Relative motion All objects in the universe move with respect to something. We say that motion is relative because there is no point in the universe that is standing still.

What is speed? A quantity divided by time is called a rate. Ex. speed, acceleration, velocity Speed is any distance divided by time. There are many ways to express speed: miles/hr, meters/sec, etc.

Average and instantaneous speed Speed can be either instantaneous or average. Instantaneous speed is the speed of an object at a particular instant. Average speed is the total distance divided by the total time.

What is velocity? Velocity is an object’s speed in a given direction. A car rounding a curve with the speed control on has a constant speed but changing velocity.

What is acceleration? Acceleration is the rate at which speed is changing or velocity divided by time, a=v/t. Acceleration can be positive, speeding up or negative, slowing down (decelerating).

Acceleration due to gravity Gravity causes all objects to accelerate towards the earth. The longer they fall, the more speed they acquire. A brick that breaks loose from the top of a building falls farther the last second than the first.

Atmospheric friction We live on a planet with a thick atmosphere. This causes objects falling through it to slow down. Without air objects will experience constant acceleration. (Ex. On the moon)

What is “g” ? Objects falling near the Earth experience an acceleration of 9.8 m/s 2 (32 ft/s 2 ) This is called “g” and is often rounded to 10 m/s 2

Useful Formulas V=d / t d = v x t t = d / v v= a x t d = ½ at 2 V avg = v f + v 0 / 2 V=velocity, a=acceleration, t=time, d=distance