Engineering Expert Witness Blog

      Last time we watched a video of Astronauts Scott and Irwin simultaneously dropping a hammer and feather to the surface of the Moon and were amazed to find that the objects struck the surface at exactly the same time.   It was history in the making, and Galileo’s theory regarding gravity was proven beyond a shadow of a doubt.

      If you watched the video of the event very closely, it might have struck you that the hammer was falling more slowly than it would had it been dropped on Earth, and you’d be right.   Let’s find out why.

      Sir Isaac Newton was a pioneer in this subject matter.  According to his book, Philosophia Naturalis Principia Mathematica, first published in 1687, every heavenly body in the universe, whether it be planet, moon, or star, generates gravity, and any object in freefall towards its surface will be subject to that gravity.   He posited that the falling object will gain speed at a constant rate as it falls, that constant speed being dictated by the acceleration of gravity factor that’s at play on the heavenly body it’s falling toward.

      For example, if the acceleration due to gravity on a small planet is, say, 2 feet per second per second, after one second of falling, an object’s velocity will be 2 feet per second.   After two seconds of falling, the object will have accelerated to a velocity of 4 feet per second.    After three seconds, the object’s velocity will have accelerated to 6 feet per second, and so on.

      In other words, the speed, or velocity, of the object’s descent will increase for every second it falls closer to the surface of the planet, a phenomenon which is measured in units of feet per second (ft/sec).   The acceleration of gravity of a falling object is the linear increase in its velocity that takes place during each succeeding second of its fall, a phenomenon which is measured in feet per second per second (ft/sec²).

      Next time we’ll discuss the formula which enables us to calculate gravitational acceleration.   We have Sir Isaac to thank for that one, too.

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