OBSERVATION: In all of the observations about the forces on carts, we have only discussed forward and backward forces. However, all of our observations have been conducted in the presence of gravity. What concepts justify our decision to ignore gravity when looking at carts that are moving on horizontal tracks? Explain how these concepts also mean that the experiments we did in FM-5 to learn about the effect of mass on the rate-of-change-of-speed would run the same way on the moon as they did on earth.

ANSWER GUIDE: The force of gravity is always pulling objects down. Explain why the carts on the tracks are not falling, why that justifies ignoring the vertical forces, and why a different amount of gravity would not change the results obtained when pushing carts with extra mass. Drawing or describing a force diagram will help.