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### Formula for Gravity

```
Date: 9/2/96 at 19:48:17
From: Anonymous
Subject: Formula for gravity

Hi! My name is Eric. I am curious if there is a way to figure out the
force of gravity on an object. For example, if I throw a ball straight
up in the air at 40 miles an hour, how long will it take for the ball
to come down?

Thanks a lot.
```

```
Date: 9/5/96 at 9:59:19
From: Doctor Jerry
Subject: Re: Formula for gravity

Hi, Eric.

Galileo and Newton figured out how to answer this question about 400
years ago. To make the problem easier, they decided to ignore the
resistance of the air. It's possible to work out the time it takes for
the ball to come down in air, but the calculation is quite a bit
harder.  So, if you don't mind too much, I'll give the easier answer.

Galileo and Newton said that the force of gravity on an object is
proportional to the acceleration it causes.  At the earth's surface,
the acceleration of the object due to the earth's pull is about
32 feet per second per second.

takes for the ball to return to the same height from which it was
released at 40 mph.  All you have to do is to divide the initial
velocity (expressed in feet per second) by 32/2 = 16.

Can you convert 40 miles per hour to feet per second?  The calculation
is 40 * 5280 / 3600 = 58 and 2/3 feet per second.

So, divide 58 and 2/3 by 16 to find out how long it takes for the ball
to come down (in a vacuum).

-Doctor Jerry,  The Math Forum
Check out our web site!  http://mathforum.org/dr.math/
```
Associated Topics:
High School Physics/Chemistry

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