You can't see it. You can't hear it. You can't taste it. You can't smell it. Yet we know it's there. Gravity, I mean.

What a strange thing! Well, not a thing, really. Certainly not a thing like tables and chairs and stuff. But definitely something. I can feel it. Can't you?

When you're a little tired, for instance, don't you feel heavy? Don't you just want to sit down, or maybe lie down?

And when you trip over a crack in the sidewalk or slip on the ice, don't you feel like you're being pulled to the ground? That's what we call falling, as you know. But isn't it more like being yanked and slammed down by a giant force? You may catch yourself, of course, and not hit the ground. Even so, you have to admit you were headed there and would have gone all the way if you hadn't acted in time. You were being pulled down and you saved yourself. That means you exerted your muscles to keep your body from hitting the ground.

In fact, that's what you do when you "carry" things. You pull up on them to keep them from falling to the ground. You just do it over a longer period of time. You hold them up and your arms get tired. You pull upwards to keep them in place. Otherwise they'd crash. The upward pull is a force. You apply upward inertial force to counter the downward force of gravity, to use Isaac Newton's conceptual model of the phenomenon.

You do the same when you throw a ball in the air, only more so. At a minimum, you do the opposite of what gravity does to pull the ball down. First of all, because you're holding the ball, you counterbalance gravity. But then you apply even more force than you need just to hold the ball up. It's the extra-added force that gets the ball moving upwards. Your inertial force overcomes the opposing gravitational force.

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