# II. Describing Motion

Ch. 3 & 4 Motion & Forces II. Describing Motion Motion Speed & Velocity Acceleration Newtons First Law Newtons First Law of Motion An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force force.

A. Motion Problem: Is your desk moving? We need a reference point... nonmoving point from which motion is measured A. Motion Motion Change in position in relation to a reference point. Reference point Motion

A. Motion Problem: You are a passenger in a car stopped at a stop sign. Out of the corner of your eye, you notice a tree on the side of the road begin to move forward. You have mistakenly set yourself as the reference point. B. Speed & Velocity Speed d rate of motion v t distance traveled per unit time distance

speed time B. Speed & Velocity Instantaneous Speed speed at a given instant Average Speed total distance avg. speed total time B. Speed & Velocity Problem:

A storm is 10 km away and is moving at a speed of 60 km/h. Should you be worried? It depends on the storms direction! B. Speed & Velocity Velocity speed in a given direction can change even when the speed is constant! C. Acceleration vf - vi a t

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

acceleration speeding up Negative acceleration slowing down D. Calculations Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. Who skates faster? GIVEN: WORK: d = 100 m t = 20 s v=?

d v t v=dt v = (100 m) (20 s) v = 5 m/s You skate faster! 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 coasters acceleration? GIVEN: WORK: vi = 10 m/s

a = (vf - vi) t t=3s vf = 32 m/s a = (32m/s - 10m/s) (3s) a=? vf - vi a t a = 22 m/s 3 s a = 7.3 m/s2 D. Calculations Sound travels 330 m/s. If a lightning bolt

strikes the ground 1 km away from you, how long will it take for you to hear it? GIVEN: WORK: v = 330 m/s t=dv d = 1km = 1000m t = (1000 m) (330 m/s) t=? t = 3.03 s d v t D. Calculations How long will it take a car traveling 30 m/s to come to a stop if its acceleration is

-3 m/s2? GIVEN: WORK: t=? vi = 30 m/s t = (vf - vi) a t = (0m/s-30m/s)(-3m/s2) vf = 0 m/s a = -3 m/s2 vf - vi a t t = -30 m/s -3m/s2 t = 10 s

E. Graphing Motion Distance-Time Graph slope A = speed steeper slope = faster speed B straight line = constant speed

flat line = no motion E. Graphing Motion Distance-Time Graph Who started out faster? A (steeper slope) Who had a constant speed? A Describe B from 10-20 min. B stopped moving A B

Find their average speeds. A = (2400m) (30min) A = 80 m/min B = (1200m) (30min) B = 40 m/min E. Graphing Motion Distance-Time Graph 400 Acceleration is indicated by a curve on a Distance-Time graph.

Distance (m) 300 200 100 Changing 0 0 5 10 Time (s)

15 20 slope = changing velocity E. Graphing Motion Speed-Time Graph = acceleration +ve = speeds up -ve = slows down slope 3 Speed (m/s)

2 straight line = constant accel. 1 line = no accel. (constant velocity) flat 0 0 2 4

6 Time (s) 8 10 E. Graphing Motion Speed-Time Graph Specify the time period when the object was... slowing down 5 to 10 seconds speeding up 0 to 3 seconds 3

Speed (m/s) 2 moving at a constant speed 3 to 5 seconds not moving 0 & 10 seconds 1 0 0 2 4

6 Time (s) 8 10

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