Pick something up with your hand and drop it. When you release it from your hand, its speed is zero. On the way down its speed increases. The longer it falls the faster it travels. Sounds like acceleration to me.But acceleration is more than just increasing speed. Pick up this same object and toss it vertically into the air. On the way up its speed will decrease until it stops and reverses direction. Decreasing speed is also considered acceleration.But acceleration is more than just changing speed. Pick up your battered object and launch it one last time. This time throw it horizontally and notice how its horizontal velocity gradually becomes more and more vertical. Since acceleration is the rate of change of velocity with time and velocity is a vector quantity, this change in direction is also considered acceleration.In each of these examples the acceleration was the result of gravity. Your object was accelerating because gravity was pulling it down. Even the object tossed straight up is falling — and it begins falling the minute it leaves your hand. If it wasn't, it would have continued moving away from you in a straight line. This is the acceleration due to gravity.In this initial experiment the bowling ball drops straight to the ground whereas the feathers float, owing to air resistance.He alludes to the earlier experiment by Galileo that tested the same hypothesis."Galileo’s experiment was simple," he explains. "He took a heavy object, and a light one, and dropped them at the same time to see which fell fastest."Although Galileo’s experiment proved two similarly shaped objects would fall at the same speed despite being different weights, he didn’t have access to a vacuum chamber in the 17th Century to conduct Professor Cox's more extravagant experiment.Professor Cox also used the bowling ball and feather to prove a hypothesis put forward by Albert Einstein.His Special Theory of Relativity argued that items would not be falling but standing still due to lack of force acting on them."Isaac Newton would say that the ball and the feather fall because there’s a force pulling them down: gravity,’ Professor Cox said."But Einstein imagined the scene very differently."The “happiest thought of his life” [as Einstein called it] was this; the reason the bowling ball and the feather fall together is because they’re not falling."They’re standing still. There is no force acting on them at all."He reasoned that if you couldn’t see the background, there’d be no way of knowing that the ball and the feathers were being accelerated towards the Earth."So he concluded they weren’t."The tweaking of Newton’s earlier theory enabled Einstein to more accurately define his own theory, which regards the relationship between space and time.
