An object which floats in water weighs 5 N in air.
a. What is the weight of the object in water?
b. What is the upthrust acting on the object in water?
c. What is the weight of the water displaced by the object?
d. The density of water is 1 g/cm'. If an object with a mass of 100 g has a weight of 1 N on
Earth, calculate the volume of water displaced by the object.
Answers
Answer:
There are at least two definitions for weight.
Weight is a force and hence it is a vector and it is of no use if we omit its direction.
If m is mass of an object and g is acceleration due to gravity, then the force mg acting downward is considered as weight by one definition.
However, nowadays, we don't say this force as weight. We say that it is the gravitational force acting on the object.
Hence there arises a new definition for weight, deleting the previous definition of mg downward.
When an object of mass m rests on a platform ( ground or any such thing) , it is at rest since the net force acting on it is zero.
The gravitational force mg acts downward, where as the normal force acts upward and which is also equal to mg.
Nowadays, we call this normal force acting upward as the weight of the object.
Hence when a body is falling freely under gravity, though there is gravitational force mg, there is no opposing upward force on it.
And we say the body is weightless, when falling.
Coming to the present question, the object floats, i.e it is at rest. An upward force of 5 N is keeping it rest by balancing the gravitational force of 5 N.
Hence its weight is 5 N. ( The net force is zero).