Free Fall

Near the surface of the Earth, gravity can be considered to be constant, independent of height. By convention, we put the y-axis of our coordinate system pointing upwards. Since the gravitational acceleration points downwards, we have for the acceleration

Since the gravitational acceleration is constant, free fall is just one example of motion in 1 dimension with constant acceleration. The kinematic equations in this special case are then (with a = -g, and y instead of x):

1. y = y₀ + v₀ t - g t²
2. v = v₀ - g t
3. v_av = (v + v₀)
4. y = y₀ + v_av t
5. v² = v₀² - 2 g (y - y₀)

In these equations, y is the height above ground at time t, and y₀ is the initial height at t=0.

In this little movie clip we drop a roll of tape, and you can observe that the distance covered per time unit (i.e. the speed) increases linearly with time. This can be verified by examining the position of the roll in each frame of the video (step through the video by clicking on the right button in the movie controller bar below the video). To make this task easier we have marked the background with black ticks that are 10 cm apart each.

© MultiMedia Physics, 1999