Multiple choiceQuestion 1 · Recall
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By the end you can describe and explain the motion of an object with a uniform velocity in one direction and a uniform acceleration at right angles to it, and solve problems on objects launched horizontally.
The main symbols used in this lesson:
| Symbol | Meaning |
|---|---|
| horizontal velocity (it stays the same) | |
| vertical velocity | |
| initial vertical velocity | |
| final velocity |
Fire a bullet horizontally and, at the same instant, drop a second bullet from the same height. Which hits the (level) ground first?
Neither. With air resistance negligible they land together, even though one has flown hundreds of metres and the other has dropped straight down.
That's the big idea of this lesson: the sideways motion and the downward motion ignore each other completely.
Treat a projectile (air resistance negligible) as two separate motions:
| horizontal | vertical | |
|---|---|---|
| force | none | weight, downwards |
| acceleration | zero | downwards |
| velocity | constant, | changes steadily: |
| equations | the suvat equations, with |
The two columns share one thing: the time . Find it from one column, then use it in the other.
Hang a lamp directly above a thrown ball: its shadow on the floor glides along at a steady speed. Put a lamp off to the side: its shadow on the wall falls exactly like a dropped ball. The real ball is doing both at once.
Where the picture breaks down: with significant air resistance the horizontal velocity no longer stays constant, and this simple two-column method stops working (that's topic 3).