Will the initial velocity always be zero if the body has uniform acceleration??
Answers
Answer:
Theoretically, yes. Empirically measurable, no.
(First, let’s assume zero air drag, for the sake of this discussion)
I will explain. If a ball is thrown straight up, from the instant it leaves your hand, it is experiencing a gravitational force down which will produce a resulting acceleration down of 9.8 m/s/s towards the earth. (It was actually experiencing that force BEFORE it left you hand too, but more on that later). This never changes while the ball is in flight. Now we need to consider if, at any time during the flight the velocity is zero.
As it turns out, logic tells us that if it is traveling up, and then at some later time is found to be traveling down, there must have been some intermediate point in time that the velocity was zero. This point in time can be determined using calculus, but it cannot be measured empirically, since the length of time that the ball is truly motionless is infinitely short. The velocity in this case must be stated as “instantaneous velocity”. Indeed, at the top of it’s flight, (if thrown vertically), a balls instantaneous velocity is zero, while it’s acceleration during the flight is constantly 9.8 m/s/s, until it hits the ground. The velocity vector changes as the ball first goes up and then down, both in amplitude and direction, but the acceleration vector does not.
To further clarify, objects sitting on the ground are clearly not accelerating. Acceleration only occurs when the sum of all the force vectors is other than zero. Once the ball is on the ground, the force (not the acceleration, but the force) that brought it back down to the ground is then experienced by the ball as weight. Let’s say the ball weighs one pound. If this is the case, and the ball isn’t moving (relative to the ground), then obviously the ground must be pushing up on the ball with the same one pound force. It isn’t moving, so the sum of the force vectors must be zero. The ground must be pushing up with the same force the ball is pushing down with.
People seem to get the terms confused, and this is understandable. Gravitational force IS NOT acceleration. Gravitation force indeed PRODUCES an acceleration in objects, at a rate of 9.8 m/s/s, but obviously this acceleration the object is experiencing occurs because the sum of the force vectors is other than zero.