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Mass, weight and density
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Mass, weight and density

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  • 1. What is a Mass? Mass is the amount of matter in a body Mass is a property that does not change with its location or shape. The SI unit of mass is the kilogram (kg) What is a Weight? The weight of an object is the gravitaional force, or gravity, acting on it Weight is a force, so its SI unit is the newton (N). The direction of weight is downward – towards the centre of the Earth.
  • 2. What is a gravitational field? A gravitational field is a region in which a mass experiences a force due to gravitational attraction What is a gravitational field strength? Gravitational field strength g is defined as the gravitaitonal force acting per unit mass.
  • 3. • The weight of an object depends on the strength of the gravitaional force acting on it. • On Earth the gravitational field strength is approximately 10 N/kg. • 1 kg mass on Earth experiences a force of 10 N
  • 4. Mass and Weight Relation The weight or gravitaitonal force acting on an object is dependent on its mass. The more mass an object has, the grater the gravitaitonal force acting on it. W = mg W = weight (N) m = mass of the object (kg) g = gravitational field strength (N/kg)
  • 5. How is mass and weight measured? • Weight is measured using electronic balance, spring balance, bathroom scale, etc. • Mass is measured using a beam balance. Spring balance and bathroom scale Beam balance
  • 6. Difference between mass and weight Mass Weight • An amount of matter • A gravitational force • A scalar quantity • A vector quantity • SI unit: kilogram (kg) • SI unit: newton (N) • Independent of the gravitational field strength • Dependent on the gravitational field strength • Measured with a beam balance or a calibrated electronic balance • Measured with a spring balance
  • 7. Examples 1. The gravitational field strength of Jupiter is 22.9N/kg. An astronaut wighs 1200 N on Earth. Assuming that the Earth’s gravitational field strength is 10N/kg, find out his weight in Jupiter. Solution: Mass of astronaut = weight / gravity = 1200 N / 10 N/kg = 120 kg Weight in Jupiter = mass x gravity = 120 kg x 22.9 N/kg = 2748 N
  • 8. 2. An object on planet A has a mass of 5 kg. Its weight is 7 N. Find out the gravitational field strength in planet A. Solution: Gravity = Weight / Mass = 7 N / 5 kg = 1.4 N/kg
  • 9. Inertia The inertia of an object refers to the reluctance of the object to change its state of rest or motion, due to its mass What is Inertia? • Mass is a measure of inertia. • An object with a greater mass will have greater inertia.
  • 10. View animation of inertia
  • 11. Density The density of a material is its mass per unit volume. What is Density? The symbol most often used for density is ρ Formula ρ = m / V ρ = density m = mass V = volume The SI unit of denstiy is kg m-3
  • 12. Example 1. A brick of salt measuring 10.0 cm x 10.0 cm x 2.0 cm weighs 433 grams. What is its density? Solution: Density = mass/Volume Step 1: Calculate Volume Volume = length x width x height Volume = 10.0 cm x 10.0 cm x 2.0 cm Volume = 200.0 cm3 Step 2: Determine density Density = mass/Volume Density = 433 g/200.0 cm3 Density = 2.165 g/cm3 Answer: The density of the salt brick is 2.165 g/cm3.
  • 13. The End