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Why does a heavier solid float on water?

Why does a heavier solid i.e. a plastic float on water?

Asked Shruthi S


An object floats or sinks in water depending upon the weight of the body and the weight of water displaced by the immersed part of the body. If the weight of the body is greater than the weight of water displaced by it, then it will sink.

If you feel that the weight of the body must be greater than an equal volume of water, and it does not sink, there must be a reason. This is what happens in the case of a ship. The ship is made up of steel. If you put a steel ball or a nail, it sinks. But when the steel is flattened and then shaped in to a boat, it floats. Why?

Because here the average density, which includes the cavity of the boat,  becomes less than the weight of water displaced. You can easily see that the ship will sink if water is allowed to enter into the cavity.

You can experiment with a steel bowl too. The bowl will float on water, it water is not allowed to enter the cavity.

So, it’s not just the density of the material, it is the average density that counts.

If the object is made in the shape of a bowl or a ship, water is not allowed to enter inside, and the weight of the ship is maintained less than the weight of water displaced by it.

Also refer to




Buoyant force concept and terminal velocity? My aerodynamics book(John Anderson Aerodynamics) say that whenever a body moves through a fluid, there are only two types of forces acting on the body. 1. Pressure force that acts perpendicular to the plane of the body and the other is the drag force that acts parallel to the objects velocity.


Forces at Work


Also i read an article that buoyant force is related to static fluids not moving fluids. Now, i was working on a concept of terminal velocity of an object moving through air in the Z direction. According to my concept, the air is continuously in motion and hence its a dynamic fluid. So, accordingly, the body falling through air will experience friction drag parallel to the velocity and pressure force perpendicular to the velocity. But as i was going through the derivation of the terminal velocity, they have counted the buoyant force in which is completely confusing me. If there is buoyant force when the body is moving through a moving fluid then when an aircraft is cruising at an altitude, why do they not mention the buoyant force? Rather they say that the vertical force is the pressure force which is also the lift force. I am really confused when to apply buoyancy and the concept of this pressure force which acts perpendicular to the plane of the body Please help me.



A pound coin floating in mercury (Photo credit: Wikipedia)

I read in one of my chapters that when a body is immersed in a container of water (kept on a weighing machine which  reads ‘W’ at first )the apparent weight lost by the body is equal to the buoyant force, and thus the reading ‘W’ increases.

Then I came across a question like this –

A beaker containing water kept on a weighing machine weighs W. A body of weight  ‘w’ is dropped in it. It is floating & experiencing a buoyant force B, then the reading on machine is –


please give me the answer and explanation
the book says answer is (a)
but I think it is (c)

(Anwesha posted this questions)

The answer is A.

The total force acting downwards is W+w and the weighing scale is providing an equal reaction. the normal reaction offered by the weighing scale is what we get as the reading.

therefore the reading on the weighing scale must be W+w

The buoyant force is acting on the object dropped and is not contributing to the normal reaction offered by the weighing scale.

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