ICSE Physics Class 8 Notes Chapter 2 Physical Quantities and Measurement
→ MASS : Is the quantity of matter contained in a body.
VOLUME : is the space occupied by body.
→ Equal mass of IRON and cotton, iron will have less volume than cotton.
→ Equal volume of Iron and cotton, the mass of iron is more than mass of cotton, because iron denser than cotton.
→ DENSITY : “Is ratio of mass of substance to volume of M kg substance”
D = \(\frac {M}{V}\) = \(\frac{\mathrm{kg}}{\mathrm{m}^{3}}\) The SI. unit of density is kg m-3
→ Density of a substance does not change with change in shape or size.
→ When a substance is heated it expands and volume increases. Hence density decreases. Water has maximum density at 4°C i.e. density of water increases from 0°C to 4°C and decreases above 4°C.
→ Volume of substance is measured by formula V = L x B x H or \(\frac {4}{3}\)πr3or by measuring cylinder.
→ Mass is measured by beam balance or spring balance.
→ RELATIVE DENSITY : of substance is the density compare with water i.e. How many times the substance is DENSER than water. Since density of water is 1 gem-3, so density of a substance in g cm-3 = relative den.c ‘ty of substance.
S.I. unit of R.D → has no units → Since it is the ratio of same quantities.
→ If a substance has density more than liquid it SINKS in the liquid and if the density of substance is LESS than liquid it floats on liquid.
→ BUOYANT FORCE : “The force exerted by liquid acting vertically upward on a body and is equal to the weight of liquid displaced by its immersed part.”
→ Weight of body: Acting vertically downward. This force has the tendency to sink the body.
→ LAW OF FLOATATION : “When a body floats in a liquid, the weight of the liquid displaced by its immersed part is equal to the total weight of the body.”
While floating wt. of floating body W = wt. of liquid displaced by its immersed part FB i.e. Apparent wt. of floating
body is zero.
Density of body is greater than density of liquid. The body sinks.
Density of body is equal to the density of liquid. The body float where ever it is left in liquid.
Density of body is less than density of liquid. The body rises to the surface and floats.