How Everything Works
How Everything Works How Everything Works

Question 1323

When you drop a baseball and a bowling ball, you say that its velocity acts faster and faster as it falls. How can you say that the acceleration is constant at 9.8 m/s2? If it is falling faster and faster wouldn't the acceleration change also until the object reaches terminal velocity and then it would be accelerating at 9.8 m/s2?
It's very important to distinguish velocity from acceleration. Acceleration is caused only by forces, so while a ball is falling freely it is accelerating according to gravity alone. In that case it accelerates downward at 9.8 m/s2 throughout its fall (neglecting air resistance). But while the ball's acceleration is constant, its velocity isn't. Instead, the ball's velocity gradually increases in the downward direction, which is to say that the ball accelerates in the downward direction. Velocity doesn't "act"—only forces "act." Instead, a ball's velocity shifts more and more toward the downward direction as it falls.

About terminal velocity: when an object descends very rapidly through the air, it experiences a large upward force of air resistance. This new upward force becomes stronger as the downward speed of the object becomes greater. Eventually this upward air resistance force balances the object's downward weight and the object stops accelerating downward. It then descends at a constant velocity—obeying its inertia alone. This special downward speed is known as "terminal velocity." An object's terminal velocity depends on the strength of gravity, the shape and other characteristics of the object, and the density and other characteristics of the air.


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