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Unit 4: Mass, Weight & Unit 4: Mass, Weight & DensityDensity

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Lesson ObjectivesLesson Objectives

At the end of the lesson, you should At the end of the lesson, you should be able to understand be able to understand

massmass gravitational fieldgravitational field gravitational field strengthgravitational field strength Weight (difference with mass)Weight (difference with mass) Solve problems using weight Solve problems using weight = mass x gravitational field strength= mass x gravitational field strength

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How heavy are you??? How heavy are you???

Does the weighing Does the weighing scales you use scales you use determine the determine the mass or weight of mass or weight of yourself? yourself?

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How heavy do you think they are?How heavy do you think they are? a newborn baby? a newborn baby? About 3 kgAbout 3 kg a car? a car? 1300–2000 kg, i.e. 1.3 –2.0 tons1300–2000 kg, i.e. 1.3 –2.0 tons an elephant? an elephant? 6200–7000 kg, i.e. 6.2–7.0 tons6200–7000 kg, i.e. 6.2–7.0 tons an airplane (e.g. Boeing 747-400)? an airplane (e.g. Boeing 747-400)? about 180 tons (empty)about 180 tons (empty) the Earth? the Earth? about 6 about 6 10 1024 24 kgkg

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MassMass How do you define mass?How do you define mass? Mass is a measure of the Mass is a measure of the amount of amount of

substancesubstance in a body. in a body. The SI unit of mass is the kilogram (kg)The SI unit of mass is the kilogram (kg)

The number and composition of atoms & molecules make up mass of a body.

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MassMass

Is there a change in the mass of Is there a change in the mass of the can when it is being crushed?the can when it is being crushed?

Can crushing experimentCan crushing experiment

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MassMass

1. Does your mass change when you 1. Does your mass change when you go to the moon?go to the moon?

2. Does your weight change when 2. Does your weight change when you are on the moon?you are on the moon?

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WeightWeight

Weight is a force and has direction Weight is a force and has direction pointing towards the centre of the pointing towards the centre of the earth (vertically downwards).earth (vertically downwards).

Its SI unit is the Newton (N).Its SI unit is the Newton (N). This force is called gravitational force This force is called gravitational force

or gravity.or gravity.

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Weightlessness

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What is gravitational field?What is gravitational field?

The Earth’s gravity is The Earth’s gravity is experienced by any object experienced by any object near it.near it.

The region surrounding the The region surrounding the Earth where gravity is Earth where gravity is experienced is called the experienced is called the gravitational fieldgravitational field..

The force experienced is The force experienced is strongest on the surface of strongest on the surface of the earth and gets weaker the earth and gets weaker further away.further away.

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What is gravitational field strength?What is gravitational field strength? Gravitational field strength g is defined as Gravitational field strength g is defined as

the the gravitational force acting per unit gravitational force acting per unit mass on an object.mass on an object.

On Earth, the gravitational field strength is On Earth, the gravitational field strength is about about 10 N kg10 N kg-1-1

A mass of 1 kg will weight 10 N on Earth.A mass of 1 kg will weight 10 N on Earth.

On Moon, the gravitational field strength is On Moon, the gravitational field strength is about 1.6 N kgabout 1.6 N kg-1-1

A mass of 1 kg will weight 1.6 N on Moon.A mass of 1 kg will weight 1.6 N on Moon.

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Gravitational Field StrengthGravitational Field Strength

Note that gravitational field Note that gravitational field strength varies slightly at different strength varies slightly at different part of the earth as the earth is not part of the earth as the earth is not 100% symmetrical.100% symmetrical.

The closer to the centre of the The closer to the centre of the earth, the greater is the earth, the greater is the gravitational field strength.gravitational field strength.

Do you expect yourself to weight Do you expect yourself to weight more at the poles of the earth?more at the poles of the earth?

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How are mass and weight How are mass and weight related?related?

The weight or amount of gravitational The weight or amount of gravitational force acting on an object is dependent on force acting on an object is dependent on its mass.its mass.

The weight W can be found byThe weight W can be found by

W = m gW = m g

where m = mass of object (in kg)where m = mass of object (in kg) g = gravitational field strength in (N kgg = gravitational field strength in (N kg-1-1))

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Common weighing instrumentsCommon weighing instruments Electronic balance, spring balance and Electronic balance, spring balance and

bathroom scales bathroom scales actually measure actually measure the weightthe weight and not the mass of an and not the mass of an object.object.

Using the scale on Moon will give Using the scale on Moon will give different readings.different readings.

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Online Simulation (Your weight on Online Simulation (Your weight on different planets)different planets)

http://www.exploratorium.edu/ronh/http://www.exploratorium.edu/ronh/weight/weight/

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Differences between mass and weight

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Worked ExamplesWorked Examples

If I have a mass of 70 kg, my weight If I have a mass of 70 kg, my weight onon

earth would be earth would be

m x gm x g

= 70 kg x 10 N / kg= 70 kg x 10 N / kg

= 700 N= 700 N

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Summary (Key ideas)Summary (Key ideas)

Definition of Mass & S.I. unitDefinition of Mass & S.I. unit Definition of Weight & S.I. unitDefinition of Weight & S.I. unit Differences between Mass & WeightDifferences between Mass & Weight W = mgW = mg Gravitational field strength (g), in N Gravitational field strength (g), in N

kgkg-1 -1 is the same as the acceleration is the same as the acceleration of free fall in m sof free fall in m s-2-2