out of the way and that displaced volume reduces the effective density of your boat. The key factors are the weight of the boat, compared to the weight of the displaced water. By shaping your material to displace more water, you can support a heavier boat. It doesn't matter if the boat is open to air or not.
Quote:
But if you said so you are saying that the textbook had taught us wrongly?

No, I'm saying you have not understood what the book is trying to teach you. When you make a boat from steel, you do not reduce the actual density of steel by reshaping it. I'm sure your book does not say that.
YRX

0
Fri 28 Sep, 2018 08:17 am
@engineer,
Its written:
(a) A lump of plasticine sinks in water because it is denser than water.
(b) When it is moulded into the shape of a boat, it floats in the water.
(c) When the lump of plasticine is moulded, its volume is increased. Thus, it becomes less dense and floats in water.
* Its exactly from the book
Can you explain.
engineer

1
Fri 28 Sep, 2018 08:40 am
@YRX,
YRX wrote:

(a) A lump of plasticine sinks in water because it is denser than water.

Absolutely correct
YRX wrote:
(b) When it is moulded into the shape of a boat, it floats in the water.

Again absolutely correct
YRX wrote:
(c) When the lump of plasticine is moulded, its volume is increased. Thus, it becomes less dense and floats in water.

The volume includes all the volume that the plasticine boat displaces. When you put the original plasticine in water, it displaced a 4x4x4 cube. When you molded it into a boat, it displaced much more water. The volume of displaced water is what has changed and that changes the effective density.

I will say that your book's explanation is slightly off and that seems to be confusing you. The plasticine's actual density does not change when you mold it, it just changes shape. It's the effective density that can be said to change. Even with that, if I was teaching you this, I wouldn't describe it that way.
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rosborne979

1
Fri 28 Sep, 2018 08:49 am
@YRX,
YRX wrote:
(c) When the lump of plasticine is moulded, its volume is increased.

The volume of plasticine does not increase. But the volume of water displaced by the plasticine (in its new shape) does increase. There's a big difference between those two things and I think the book you are reading is being inaccurate in its statement, and that is leading to your confusion.
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