BeginnerTopic 3 of 106Engineering Mechanics
Mass vs weight
Mass is the amount of matter; weight is the gravitational force on it, W=mg.
In depth
Mass is the amount of matter and resists acceleration; weight is the gravitational force W=mg and varies with location. Mass is measured in kilograms, weight in newtons. A scale actually measures force.
Key points
- Mass is constant everywhere
- g≈9.81 m/s² on Earth and 1.62 on the Moon
- Weight is a vector pointing to the centre of gravity
ExampleA 10 kg object weighs 98.1 N on Earth but only 16.2 N on the Moon.
What is Mass vs weight?
Mass is the amount of matter; weight is the gravitational force on it, W=mg.
Why is it important to understand mass vs weight?
Confusing mass and weight leads to wrong units and wrong loads, especially when a design moves to the Moon or to a different gravity.
Why does my weight change on another planet but my mass does not?
Mass is intrinsic to matter, while weight is the gravitational force on it. Weight scales with the local gravity, so the same mass weighs less on the Moon.
Why do scales in shops show kilograms?
Scales that show kilograms are calibrated to read mass at standard gravity, so they convert the measured force into a mass value.
How is Mass vs weight used in practice?
Typical applications include calculating the load on a crane hook from the mass of a container, checking the weight of a satellite component for launch limits and estimating the force needed to lift a tank of water.
What should I learn before Mass vs weight?
Study "Newton's laws" first (topic 2 of 106 in Engineering Mechanics). It covers the groundwork this topic builds on.
What are the key points of Mass vs weight?
Mass is constant everywhere. g≈9.81 m/s² on Earth and 1.62 on the Moon. Weight is a vector pointing to the centre of gravity.
Can you give an example of Mass vs weight?
A 10 kg object weighs 98.1 N on Earth but only 16.2 N on the Moon.
What comes after Mass vs weight?
The next topic is "Free-body diagrams", which builds directly on this one.
How difficult is Mass vs weight?
It is classed as Beginner in Engineering Mechanics (topic 3 of 106). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.