Earth's gravitational acceleration on any object at its surface
is 9.8 meters per second squared.  Astronomers often round this number
off to 10 meters per second squared; it's close enough for most
calculations.

   (a) A top-of-the-line Harley-Davidson can go from 0 to 75 mph in
       about four seconds on a dry racetrack.  How does this vehicle's
       rate of acceleration compare to Earth's gravity?  Remember to
       convert miles per hour into meters per second.
   (b) If you push this Harley horizontally off a cliff, how fast will
       it be going in four seconds?
   (c) How far will the Harley have fallen in four seconds?  Remember 
       that the acceleration of gravity is constant with time, so the
       Harley's average velocity is half the sum of the initial and
       final velocities.
   (d) Let's say the Harley hits the ground after falling for four
       seconds.  If this Harley weighs 450 kg, how much gravitational
       force is Earth exerting on it when it's one inch off the ground?