A tennis ball drops from rest at the top of a tall glass cylinder—first with the air pumped out of the cylinder so that there is no air resistance, and again after the air has been readmitted to the cylinder. You examine multiflash photographs of the two drops. Can you tell which photo belongs to which drop? If so, how?

Gravity is the sole force acting on an item during a unique sort of motion called free fall. Objects that are claimed to be falling freely do not really experience much air resistance; instead, gravity is the only force acting on them as they descend. Regardless of their mass, all objects will fall with the same rate of acceleration under these circumstances.

When the tennis ball falls inside a glass cylinder with a vacuum no air resistance is present for a tennis ball. The tennis ball in a vacuum falls nearly about the gravitational acceleration inside the cylinder after each jump from the bottom of the cylinder.

The tennis ball in the cylinder feels air resistance after readmission of air in the cylinder so it falls with less acceleration than gravitational acceleration. The speed of a tennis ball with air decreases after each jump from the bottom of the glass cylinder.

The tennis ball falls from the top and jumps from the bottom of the cylinder more frequently in a vacuum so a multiflash photograph of a tennis ball is more regular.

The frequency of falling and jumping off a tennis ball from the top and bottom of a glass cylinder with air is less than in a vacuum so a multiflash photograph of a tennis ball with air becomes blurred.