 Mass, Weight and Density Sec 3 Science Physics

Mass  Qn: Mass is the (a) same as weight (b) amount of substance in a body. (c) amount of gravity acting on a body. Mass is the measure of the amount of substance in a body.

Mass  Qn: Mass is a (a) scalar.(b) vector. Mass is a scalar because it does not have a direction component.  Qn: SI unit of mass is (a) grams(b) kilograms SI unit of mass is kg.

Mass  Mass is measured with  a beam balance or electronic balance.

MASS  Qn: What happens to your mass when you (a)are brought to the moon? (b)are perspiring? (c)change your position from standing to sitting? Ans : (a) Mass does not change with location because the amount of substance in a body remains the same even when it is brought to a different place. (b) When you perspire, you lose water hence the total amount of substance in your body decreases and you have a lower mass. (c) When you change your position, the amount of substance remains the same hence mass does not change.

Mass and Inertia  Newton’s First Law A body remains in its state of rest or motion unless it is acted by a resultant force.  A body resists a change in their state of rest or motion and the resistance is dependent on its mass.  Inertia is the resistance of a mass to change its state of motion or rest.  The larger the mass, the larger the inertia.

Mass and Inertia Application of inertia  Table cloth trick Table cloth trick  Qn: Will placing heavier plates on the table cloth increase or decrease the success rate of the experiment?  Heavier plates have a larger inertia hence it will be more reluctance to change its state of rest and will remain on its original position. Hence success rate will increase.  Egg experiment Egg experiment  Can you explain how this trick work to your partner?  When the support is removed suddenly, due to the inertia of the egg, they will remain at its rest position and not move together with the support.

Mass and Inertia  QN: What happens to the passengers in a bus if the driver applies the brake suddenly? Explain your answer.  ANS: The passengers will be thrown forward. Their inertia resist a change in motion hence they will continue to move forward even though the bus has stopped.

Gravitational Force  The force that exist between two masses is called Gravitational Force.

Gravitational force of the Earth  As a result of gravitational force between the Earth and an object, when the object is dropped on earth, it will fall downwards towards the earth due to the gravitational attraction.

Gravitational Field  Gravitational field is a region in which a mass experiences gravitational force.

Gravitational Field Strength  Gravitational field strength is the gravitational force per unit mass.  The gravitational field strength on earth is 10 N/g.  That means every 1 kg of mass on earth will experience 10 N of gravitational force.  The gravitational field strength of the moon is 1/6 that of the earth’s (i.e N/kg) because moon has a smaller mass than earth.

Weight  Weight is the gravitational force acting on a body.  SI unit = Newton (N)  W = mg (W = weight (N), m = mass (kg), gravitational field strength (N/kg)

Weight  Weight of an object is measured using a spring balance or newton meter.

Weight  Qn: If the mass of your school bag is 8 kg, what is its weight on earth and on the moon? On earth - W = mg = 8 x 10 = 80 N On the moon – W = mg = 8 x 10/6 = 13.3 N  Qn: What is the weight of an apple that has a mass of 200 g? W = mg = (200/1000) x 10 = 2 N

Weight  Qn: Why is there a difference in the person’s weight on the moon and on earth although there is no change in his mass?  ANS:  The moon has a smaller gravitational field strength (g).  Since W = mg, the weight of the man on the moon will be smaller too.

Weight  Do Q 11 to Q 15 of your notes

Density  Density of a substance is its mass per unit volume.  SI unit = kg/m 3  Other common unit = g/cm 3

Density  1000 kg/m 3 = 1 g/cm 3  Qn: The density of iron is 7.86 g/cm 3. What is its density in kg/m 3 ?  Ans: 7.86 g/cm 3 = 7.86 x 1000 = 7860 kg/m 3

Density Qn: Do you think the gold bars, gold chain and gold coin have the same density? Ans: Yes, the density of a substance is a constant that does not depend on its shape or volume.

Density  Glass has a density of 2500 kg/m 3. That means 1 m 3 of glass has a mass of 2500 kg.  Silver has a density of 10.5 g/cm 3. That means 1cm 3 of silver has a mass of 10.5 g.

Density  Qn: The density blocks are made of different materials but they have the same volume. How do you determine which block has the largest density?  Ans: Density is mass/volume and their volume are the same, so the larger the mass, the larger the density. Measure the mass of the blocks, the block with the largest mass will have the largest density.

Density Qn: All the rods on the right have the same mass. Which rod has the smallest density? Ans: The longest rod has the smallest density. Density = mass / volume, since mass is constant, density will be inversely proportional to density. Hence, the smallest density rod will have the largest volume (longest rod).

Density Qn : The density of Olive oil is 0.92 g/cm 3. What is the volume of 500 g of Olive oil? Ans: Volume = mass / density = 500 / 0.92 = 544 cm 3 Qn: The mass of an empty beaker is 110 g. The total mass of the beaker is 158 g when 50 cm 3 of a liquid is poured into it. What is the density of the liquid? Ans: Density = mass / volume = (158 – 110) / 50 = 48/50 = 0.96 g/cm 3

Density Qn: A room is 4 m long, 2.5 m wide and 2 m high. If the density of air in the room is 1.3 kg/m 3, what is the mass of air in the room? Ans: Mass = density x volume = 1.3 x (4 x 2.5 x 2) = 26 kg

Density  When 2 liquids of different densities are poured into the same container, the denser one will sink and the less dense liquid will float.  When a solid is placed in a liquid, it will float if its density is lower than the liquid’s density, and it will sink if it is denser than the liquid.

Density Qn: Why is the woman able to float in the dead sea effortlessly? Ans: The density of dead sea is very high (due to its high salt content) and human body is less dense than the dead sea hence she is able to float in the dead sea. Floating in dead sea

 Qn: Why does normal coke sink in water while diet coke floats in water?  Ans: Diet coke contains less sugar than normal coke hence its density is less. Since it contains a lot of gas, its density will be smaller than that of water and will float. The Coke Experiment

Mass  Do Q16 to Q20 of your notes