Quote:

Originally Posted by **Waveman28**

At this point, F=ma so the force is 9.8N. How can this be if the ball is not even moving?

This is not a trivial question at all. This kind of questions led great minds to discuss the nature of motion, speed and time for millennia - in fact, from antiquity (see

Zeno's paradoxes) to the days of Isaac Newton.

But thanks to him, we know the answer. Look carefully at the words "at this point" in your post (as quoted here). The ball is "not moving" only for an infinitely short moment. It does not spend any length of time remaining motionless, it just slows down to zero and immediately starts accelerating in the opposite direction.

Its speed is continuosly changing. Take any specific value of speed within a realistic range, say +1 m/s (assume the positive direction is downwards). Now ask for how long the ball will have exactly that speed. The answer will also be "for an infinitely short moment". The same goes for speed zero.

It's like asking where a flying arrow is. Suppose for simplicity that it follows a straight line, and there is a point zero somewhere in its path. Ignore the length of the arrow - think of one point of it, say, the tip. Is there a time when the tip is at point zero?

Yes and no.

The tip only spends an infinitely short length of time at precisely point 0, so you could say no, it spends no time there.

On the other hand, yes, the tip of the arrow does "show up" at point 0, and anybody doubting that is welcome to try and stand there.

Does this help?