Drop a ball, releasing it from rest so that it falls straight down. Use a simple model - neglect air resistance. What does the ball's free-body diagram look like as the object falls?

Remember to ask yourself the question "Is my free-body diagram consistent with the ball's motion?". Really what we should be doing is adding up all the forces on the free-body diagram and looking at the net force. Is the net force consistent with the ball's motion?

Let's consider the motion diagram, in which we look at the ball's position at equal time steps.

The more time passes the faster the ball falls, so the more space there is between successive positions of the ball. What does the arrow beside each image of the ball represent?

The arrows above show the ball's velocity at each time step. In other words, they show the ball's speed and direction of motion at each time step. How does the velocity change from one time step to the next?

What you should observe is that the velocity changes by the same amount at each time step. The rate of change of the velocity is constant.

Let's connect that to the free-body diagram. Our free-body diagram should show just one constant force, the downward force of gravity acting on the ball, representing the interaction between the Earth and the ball.

So, the net force is constant and directed down. The rate of change of velocity is constant and directed down. Coincidence?

What is we repeated this experiment on the Moon? What would change?

Did we see the same kind of connection between the net force and the rate of change of velocity in the previous examples we considered?