(a) A hydrometer is based on the principle of floatation which states that a body floats in a liquid when the weight of the body is equal to the weight of the liquid displaced by the immersed part of the body.
(b) In the case of a sharp knife, the force is exerted on a smaller area of cross-section. Hence, the pressure exerted by a sharp knife is more than the pressure exerted by blunt knife.