An athlete throws a ball of mass ?m? directly upward, and it feels no appreciable air resistance. Draw a free-body diagram of this ball while it is free of the athlete’s hand and (a) moving upward; (b) at its highest point; (c) moving downward. (d) Repeat parts (a), (b), and (c) if the athlete throws the ball at a 60° angle above the horizontal instead of directly upward.
Solution 26E When the ball is free from athletes hand and if we neglect the air resistance, then, the only force that is acting on the ball is the gravitational force which will act downwards and will be equal to w = mg Now the force will remain same whether the ball is going up, at the highest point or coming down. This is because the gravitational force is constant if mass do not change and will always act downwards. The only thing that will change during this motion is the magnitude and direction of velocity and position. Now in a free body diagram we only show the force that is acting on the body. Since in all the three cases stated in the problem, the force is same, the free body diagram will be same. The diagram is shown below. Now, even if we launch the ball with an angle, the gravitational force will still be same and will act in the same direction (that is downwards). Thing that is different from the previous case is the direction of velocity and positions, but force will still remain same. Hence the free body diagram of the ball will not change.