2Lesson ObjectivesAt the end of the lesson, you should be able to understandmassgravitational fieldgravitational field strengthWeight (difference with mass)Solve problems using weight= mass x gravitational field strength
3How heavy are you???Does the weighing scales you use determine the mass or weight of yourself?
4How heavy do you think they are? a newborn baby?About 3 kga car?1300–2000 kg, i.e. 1.3 –2.0 tonsan elephant?6200–7000 kg, i.e. 6.2–7.0 tonsan airplane (e.g. Boeing )?about 180 tons (empty)the Earth?about 6 1024 kg
5Mass 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.
6Can crushing experiment MassCan crushing experimentIs there a change in the mass of the can when it is being crushed?
7Mass 1. Does your mass change when you go to the moon? 2. Does your weight change when you are on the moon?
8WeightWeight 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.
10What 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.
11What 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-1A mass of 1 kg will weight 10 N on Earth.On Moon, the gravitational field strength is about N kg-1A mass of 1 kg will weight 1.6 N on Moon.
12Gravitational 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?
13How 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 byW = m gwhere m = mass of object (in kg)g = gravitational field strength in (N kg-1)
14Common 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.
15Online Simulation (Your weight on different planets)
17Worked 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
18Summary (Key ideas) Definition of Mass & S.I. unit Definition of Weight & S.I. unitDifferences between Mass & WeightW = mgGravitational field strength (g), in N kg-1 is the same as the acceleration of free fall in m s-2