Competency #1
Answer and Explanation
The ball has left the hand, so there is no applied force acting at this time. The ball is not in contact with any other objects or surfaces, so there are no friction or support forces acting. Since the moon has no atmosphere, the ball experiences no air resistance. There is a force of gravity between the ball and the moon. The force diagram for this case is drawn like this:
Newton's 2nd Law can be stated as:
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Note that acceleration and force are both vectors, which means that the direction is an important consideration.
It is important that we analyze the vertical and horizontal directions independent of one another.
First, we apply Newton's 2nd law in the vertical direction:
Since the only force acting in the vertical direction is acting downward, the vertical net force must be downward. If the net force is downward, then the acceleration must be downward.
Next, we apply Newton's 2nd law in the horizontal direction:
There are no forces acting in the horizontal direction. Therefore, the net force in the horizontal direction is zero. If the net force is in the horizontal direction is zero, the acceleration in the horizontal direction must be zero.