Q1.
Suppose you are travelling on a ship which is moving at a constant speed along a straight line on a calm sea. Suppose there is no window on the ship. Is there any way that you can determine whether the ship is moving or is stationary?
Answer
No. Shut inside the cabin, there is no experiment you can perform that will tell you whether the ship is moving or standing still — provided the speed really is constant, the path really is straight and the sea really is calm.
Why not? Everything inside the cabin — you, the floor, the air, a glass of water — is moving together with the ship at exactly the same speed. So the position of each of these things with respect to the ship never changes, and the ship is the only reference point you have left.
| Test you might try | Ship standing still | Ship moving steadily |
|---|---|---|
| Drop a coin from your hand | Falls straight down at your feet | Falls straight down at your feet — same result |
| Pour water into a glass | Falls straight into the glass | Falls straight into the glass — same result |
| Throw a ball up and catch it | Comes back to your hand | Comes back to your hand — same result |
| Look at the water surface in a bowl | Flat and level | Flat and level — same result |
When you can tell. The moment the motion stops being steady and straight, your body feels it at once:
- the ship speeds up or slows down — you are pushed backwards or forwards;
- the ship turns — you lean to one side, and hanging objects swing;
- the sea is rough — the cabin rolls and pitches;
- you open a window — now the sea and the shore are available as an outside reference point, and the answer becomes obvious.
The big idea: steady motion in a straight line cannot be detected from inside. This is why you can read a book comfortably in a train running smoothly at 100 km/h, but the same book slides off your lap when the train brakes. Galileo described exactly this puzzle — using a ship, just as your textbook does — about four hundred years ago.
Did you know? This is also why we do not feel the Earth's motion. The Earth carries us around the Sun at roughly 30 km every second, but it does so smoothly, and everything around us — air, buildings, clouds — travels along with us.