This is a demonstration of the
acceleration of gravity on a ball that I’m going to throw straight up. Just like that. While it’s in the air, during the first
leg of its journey, it’s going up. So here’s the ball when I release it. Its velocity vector is headed in the vertical
direction. The velocity decreases. As it’s rising, it slows down because gravity
is slowing it down, pulling it in the direction opposite its motion. When it reaches the top, its velocity is zero,
it’s momentarily at rest, and then gravity starts pulling it down. And now gravity is acting in the direction
of the velocity. Here’s the velocity vector in black, and
gravity is pulling it down. In green are shown the gravitational acceleration
vectors, and these are always down. So the gravity is acting down while its on its way up; the gravity is acting down while its on its way down; and the gravity is also acting down when it’s at the top,
when its velocity is zero. Were it not so, if the gravity didn’t act
on it when it was at the top, it would get up to the top and stay there forever. So that’s how you know that even though
the velocity vanishes, is zero, at the top, gravity still must be acting because the velocity vector
is changing: it’s going from up to down. So now, when you tell your grandkids about
what happens to a ball, you’ll tell them that at the peak
of its vertical journey its velocity is zero, but its acceleration is still 9.8 m/s^2 pointing down.

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Dennis Veasley

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