Weight, Mass & Gravity

• What is mass? 

  • Mass is the measure of the amount of matter in an object
  • Consequently, this is the property of an object that resists change in motion
  • The greater the mass of a body, the smaller the change produced by an applied force
  • The SI unit for mass is the kilogram (kg)
• What is weight?

  • Weight is the effect of a gravitational field on a mass
  • Since it is a force on an object due to the pull of gravity, it is measured in Newtons (N) and is a vector quantity
  • The weight of a body is equal to the product of its mass (m) and the acceleration of free fall (g)
• Measuring Mass and Weight

  • The weight (and hence mass) of two objects can be compared using a balance

   
• A balance can be used to compare two different weights:

  
• Defining Weight

  • Weight is defined as:The force acting on an object due to gravitational attraction
  • Planets have strong gravitational fields
  • Hence, they attract nearby masses with a strong gravitational force
• • Because of weight:
  • Objects stay firmly on the ground
  • Objects will always fall to the ground
  • Satellites are kept in orbit
• Some of the phenomena associated with gravitational attraction and the weight force:

  
• Acceleration due to Gravity:

  • An object’s mass always remains the same, however, its weight will differ depending on the strength of the gravitational field on different planets
  • For example, the gravitational field strength on the Moon is 1.63 N/kg, meaning an object’s weight will be about 6 times less than on Earth
• On the moon, your mass will stay the same but your weight will be much lower:

  
• • The value of g (gravitational field strength) varies from planet to planet depending on their mass and radius
  • A few examples of varying gravitational field strength are shown below
• Gravitational field strength of the planets in our solar system:

  
• Free Fall:

  • An object in free fall is falling solely under the influence of gravity
  • On Earth, all free-falling objects accelerate towards Earth at a rate of 9.81 m s−2
  • In the absence of air resistance, all bodies near the Earth fall with the same acceleration regardless of their mass