Answer:
Acceleration defines change of velocity of a body in a unit time interval. It measures how much velocity is changing in a given time interval. If the body has uniform velocity, it means velocity is not changing. So the change in velocity is zero. In other words, its acceleration is zero.
Explanation:
Acceleration roughly means “change in velocity.” If this change in velocity is consistently in one direction, the body will reach a point at which its velocity is zero. Another consideration is when you drop a basketball onto the pavement. The ball falls, hits the ground, and comes back up. Because its direction of motion changed, you know that its velocity MUST have been 0 m/s at some point during the bounce. This doesn't mean there was no acceleration at this point. Let's suppose that the object has no acceleration when its velocity is zero. The ball would fall, strike the ground, and stick to it like superglue. Obviously, the ball doesn't behave in this fashion. Instead, the ball rises shortly after hitting the ground, which means the acceleration due to the force applied by the ground increases as the ball is squished into the ground until it is greater than the acceleration due to gravity. At this point, the ball starts slowing down, until it stops—but the upward acceleration still exists. Thus, the ball quickly rises immediately after it becomes stationary and bounces off the ground. Another example is when you're in a car, stopped at a red light. When it turns green, you accelerate; however, because you started stationary, there MUST be a point at which your velocity was zero and your acceleration was nonzero. If acceleration had to be zero when the velocity of the car is zero, the car wouldn't be able to move—ever. And, as many others have stated, a vertically fired projectile still has an acceleration of-9.81 m/s² due to gravity at the highest point of its flight. If it did not, it would not turn around and fall to the earth. It would just hover in the sky indefinitely, which would actually be kinda cool.
