In Experimenting...Boys needs to find out which would hit the ground first, small ball or big ball?

Question

a. what variable should they manipulate?

b. what factors should they make the same?

Answer 1

The sizes of the balls makes no difference, since their acceleration is the same.  So if two balls of different sizes (mases) were dropped simultaneously, they would hit the ground at the same time.  This of course does not take into consideration the shapes of the balls, air resistance, and other variables that might affect acceleration toward the ground.

Answer 2

These are some variables.

Mass of the balls.  Size of the balls.  Surface roughness.

If you can get enough balls, there are several combinations that makes sense to compare.

Timing balls dropped straight down is pretty tough.  A stopwatch is typically only good to a tenth of a second.  Maybe you could drop everything at once, and see which ball gets to the ground first with a video camera, played back in slow motion, or even frame by frame.

Another way to do it is roll balls down a ramp.