MASS, WEIGHT AND DENSITY

MASS Mass is a measure of substance in a body. SI unit of mass is Kilogram (kg) and other units are gram (g), milligram (mg) and tonne. Mass can be measured using beam balance and electronic balance.

Mass of the body will remain same everywhere. Mass of a body resists to change from its state of rest or motion, this is called inertia. A body is resists to change its motion due to inertia. Greater the mass greater inertia. So that’s why the heavier objects are difficult stop when it is in motion.

WEIGHT Weight of a body is attractive force exert on it due to gravity. SI unit of weight is newton (N). Weight of an object can be measured by using spring balance (newton meter). Weight of a body varies in different places. It depends on gravitational field strength.

Gravitational field Gravitational field is a region in which a mass experiences a force due to gravitational attraction. Gravitational field on the surface of the Earth is 10N/kg. This means that a gravitational force of 10N is exerted on an object of mass 1kg. Near to the surface of the moon is 1.6 N/kg. Relationship between mass, weight and gravitational field Weight = mass × gravitational field strength. w = mg

weight on the moon = mg = 5 × 1.6 =8N Example Question 1 The mass of an object on the Earth is 25Kg. Calculate the weight of the object? Ans: w = mg = 25 × 10 = 250N Example Question 2 On the Earth, the gravitational field strength is 10 N/kg. On the Moon, the gravitational field strength is 1.6 N/kg. If an object has a weight of 50 N on Earth, what is its weight on the Moon? m = w/g = 50/10 = 5kg weight on the moon = mg = 5 × 1.6 =8N

Density is the Mass per unit Volume What is Density? If you take the same volume of different substances, then they will weigh different amounts. Wood Water Iron 1 cm3 1 cm3 1 cm3 IRON 0.50 g 1.00 g 8.00 g Q) Which has the greatest mass and therefore the most dense? Density is the Mass per unit Volume

Density Equation: = m V m V  Mass Density = Volume g or kg m V  Mass Density = Volume Unit of density: kg/m3 or g/cm3 cm3 or m3 = m V If the different size of object has same density means all the objects is made up of same material.

V 10 V V  2.5 Example questions: Q1. Mass of aluminium block is 27g and volume of it is 10cm3 Calculate the density of the block.  = m = 27 = 2.7 g/cm3 V 10 Q2. What is the mass of the air in a room measuring 5m × 2m × 3m? Density of the air is 1.3 kg/m3  = m , m=  × V = 1.3 × 30 = 39 kg V Q3. Mass of the glass block is 125g. Density of the glass block is 2.5 g/cm3. Calculate the volume of the block.  = m, V = m = 125 = 50 cm3 V  2.5

DENSITY OF A REGULAR SOLID Find the Mass of the solid on a balance. Measure the length, breath and height and calculate the Volume. (ie V = l x b x h ) Calculate the Density. m = 240 g 2.0 cm  = m = 240 =10.0 g/cm3 V 24 3.0 cm 4.0 cm

DENSITY g/cm3 Aluminium 2.70 Iron 7.86 Brass 8.50 Wood 0.50 Slate 2.80 Glass 2.50 Lead 11.3 Marble 2.70 Wax 0.89

DENSITY OF AN IRREGULAR SOLID Find the Mass of the irregular solid by using electronic balance. Add the known volume of water into the measuring cylinder and this initial volume. Put the irregular solid into measuring cylinder. Measure the volume of water and this final volume. Find the volume of the solid by subtracting initial volume of the water from final volume of the water. (80 − 50 = 30 cm3 ) Calculate the density of the solid using the formula:  = m V m = 360 g 80 cm3 50 cm3  = m = 360 =12 g/cm3 V 30

Example question:1 A lump of metal has a mass of 210 g. It is lowered into a measuring cylinder containing water. The level of the water rises from 35 cm3 to 140 cm3. What is the density of the metal?  

Example question:2 A measuring cylinder contains 118 cm3 of water. When a small object is fully immersed in the water, the reading goes up to 132 cm3. The object has a mass of 42 g. What is the density of the object?  

DENSITY OF AN IRREGULAR SOLID OR use a Eureka Can to find the Volume. Find the mass of the solid on a balance. Add water until just overflowing. Place a Measuring Cylinder under the spout. Add the Object. Collect the Water and read off the Volume. Calculate Density m = 440 g 40.0 cm3  = m = 440 =11.0 g/cm3 V 40

DENSITY g/cm3 Aluminium 2.70 Iron 7.86 Brass 8.50 Wood 0.50 Slate 2.90 Glass 2.50

DENSITY OF A LIQUID Find the Mass of an empty Measuring Cylinder. Mass of Liquid = Mass of Measuring Cylinder and Liquid – Mass of empty Measuring Cylinder DENSITY OF A LIQUID Find the Mass of an empty Measuring Cylinder. Add a certain Volume of Liquid. Find the Mass of the Measuring Cylinder and Liquid Calculate the Mass of Liquid. How? Calculate Density of Liquid. 45.0 g 25.0 g 20.0 cm3 45 – 25 = 20 g  = m = 20 =1.00 g/cm3 V 20

Mass of empty Measuring Cylinder Liquid Mass of empty Measuring Cylinder in g Mass of Measuring Cylinder and Liquid Mass of Liquid Volume in cm3 Density in g/cm3

DENSITY OF A GAS Remove the air from a flask of a known Volume, using a vacuum pump. Find its Mass. Add the gas to be tested. Reweigh. The difference is the Mass of gas. Calculate Density. To vacuum flask 150.0 g 170.0 g 1000 cm3 170 -150 = 20.0g  = m = 20 =0.0200 g/cm3 V 1000