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I feel something is missing here, and I could not find an answer to something initially i thought very simple! Looking at the density of plastics (polymers if you wish), most of them are denser than water (acrylics ~ 1.2, nylon 1.15 ... of water ), how comes they don't sink!

I feel i should go back to year 3 of school since this puzzled me!

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There are different types of plastics, with different densities. HDPE (high density polyethylene), LDPE (low density polyethylene), and PP (polypropylene) which make up containers and plastic bags for example do float, as their density is less than that water. Higher density plastics such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid), do sink.

Plastics in the Oceans is worth reading.

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In general, for something to float on water, it must displace an equal or larger mass of water (if the object is moving it can displace more) than the mass of the object, or be light enough that it doesn't break surface tension.

The given values for the material density might be solid pellets or bricks of plastic, and the floating bits you see might have different density due to shape.

Remember that density is M/V.

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  • $\begingroup$ It actually must displace the same mass of water compared to the mass of the object. If an object displaces less than its mass of water when fully submerged, that means it is more dense than the water (it can only displace it's volume) and will sink if left alone. $\endgroup$ – JMac Feb 9 '17 at 12:27
  • $\begingroup$ Clarified what I'd meant $\endgroup$ – Jake Watrous Feb 9 '17 at 13:02
  • $\begingroup$ Still not right. It displaces the same mass, not a larger mass, that is my point. How would an object exert enough force to displace more than it weighs? $\endgroup$ – JMac Feb 9 '17 at 13:03
  • $\begingroup$ @JMac: You are mixing up mass and volume. An object displaces the same volume of water. If the mass of the object is less than the same volume of water, it floats. $\endgroup$ – Oscar Bravo Feb 9 '17 at 13:20
  • $\begingroup$ @OscarBravo A floating object will displace the objects mass worth of water. A submerged object will displace the objects volume in water. The joining case is an object of the same density as the water. In that case the volume of the object and the mass of the object will be the same as the volume and mass of water displaced. $\endgroup$ – JMac Feb 9 '17 at 13:33
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Even though it may be denser, due to its shape it doesn't sink. Also note that it may not be heavy enough to break water surface tension

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Also salt water is more dense than pure water. So things float better in the oceans.

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  • $\begingroup$ Which is true, but I think that 3.5% salt probably doesn't highten its density so much. $\endgroup$ – peterh Feb 9 '17 at 18:40

protected by Qmechanic Feb 9 '17 at 13:43

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