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

A. Motion Problem: Is your desk moving? 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 Relative Motion - Movement in relation to a frame of reference. Reference point Motion

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

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

Measuring displacement * 07/16/96 Measuring displacement Displacement is independent of the path taken by the object. Involves the starting point, ending point, and direction. Vector quantity Example: A man travels 4 blocks North and then returns 4 blocks South. What is his displacement? D = 0 blocks Distance = 8 blocks *

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

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

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

Velocity Problems

Graphing d/t

E. Graphing Motion slope = speed steeper slope = straight line = * 07/16/96 E. Graphing Motion Distance-Time Graph A B slope = steeper slope = straight line = flat line = speed faster speed constant speed no motion *

E. Graphing Motion Who started out faster? A (steeper slope) Distance-Time Graph A B 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 = (2400m) ÷ (30min) A = 80 m/min B = (1200m) ÷ (30min) B = 40 m/min

Acceleration

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

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

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

t a D. Calculations a = (vf - vi) ÷ t t = 3 s 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: vi = 10 m/s t = 3 s vf = 32 m/s a = ? WORK: a = (vf - vi) ÷ t a = (32m/s - 10m/s) ÷ (3s) a = 22 m/s ÷ 3 s a = 7.3 m/s2 a vf - vi t

t a D. Calculations t = ? t = (vf - vi) ÷ a t = (0m/s-30m/s)÷(-3m/s2) How long will it take a car traveling 30 m/s to come to a stop if its acceleration is -3 m/s2? GIVEN: t = ? vi = 30 m/s vf = 0 m/s a = -3 m/s2 WORK: t = (vf - vi) ÷ a t = (0m/s-30m/s)÷(-3m/s2) t = -30 m/s ÷ -3m/s2 t = 10 s a vf - vi t

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

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

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