Chapter 4 - understanding motion, energy & gravity (sept 23) Speed = change in position/location in some amount of time = m/s. Velocity - not the same as speed. A change in location and direction in some amount of time. Units are m/s - same as speed. Ex. driving 100km/h west is not the same velocity as driving 100km/h north, although the speeds are the same. Acceleration = change in velocity in some amount of time = (m/s)/s = m/s^2. Changing direction of motion at a constant speed is also an acceleration. Ex. dropping a motionless object; v = 0m/s @ 0 s; v = 10 m/s @ 1 s --> a = (10m/s - 0m/s)/s. Note: momentum has direction because velocity has direction. Changing mass also changes momentum (less common) A force is needed to change momentum. Mass = sum of all the matter (molecules and atoms) in an object.