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Notes from the field ·

Which Weighs More: A Kilogram of Feathers or a Kilogram of Rocks?

By Ahmed, founder

“Which weighs more, a kilogram of feathers or a kilogram of rocks?” Most children say rocks, then laugh when the answer is “neither, they weigh the same.” The real lesson is why it feels different.

Mass and volume

A kilogram is a measure of how much matter there is, called mass. A kilogram of feathers and a kilogram of rocks contain the same amount of matter. The feathers take up far more space, called volume. Rocks pack a lot of matter into a small space, which is a way of saying they’re denser.

So what’s different isn’t the mass. It’s how tightly the matter is packed.

Make a balance

You need: a coat hanger, two paper cups, string, tape and a doorknob or hook to hang it on.

  1. Tape a cup on each end of the hanger with string so they hang at the same height.
  2. Hang the hanger from a hook. Adjust until it balances.
  3. Put an object in one cup and another in the other. The heavier side drops.

Balances compare masses directly, and they work anywhere, because they don’t depend on gravity strength.

Estimate, then check

  1. Pick five classroom objects. Rank them by predicted mass.
  2. Use the balance to compare pairs.
  3. Estimate each in grams, then check on a kitchen scale.

Good benchmarks: a paper clip is about 1 gram, and a litre of water is about 1 kilogram.

Hefting

Fill three bags that look very different (cotton balls, a few small stones, a bag of sand) so they might have the same mass. Have students heft them, guess the order and then check. Look at which ones are surprising.

Common mix-ups

  • “Bigger things weigh more.” A balloon is larger than a coin and has less mass.
  • “Weight and mass are the same.” They’re related, but weight is the pull of gravity on mass. In everyday language we use them interchangeably, and the distinction matters in science later on.
  • “The feathers are lighter.” Per bag, per kilogram, they match.

The full lesson is Which weighs more, a kilogram of feathers or a kilogram of rocks? in Measure It. Explore density with our free density lab. Sign up for three lessons free.