Ch 11 Motion II Describing Motion u Speed

Ch. 11 Motion II. Describing Motion u Speed & Velocity u Acceleration u

A. Motion l Problem: § Is your desk moving? l To describe motion completely, we need a frame of reference – a system of objects that are not moving with respect to one another.

A. Motion l Relative Motion § Movement in relation to a frame of reference. Reference point Motion

Quick Write l Explain a time where you experienced confusion about your motion.

Measuring Distance l Distance – the length of a path between two points. l Units – m (meters) km (kilometers) cm (centimeters)

Measuring displacement l Displacement is independent of the path taken by the object. l Involves the starting point, ending point, and direction. Vector quantity l Example: A man travels 4 blocks North and then returns 4 blocks South. What is his displacement?

B. Speed & Velocity l Speed d § rate of motion v t § distance traveled per unit time

B. Speed & Velocity l Instantaneous Speed § speed at a given instant § Ex. The speed measured by a police officer using a radar gun l Average Speed § speed over an entire trip __ v = d t

B. Speed & Velocity l Velocity § speed in a given direction § can change even when the speed is constant!

Velocity Problems

Graphing d/t

E. Graphing Motion Distance-Time Graph A l slope = speed l steeper slope = faster speed B l straight line = constant speed l flat line = no motion

E. Graphing Motion Distance-Time Graph A l l l B l Who started out faster? § A (steeper slope) Who had a constant speed? §A Describe B from 10 -20 min. § B stopped moving Find their average speeds. § A = (2400 m) ÷ (30 min) A = 80 m/min § B = (1200 m) ÷ (30 min) B = 40 m/min

Acceleration

C. Acceleration vf - vi a t l Acceleration § the rate of change of velocity § change in speed or direction a: v f: v i: t: acceleration final velocity initial velocity time

C. Acceleration l Positive acceleration § “speeding up” l Negative acceleration § “slowing down”

Quick Write l Given an example of something that you have seen in your daily life that is accelerating. l Describe its motion.

D. Calculations A roller coaster starts down a hill at 10 m/s. Three seconds later, its speed is 32 m/s. What is the roller coaster’s acceleration? GIVEN: WORK: l vi = 10 m/s t=3 s vf = 32 m/s vf - vi a=? a t a = ( v f - v i) ÷ t a = (32 m/s - 10 m/s) ÷ (3 s) a = 22 m/s ÷ 3 s a = 7. 3 m/s 2

D. Calculations How long will it take a car traveling 30 m/s to come to a stop if its acceleration is -3 m/s 2? GIVEN: WORK: l t=? vi = 30 m/s vf = 0 m/s a = -3 m/s 2 t = (vf - vi) ÷ a t = (0 m/s-30 m/s)÷(-3 m/s 2) vf - vi a t t = -30 m/s ÷ -3 m/s 2 t = 10 s

E. Graphing Motion Distance-Time Graph l Acceleration is indicated by a curve on a Distance-Time graph. l Changing slope = changing velocity

E. Graphing Motion Speed-Time Graph = acceleration § +ve = speeds up § -ve = slows down l slope l straight line = constant accel. line = no accel. (constant velocity) l flat

E. Graphing Motion Speed-Time Graph Specify the time period when the object was. . . l slowing down § 5 to 10 seconds l speeding up § 0 to 3 seconds l l moving at a constant speed § 3 to 5 seconds not moving § 0 & 10 seconds