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water

ritika bhardwaj

Updated Aug. 21, 2020

Transcript

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Date 1

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Topic:

Why does a block of plastic released under water come up to

the surface of water?

Subject: Science

Class: 9th

Density is basically the concentration of mass per unit volume of a body. So, density = mass / volume.

Now, every substance on our planet has some density since it has mass and occupies some space. And, no matter how one changes the mass or the volume of that substance, it's density remains the same because it's a fixed standard value for that substance.

You might be well aware that the density of water is approximately 997 kg/m³. Now, density plays an important role in many natural and man-made phenomena that occur in this world. One such phenomenon is known as "buoyancy".

When an object having a uniform fixed density is released in a substance having a different density, then there are two cases possible :

Either the object sinks completely.

Or the object floats on the surface of that liquid.

When an object is immersed in a liquid of different density, then the object experiences a force in the upward direction apart from its weight, which is acting in the downward direction. This upward force is known as the "buoyant force".

if the density of the object is less than the density of the liquid, the buoyant force is more than the weight acting downwards. So, the net force is in the upward direction and hence the object begins to rise.

Whereas, if the density of the object is more than the density of the liquid, the weight is more than the buoyant force. In this case, the net force acts in the downward direction and hence the object begins to sink.

The average density of plastic is approximately 920 kg/m³. This is less than the density of water. So, the net force acts in the upward direction and the plastic block submerged in the water begins to rise. Eventually, the block reaches the surface and floats on the surface of the water.