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Calculating Weight | Physics for Grade 10 PDF Download

Mass v Weight

  • Mass (measured in kilograms, kg) is related to the amount of matter in an object
  • Weight (measured in newtons, N) is the force of gravity on a mass
    • The weight of an object and the mass of an object are directly proportional
    • The size of this force depends on the gravitational field strength (often called gravity, g, for short)

Exam Tips

It is a common misconception that mass and weight are the same, but they are in fact very different

  • Since weight is a force - it is a vector quantity
  • Since mass is an amount - it is a scalar quantity

Calculating Weight

  • Weight, mass and gravitational field strength are related using the equation:
    Calculating Weight | Physics for Grade 10
  • g is known as the acceleration due to gravity or the gravitational field strength
  • On Earth, this is equal to 9.81 m/s2 (or N/kg)

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/s2
  • In the absence of air resistance, all bodies near the Earth will fall with the same acceleration regardless of their mass

Mass v Weight

  • 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
    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
    Gravitational field strength of the planets in our solar system

Solved Example

Example: A student estimates they would have a weight of 190 N on Mars.The gravitational field strength on Earth is 9.8 N/kg. The gravitational field strength on Mars is 3.8 N/kg.Calculate the weight of the student on Earth.

Step 1: List the known quantities
Weight on Mars, WM = 190 N
Gravitational field strength on Mars, gM = 3.8 N/kg
Gravitational field strength on Earth, gE = 9.8 N/kg

Step 2: Write out the equation relating mass and weight and rearrange for mass
W = mg
Divide both sides by g:
m = W/g

Step 3: Calculate the student’s mass
The student’s mass is the same anywhere in the universe
Calculating Weight | Physics for Grade 10
Calculating Weight | Physics for Grade 10

Step 4: Calculate the student’s weight on Earth
WE = m × gE = 50 × 9.8 = 490 N

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