# Unit 4: Mass, Weight & Density

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Unit 4: Mass, Weight & Density

Lesson Objectives At the end of the lesson, you should be able to understand mass gravitational field gravitational field strength Weight (difference with mass) Solve problems using weight = mass x gravitational field strength

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

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

Mass How do you define mass?
Mass is a measure of the amount of substance in a body. The SI unit of mass is the kilogram (kg) The number and composition of atoms & molecules make up mass of a body.

Can crushing experiment
Mass Can crushing experiment Is there a change in the mass of the can when it is being crushed?

Mass 1. Does your mass change when you go to the moon?
2. Does your weight change when you are on the moon?

Weight Weight is a force and has direction pointing towards the centre of the earth (vertically downwards). Its SI unit is the Newton (N). This force is called gravitational force or gravity.

Weightlessness

What is gravitational field?
The Earth’s gravity is experienced by any object near it. The region surrounding the Earth where gravity is experienced is called the gravitational field. The force experienced is strongest on the surface of the earth and gets weaker further away.

What is gravitational field strength?
Gravitational field strength g is defined as the gravitational force acting per unit mass on an object. On Earth, the gravitational field strength is about N kg-1 A mass of 1 kg will weight 10 N on Earth. On Moon, the gravitational field strength is about N kg-1 A mass of 1 kg will weight 1.6 N on Moon.

Gravitational Field Strength
Note that gravitational field strength varies slightly at different part of the earth as the earth is not 100% symmetrical. The closer to the centre of the earth, the greater is the gravitational field strength. Do you expect yourself to weight more at the poles of the earth?

How are mass and weight related?
The weight or amount of gravitational force acting on an object is dependent on its mass. The weight W can be found by W = m g where m = mass of object (in kg) g = gravitational field strength in (N kg-1)

Common weighing instruments
Electronic balance, spring balance and bathroom scales actually measure the weight and not the mass of an object. Using the scale on Moon will give different readings.

Online Simulation (Your weight on different planets)

Differences between mass and weight

Worked Examples If I have a mass of 70 kg, my weight on earth would be
m x g = 70 kg x 10 N / kg = 700 N

Summary (Key ideas) Definition of Mass & S.I. unit
Definition of Weight & S.I. unit Differences between Mass & Weight W = mg Gravitational field strength (g), in N kg-1 is the same as the acceleration of free fall in m s-2