This document defines key terms related to motion and forces, including force, balanced vs unbalanced forces, inertia, mass, acceleration, and Newton's three laws of motion. It explains that an object at rest stays at rest and an object in motion stays in motion unless acted on by an unbalanced force, and that applying more force results in greater acceleration depending on an object's mass. It also notes that for every action there is an equal and opposite reaction force.

1. Describing Motion Newton’s Laws

11. The truck is in motion . What is the force that causes it to stop? The push of the stopped car. The car is at rest . What is the force that causes it to move? The push of the truck.

12. What about the ladder on top of the truck? The ladder is in motion because the truck is in motion. When the truck stops, the ladder stays in motion. The truck is stopped by the force of the car, but the ladder is not. What force stops the ladder? Gravity .

13. The truck is in motion , the car is at rest . How do each of these vehicles accelerate ? The truck stops moving. The car starts moving. Which one will be the hardest to accelerate ? The truck because it has the most mass.

14. Why does the car move [accelerate] when it is hit by the truck? The heavy and moving truck has more force than the small, at rest car. Why does the truck stop moving when it hits the car? The force of the car pushing back on the truck, plus the force of friction between the massive truck and the road slow down, the stop the truck.

15. The truck hits the car. An action force stops the truck. What is the equal and opposite reaction force? The force that pushes the car forward.