Physics Lingo How to talk to a Physicist.

Distance and Displacement Distance is the length of the actual path taken by an object. Consider travel from point A to point B in diagram below: A B s = 20 m Distance s is a scalar quantity (no direction): Contains magnitude only and consists of a number and a unit. (20 m, 40 mi/h, 10 gal)

Distance and Displacement Displacement is the straight-line separation of two points in a specified direction. A vector quantity: Contains magnitude AND direction, a number, unit & angle. (12 m, 30 0 ; 8 km/h, N) A B D = 12 m, 20 o 

Distance and Displacement For motion along x or y axis, the displacement is determined by the x or y coordinate of its final position. Example: Consider a car that travels 8 m, E then 12 m, W.For motion along x or y axis, the displacement is determined by the x or y coordinate of its final position. Example: Consider a car that travels 8 m, E then 12 m, W. Net displacement D is from the origin to the final position: What is the distance traveled? 20 m !! 12 m,W D D = 4 m, W x 8 m,E x = +8x = -4

The Signs of Displacement Displacement is positive (+) or negative (-) based on LOCATION.Displacement is positive (+) or negative (-) based on LOCATION. 2 m -1 m -2 m The displacement is the y-coordinate. Whether motion is up or down, + or - is based on LOCATION. Examples: The direction of motion does not matter!

Definition of Speed Speed is the distance traveled per unit of time (a scalar quantity).Speed is the distance traveled per unit of time (a scalar quantity). v = = stst 20 m 4 s v = 5 m/s Not direction dependent! A B s = 20 m Time t = 4 s

Definition of Velocity Velocity is the displacement per unit of time. (A vector quantity.)Velocity is the displacement per unit of time. (A vector quantity.) v = 3 m/s at 20 0 N of E Direction required! A B s = 20 m Time t = 4 s D=12 m 20 o

Average and Instantaneous v xx tttt x2x2x2x2 x1x1x1x1 t2t2t2t2 t1t1t1t1 xx tt Time slope Displacement, x Average Velocity: Instantaneous Velocity:

Example 1. A runner runs 200 m, east, then changes direction and runs 300 m, west. If the entire trip takes 60 s, what is the average speed and what is the average velocity? Recall that average speed is a function only of total distance and total time: Total distance: s = 200 m m = 500 m Avg. speed 8.33 m/s Direction does not matter! start s 1 = 200 m s 2 = 300 m

Example 1 (Cont.) Now we find the average velocity, which is the net displacement divided by time. In this case, the direction matters. x o = 0 t = 60 s x 1 = +200 m x f = -100 m x 0 = 0 m; x f = -100 m Direction of final displacement is to the left as shown. Average velocity: Note: Average velocity is directed to the west.

Example 2. A sky diver jumps and falls for 600 m in 14 s. After chute opens, he falls another 400 m in 150 s. What is average speed for entire fall? 600 m 400 m 14 s 150 s A B Average speed is a function only of total distance traveled and the total time required. Total distance/ total time:

Position vs. time graphs (x vs. t) Shows the change in position for an object over a period of time.Shows the change in position for an object over a period of time. The slope of the line at any point shows the instantaneous velocity of the object.The slope of the line at any point shows the instantaneous velocity of the object. What is the instantaneous velocity of this object from t=1 s to t=3 s?What is the instantaneous velocity of this object from t=1 s to t=3 s? –20 m/s