FORCES 2 : WEIGHT vs MASS By the end of this presentation, you should be able to : Explain the difference between weight and mass? Describe the difference between the weight of an object on the earth, compared to the same object on the moon (or on another planet), or in outer space. Calculate the weight of an object if given its mass and the gravitational field strength.

What is gravity? Gravity is a natural force field that exists around ALL objects. Yes, you have your own gravitational field around you! The strength of this field is controlled by the mass of the object. The heavier it is, the stronger the field. Gravity is always attractive, and tries to pull you towards the centre of the Earth, wherever you are standing.

More about the moon and planets in our Solar System Some more facts about gravity The closer you are to the planet, the stronger the gravitational pull. The Moon’s gravitational field is 6 times weaker than the Earth’s. More about the moon and planets in our Solar System

The strength of gravitational fields in different places The gravitational field strength on the Earth is 10N/kg. This means a 2kg bag of flour on your foot, will exert a 20N force on your foot. This is its WEIGHT. The table shows how the gravitational fields of the other planets compare to that of the Earth. Go to ZoomSchool

Mass v.s. Weight Your mass is determined by how much matter or atoms make up your body. It is measured in kilograms (kg). This will not change with location i.e. where you are in the Solar System, but it might change… if you go on a food binge! Your WEIGHT depends on how much you are being pulled by the planet. So it depends on your MASS and the GRAVITY on the planet. We can write weight = mass x gravitational field strength Weight is a force so is measured in newtons

Changing mass to weight (using gravity) Remember the equation Weight = Mass x Gravity Put the quantities into a triangle for easy manipulation The astronaut has a mass of 60kg. On the moon, g=1.7N/kg On Earth, g=10N/kg W m x g What is his weight on Earth? W = 60 x 10 = 600N 2. What is his weight on the Moon? W = 60 x 1.7 = 102N