Mass, Weight and Gravity
In I/GCSE Physics, the building block of all forces and thus motion: Mass. The first thing to clarify is a very common misconception – Mass and Weight are not the same thing! This is unfathomably important to remember:
- Mass is a universal value which is measured in Kilograms and does not change depending on the forces acting upon it.
- Weight is most commonly a planetary confined value that does change depending on the Gravitational Field acting upon it. Weight is also measured in Kilograms... You can see how this will be annoying, just always look for Weight or Mass.
In order to do that we are going to have to understand a little more about Forces: a Force acts as a push or pull on an object, whether that be away from or towards the object applying the Force. The Earth has a Gravitational Field Strength (GFS) of 10N or ten Newtons, meaning that the Earth’s Gravitational Field will apply ten Newtons of force on an object, in this case it will be a pulling Force; this is what we must know in order to calculate weight on earth.
Weight is relative to a planet’s Gravitational Field. Meaning, the same object will have a different weight depending on the GFS of the planet. Let’s calculate it: we know that the GFS of Earth is 10N, meaning that for every one kg of mass there will be 10N of Force. Hence, we must times our Mass by 10 in order to calculate our Weight on Earth.
Say we have an object with a mass of 2kg, what would it weigh on Earth?
1. All we must do is multiply it by 10 and we have the Weight on Earth; 2 × 10 = 20.
2. Therefore, our object weighs 20kg on Earth.
a) The GFS of the moon is 1.6, find how much you would weigh on the moon?
b) Find how much you would weigh in space (your Mass).
Formula for Weight: W = mg (Weight = Mass × GFS)
N.B. ‘g’ ≠ Gravity! If you state this in the exam you will be marked down! ‘g’ = Gravitational Field Strength.
End of this topic!