Open the data from Trials II and III. Answer the following questions in Notes on your handheld computer.

1. How does a propeller create a force that pushes on a cart?

2. Would a propeller work in outer space, where there's no air? If not, how could you make something move in outer space?

3. Look at your position versus time graph in Trial II. Is the velocity of the cart constant? How can you tell?

4. Look at your velocity versus time graph in Trial III. For each case, describe the graph and then write why you think it has that shape.

   1. no friction, push cart

   2. add friction, push cart

   3. add friction, run propeller

5. Do you think the graphs are evidence that friction exerts a force like the propeller, but in the opposite direction?

6. The rate of change of velocity is the slope of the velocity versus time graph. What is happening to the cart when the slope of the velocity graph is not zero? What is done to the cart to increase or decrease its rate of change of velocity?