Race two jars--one full of water and the other empty (actually, it's full of air). Take a guess as to which jar will finish first.
two identical jars with lids
Fill one of your jars with water.
Put lids on both of the jars. Make sure the lid on the jar full of water is on tight.
Place a three-ring binder on a level floor and start both of the jars from the top of the 'ramp'.
Which one gets to the bottom of the ramp first?
Which one rolls the farthest?
Were you surprised at the outcome?
When the race begins, the jar full of water moves down the ramp faster than the empty jar. This happens because its weight is evenly distributed throughout its volume, thanks to the water inside it. The empty jar's weight is all in the glass perimeter so it doesn't roll quite as fast. But as the jars begin rolling on the flat surface, the greater weight of the full jar causes friction between the jar and the floor as well as friction between the water and the inside of the jar. The full jar slows down, allowing the lighter, empty jar to take the lead!
Friction is the resistance to motion of surfaces that touch; resistance of a body in motion to the air, water, or other medium through which it travels or to the surface on which it travels. Oil reduces friction. Bodies moving through a vacuum encounter no friction. A sled moves more easily on smooth ice than on rough ground because there is less friction.