| A | B |
|---|
| terminal velocity | the constant velocity of a falling object when the size of the upward force of air resistance matches the size of the downward force of gravity |
| free fall | the condition an object is in when gravity is the only force acting on it |
| projectile motion | the curved path an object follows when thrown or propelled near the surface of the Earth |
| inertia | the tendency of all objects to resist any change in motion |
| momentum | a property of a moving object that depends on the object's mass and velocity |
| friction | a force that opposes motion between two surfaces that are touching |
| gravity | a force of attraction between objects that is due to their masses |
| law of universal gravitation | the law that states that all objects in the universe attract each other through gravitational force; the size of the force depends on the masses of the objects and the distance between them |
| weight | a measure of the gravitational force exerted on an object, usually by the Earth |
| mass | the amount of matter that something is made of |
| acceleration | the rate at which velocity changes; an object accelerates if its speed changes, if its direction changes, or if both its speed and its direction change |
| negative acceleration | acceleration in which velocity decreases; also called deceleration |
| speed | the rate at which an object moves; speed depends on the distance traveled and the time taken to travel that distance |
| average speed | the overall rate at which an object moves; average speed can be calculated by dividing total distance by total time |
| motion | an object's change in position over time when compared with a reference point |
| reference point | an object that appears to stay in place in relation to an object being observed for motion |
| velocity | the speed of an object in a particular direction |
| resultant velocity | the combination of two or more velocities |
| force | a push or a pull; all forces have both size and direction |
