How do we describe motion? Different observers can describe the motion of the same object differently Motion is relative – the description depends on the observer
How do we describe motion? To describe motion unambiguously, we need An object of reference (an origin) Coordinate axes (positive and negative) A reference clock
How do we describe motion? Physics uses specific vocabulary that is associated with specific mathematical definitions to describe motion: distance, displacement, speed, velocity, and acceleration
Definitions Some definitions: Scalar – are quantities that are fully defined by a magnitude alone (how much, or how big) Examples: length, width, temperature, time,
Definitions Some definitions: Vectors – are quantities that require both a magnitude and direction to be fully defined Example: directions on a map, velocity, acceleration…
Distance vs. displacement I walk three steps to the right, then turn around and walk four steps to the left. What is my distance? What is my displacement?
Definitions Distance refers to how much ground an object has covered during its motion. Distance is a SCALAR Distance is always positive
Definitions Displacement refers to an objects change in position (i.e., how far away from the starting position) Displacement is a VECTOR You need to know the magnitude (how far) and the direction (which way)
Example: Joe walks 4 meters to the right, then turn and walks 8 meters to the left. What is his distance? What is his displacement?
A straight track is 1600m in length. A runner begins at the starting line, runs due east for the full length of the track, turns around and runs halfway back. The time for this run is five minutes. What is the runner’s average speed? What is the runner’s average velocity?
Two buses depart from Chicago, one going to New York and one going to San Francisco. Each bus travels at a speed of 30 m/s. Do they have equal velocities?