Re: Do all objects fall at the same speed?

Hi Carina,

It's true that, if you do experiments in your classroom, different objects DO fall at different speeds through the air. A baseball falls very fast; a piece of paper falls slowly; a feather falls very very slowly.

There are two forces at work when you drop an object. One force is gravity, another is air resistance. Gravity is very simple: it pulls on everything, and makes everything speed up in the same way. Sometimes gravity has to pull hard (to speed up a heavy rock) and sometimes gravity has to pull gently (to speed up a light pebble). But, no matter what, gravity always uses just the right force, and makes every object try to speed up (and thus move downwards) in the same way. So, if gravity were the only force, everything would fall in exactly the same way.

If you are dropping things off of tables at school, though, they have to drop through a thick soup of air. Perhaps you are used to thinking of the atmosphere as light, wispy, and not very forceful. But it is dense enough to hold up large balloons. It is thick enough for birds and helicopters to push on. It is viscous enough to slow down a parachute jumper. When you do experiments with gravity, the air will get in the way and slow things down.

Remember I mentioned that gravity always picks the right force, to make things move at the same speed? Air does not do this. Air pushes hard on bigger objects, gently on smaller objects. It pushes harder when an object is moving fast. It really doesn't care how heavy, or how hard-to-move, the object is. So, when air and gravity are acting together, things do NOT fall in the same way.

• If you drop a feather, GRAVITY is pulling gently (since the object is light and easy to move), but AIR is pushing strongly (since the object has a lot of area). The air wins, and the object falls slowly.
• If you drop a whiffle ball, GRAVITY is pulling strongly (since the object is heavy) and AIR is also pushing pretty strongly (since the object has a lot of area). The whiffle ball falls, but not so fast.
• If you drop a lead fishing weight, GRAVITY is still pulling strongly since the object is heavy. But since it is very small, the AIR pushes gently. Gravity wins, and the object is able to fall fast.
• If you drop a huge lead brick, GRAVITY is now pulling really terribly strongly. The AIR may be pushing gently or strongly, it hardly matters since the gravitational force is so strong. Amazingly, despite begin 10000 times heavier than the fishing weight, the brick and the weight will fall at the same speed! (warning: dropping bricks is fun, but it tends to punch holes in the floor.)

If you were able to do your experiments on the moon, you could play with gravity WITHOUT air. Then, you would find that everything falls at the same speeds. On earth, if you can do an experiment in which you move things SIDEWAYS, you can perhaps learn about air resistance and ignore gravity. (For example: take a Superball and throw it down a hallway. How far can you make it go before landing? Now, tape up the same Superball inside of a big styrofoam cup, or a folded sheet of paper. How far can you throw it now? Since you are throwing the ball (mostly) sideways, gravity is less important, and the distance will depend a lot on air resistance.

I hope this helps,

-Ben Monreal