Answer
Let up be the positive direction. Then down is the negative direction.
(a) A person throws a ball up in the air and the ball has left the person's hand. As the ball is moving up in the air, the velocity is positive. Because of gravity, the acceleration vector points toward the ground so the acceleration is negative.
(b) An object is falling freely down toward the ground. Then the velocity is negative. Because of gravity, the acceleration vector points toward the ground so the acceleration is also negative.
Work Step by Step
Let up be the positive direction. Then down is the negative direction.
(a) A person throws a ball up in the air and the ball has left the person's hand. As the ball is moving up in the air, the velocity is positive. Because of gravity, the acceleration vector points toward the ground so the acceleration is negative.
(b) An object is falling freely down toward the ground. Then the velocity is negative. Because of gravity, the acceleration vector points toward the ground so the acceleration is also negative.
