Question

this discussion question, like the last one, deals with one - dimensional kinematics. a friend of yours contends that the acceleration of a ball tossed up into the air is positive on the way up, negative as it is falling down and zero at the top of the trajectory. what is your opinion on the matter? justify your response based on kinematic principles. make an initial posting before replying to other threads. reply to at least one of your classmates.

Explanation:

The friend's opinion is incorrect. When a ball is tossed upward, the only significant force acting on it (neglecting air resistance) is gravity. By Newton's second law, \( F = ma \), the acceleration due to gravity (\( g \), approximately \( 9.8\ m/s^2 \) downward) is constant throughout the motion. On the way up, the velocity is upward, but acceleration is downward (so if upward is positive, acceleration is negative). While falling down, velocity is downward (positive if downward is positive, or negative if upward is positive), but acceleration is still downward (consistent with the sign convention). At the top of the trajectory, velocity is zero, but acceleration is still \( g \) downward (not zero) because gravity still acts on the ball. Kinematic principles state that in free - fall (near - Earth's surface, neglecting air resistance), acceleration is constant and equal to the acceleration due to gravity, directed downward, for the entire motion of the ball.

Answer:

The friend's opinion is wrong. The ball's acceleration is constant (equal to \( g\approx9.8\ m/s^2 \)) and directed downward throughout its motion (upward, at the top, and downward), because gravity (the main force) acts constantly downward.