Guest post: What's the Difference between Weight and Mass?

Question posed by William, and answered by his mum, Jennie, who is a primary school teacher in the midlands. This is a topic I've been thinking of covering for a while as a lot of people seem to mix the two up, but I don't need to now because Jennie's done it for me. Of course, if the post below throws up any further questions, you know what to do...

What is the difference between weight and mass?

This is not a question I was expecting while driving my children back from a trip to Grandma’s house. And not a question I expected from my nine year old son. I have to confess that initially it left me wrestling with a tricky concept which I had no idea how to explain accurately but simply enough for him to understand. I don’t like to fob children off with “go and google it” if I can help it, and I saw this as a challenge to my teaching skills.

It occurred to me later that many adults could do with a similarly plain speaking explanation, so here it is:

Take a look at your body. This may be a trauma if you have one like mine*, but take a good look.
Now imagine you are on the moon. Look at your body. Apart from the fact you would be wearing a space suit, it is unchanged, the size and shape is exactly as it was on earth. Now get back in your spacecraft and go for a trip into deep space. Look at your body. Still the same? Yep. No change in size or shape. This is your “mass”. It is the amount of “stuff” your body is made up of.

But what if you weighed yourself in these three different locations?

You probably know roughly what your “weight” is here on Earth. But what would it be on the moon? You would weigh significantly less. And in space you would be weightLESS, your body would not weigh anything at all**. Remember, your body mass (its shape and size) is unchanged. So what causes the difference in your weight in these three places?

The answer is gravity.

Gravity is the downward pull on our mass, and the more gravity we are subjected to, the heavier we become. Think about that feeling on a rollercoaster, how heavy you suddenly feel as it shoots you skyward, and inversely, how light you feel as it plummets back to the ground***. So although your mass stays the same on the Earth, the moon, and in space, your weight will vary because the pull of gravity is different in each location.

As I reminded a friend who was fretting about her weight...Weight is just a number that would not matter if you lived in space!

The trouble is, your mass still would.

* Tom says: Or mine.

**  Tom says: Welllll... not exactly. Astronauts in space are not, strictly speaking entirely without weight ("weightlessness" is a bit of a misnomer), but for the purposes of this post it's not too far off the mark to think of it like that, so I'll let it swing. Feel free to ask for a post about it, though ;-)

***Tom says: The changes in how heavy you feel on a rollercoaster aren't due to changes in gravity. I know this isn't what Jennie's saying, which is why I've left it in, but I thought I should add a note to clarify: it's a decent analogy to explain how you might feel in a different gravitational field (standing on the Moon, for example), but the cause is different.