Video Transcript
A ball is thrown upward. After the ball has left the
thrower’s hand, it has uniform acceleration. In which direction is the
acceleration? Is it (A) downward or (B)
upward?
When the ball leaves the thrower’s
hand, the ball moves upward. The speed of the ball must be
greater than zero for the ball to move. Let us imagine what would happen to
the ball if after it left the thrower’s hand, the ball had uniform acceleration
upward. The upward speed of the ball would
increase as the ball traveled upward. The ball would move faster upward
the further upward it traveled. The ball would never change
direction and so would never start to move downward.
Let us instead imagine what would
happen to the ball if after it left the thrower’s hand, the ball had uniform
acceleration downward. As soon as the ball leaves the
thrower’s hand, the ball is accelerated downward. This does not mean that the ball
immediately travels downward however, as the ball was traveling upward when it left
the thrower’s hand. What would happen is that the ball
would travel upward but would slow down as it traveled. The ball would continue to slow
down until the speed of the ball was zero. The ball would then increase in
speed again but now traveling downward. The ball would then fall to the
ground.
When a ball is thrown, we observe
the ball to eventually fall back to earth, not to move ever faster away from
earth. The acceleration of the ball is
then downward.
