This negative makes me think we may have made a mistake, so let's look at our variables.
Well, we said the initial velocity was 20 meters per second. Yeah, that was absolutely true.
We threw it up at 20 meters per second. We said the final velocity was 0.
We can't really argue that. At the top of the trajectory, it does have to stop for a brief instant.
And we said the acceleration was 10 and that's meters per second squared.
When on earth isn't that always the acceleration? Well, here's where we have to be careful.
When we said that the initial velocity was +20, we made a choice.
We said, "Hey, in this problem, up is positive."
Now, we were free to make that choice. We could've said, "Down was positive too."
But once we make our choice, we're no longer free.
Acceleration. Objects always accelerate downward. They accelerate towards the earth.
They don't spring up off the earth.
If we've chosen up is positive, we have to choose A as negative since it points downwards.
And if we carry that out, we get a +20 here and so the maximum height is 20 meters.
Remember whenever you make a calculation in physics,
you are calculating something about the real world.
If you're number doesn't make sense in the real world, you did something wrong.
This is nice because it's a built-in sanity check.
We can always check to see that our number isn't totally crazy,
and we know if it is totally crazy we made a mistake.
If it's not, we still may have made a mistake but we feel more confident with a better answer.
One more question before we move on.
And this time you throw a ball again straight up at 20 meters per second.
But now I want to know how long does it take to reach the highest point. Enter your answer here.
