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Heavy things are harder to move

Forces and motion · level 1, lesson 4 of 6

A question to start

At the shop, an empty trolley rolls off with a little push. A trolley full of bottles and potatoes needs a big push. Why?

The idea

The more stuff there is in something, the harder it is to get it moving, and the harder it is to stop. A full trolley has much more stuff in it than an empty one, so the same push gets it going much more slowly.

Scientists call the amount of stuff in something its mass. We measure mass in kilograms (kg). A bag of sugar is 1 kg. A big dog might be 30 kg. The rover Perseverance, driving on Mars, is about 1,000 kg, as much as a small car.

empty: rolls farfull: hardly moves

It works for stopping as well. A rolling football is easy to stop with your foot. A rolling bowling ball, going just as fast, is much harder to stop, and it hurts.

Worked examples

1. Two boxes. An empty box and a box full of books, both on a smooth floor. The same push sends the empty box sliding fast; the full box only creeps.

2. Stopping a bike. A grown-up on a heavy bike needs to squeeze the brakes harder, or for longer, than a child on a small bike going at the same speed.

Try it

  1. Which is harder to get moving: an empty trolley or a full one?
  2. What do scientists call the amount of stuff in something?
  3. What unit do we measure it in?
  4. Which is harder to stop, if both roll at the same speed: a football or a bowling ball?
Answers
  1. The full trolley.
  2. Its mass.
  3. Kilograms (kg).
  4. The bowling ball: it has more mass.

Common mistake

"Big means heavy." A balloon can be bigger than a brick but has far less mass. It is the mass, the amount of stuff, that makes something hard to move.

Where it leads