Gravity
 

Falling objects 

Two objects, such as a pebble and a block, released from the same height above ground will fall at the same rate and reach the ground at the same time. If air resistance is removed all objects fall to Earth at the same rate. Children will find this concept difficult to test, as air resistance cannot be removed. They will discover that a pen and a feather will fall at different rates, as the surface area of the feather is much greater than that of the pen; the feather is therefore affected to a much greater extent by air resistance.