Gravity Explained Simply: Why a Fired Bullet Lands Just as Fast as a Dropped One
Imagine: you fire a bullet horizontally from a gun. At the exact same moment, someone next to you simply drops a second bullet from the same height. Which bullet reaches the ground first?
Answer: both at the same time (ignoring air resistance, as is usually done in physics class).
The four terracotta dots show the dropped bullet at four equal moments in time, the four brick-red dots the fired bullet at the same moments. At every moment, both have fallen exactly the same distance - only the brick-red bullet is also moving to the right.
Why is that?
Motion in space can be broken down into two independent directions: horizontal and vertical.
- Horizontal: the fired bullet simply flies straight ahead at constant speed (no force acts in this direction, ignoring air resistance).
- Vertical: exactly the same gravitational force pulls both bullets downward - regardless of how fast, or whether, they move horizontally.
Gravity "knows nothing" about the bullet's horizontal speed. It pulls both bullets downward at exactly the same rate. That's why both bullets cover the same falling distance in the same time - and hit the ground at the same moment.
đź’¬ Mia asks Grandpa Theo
Wait - why doesn't the fired bullet fall faster than the one I simply drop? It flies off so much faster!
Yes, but only sideways! Downward, gravity pulls both bullets exactly equally hard. The fast sideways motion doesn't change that at all.
Wow, I wouldn't have guessed that! Does it have a name?
Physicists call this the independence of motion directions. Horizontal and vertical are like two separate stories happening at the same time!
Note: in the real world, air resistance slows fast projectiles down slightly, which changes the result a tiny bit. But for physics class (and almost all everyday cases), air resistance is ignored - the principle stays the same.
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