How Everything Works
How Everything Works How Everything Works

Question 763

If the net force on an object is zero and it has no acceleration, then what causes it to have velocity? Doesn't a force give it velocity? And doesn't this make the gravitational and support forces unequal? - EH
The great insights of Galileo and Newton were that an object doesn't need a force on it to have a non-zero velocity. Objects tend to coast along at constant velocity when they are free of forces, or when the net force on them is zero. Inertia keeps them going even though nothing pushes on them. While it takes an acceleration and thus a non-zero net force to get an object moving in the first place, it will continue to move even if the net force on it drops to zero. So while I was lifting the bowling ball upward at constant velocity, the net force on the bowling ball was truly zero—it was coasting upward because its weight and the support force from my hand were canceling one another. However, to start the bowling ball moving upward, I had to push upward on it harder than gravity pushed downward. For a short time, the bowling ball experienced an upward net force and it accelerated upward. After that, I stopped pushing extra hard and let the bowling ball coast upward at constant velocity.

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