It is easier to move our arms and legs when we are inside water than in air.Why?

Since the density of water is greater than that of air, the upthrust on our limbs is greater when our body is immersed in water. The apparent weight of the limbs is less in water than in air. Hence, it is easier to move our arms and legs in water than in air. 

Air resistance and water resistance are kinds of friction. Solid things, like your chair, push up on you. Upwards forces from water or air are called upthrust.This force is called the upthrust. A piece of wood floating on water, and a helium balloon floating in the air, are both effects of upthrust. ... It states that: the upthrust is equal to the weight of the fluid displaced.Therefore, at sea level, air is 784 times less dense than water.Upthrust = apparent loss of weight of object = weight in air - weight in liquid.

If the upthrust is larger than the weight of the object, the object will rise. This happens when you jump into a swimming pool and then rise back up to the surface. Under the surface, the weight of water you displace - the upthrust - is more than your weight, so there is a resultant upwards force.A non-contact force is a force which acts on an object without coming physically in contact with it. The most familiar non-contact force is gravity, which confers weight. In contrast a contact force is a force applied to a body by another body that is in contact with it.

The upthrust on an object stays the same no matter how deep it goes (as long as it is completely submerged). This is because the upthrust depends only on the DIFFERENCE between the forces on the top and bottom of the object.The three types of buoyancy are positive buoyancy, negative buoyancy, and neutral buoyancy. Positive buoyancy is when the immersed object is lighter than the fluid displaced and this is the reason why the object floats.The effect of upthrust is that weight of the body immersed in a liquid appears to be less than its actual weight. There are two factors on which upthrust depends - Volume of the body and Density of the Liquid. Larger the volume of body submerged in liquid, greater is the upthrust.

Compressed air takes less volume, so has higher density. ... Determining its weight, and the volume of water it displaces, will determine whether it will float or sink. Note that if more air is added to the tank, it will get slightly heavier (and the volume will remain constant), and will therefore be less buoyant.

A body at rest in a fluid is acted upon by a force pushing upward called the buoyant force, which is equal to the weight of the fluid that the body displaces. If the body is completely submerged, the volume of fluid displaced is equal to the volume of the body. If the body is only partially submerged, the volume of the fluid displaced is equal to the volume of the part of the body that is submerged.

Archimedes' principle is very useful for calculating the volume of an object that does not have a regular shape. The oddly shaped object can be submerged, and the volume of the fluid displaced is equal to the volume of the object. It can also be used in calculating the density or specific gravity of an object. For example, for an object denser than water, the object can be weighed in air and then weighed when submerged in water. When the object is submerged, it weighs less because of the buoyant force pushing upward. The object's specific gravity is then the object's weight in air divided by how much weight the object loses when placed in water. But most importantly, the principle describes the behaviour of any body in any fluid, whether it is a ship in water or a balloon in air.