Chapter 12 What is Motion Describing Motion u

Chapter 12 What is Motion? Describing Motion u Frame of Reference Motion u Speed & Velocity u Acceleration u Momentum u

Motion l Problem: § Is your desk moving? l We need a frame (point) of reference. . . § a nonmoving point from which motion is measured

Motion l Motion § Change in position in relation to a point of reference. Reference point Motion

Motion Problem: l 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. l You have mistakenly set yourself as the reference point.

Speed d l Speed s t § rate at which an object moves § distance traveled per unit time

Speed l Instantaneous Speed § speed at a given instant • ex - the car is going 35 mph right now l Average Speed § The average speed of the entire journey

Velocity l 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 storm’s direction!

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

Calculations of Speed & Velocity Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. Who skates faster? GIVEN: WORK: l d = 100 m t = 20 s s=? d v t s=d÷t s = (100 m) ÷ (20 s) s = 5 m/s You skate faster!

Calculations of Speed & Velocity 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: l v = 330 m/s t=d÷v d = 1 km = 1000 m t = (1000 m) ÷ (330 m/s) t=? t = 3. 03 s d v t

Graphing Speed & Velocity distance vs. time graph l the slope is = to l speed l steeper slope means l faster speed l straight line means l constant speed l flat line means l no motion Distance-Time Graph l A B

Graphing Speed & Velocity Distance-Time Graph l A 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

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

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

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

Acceleration 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

Acceleration 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

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

Graphing Acceleration 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

Momentum l l All moving objects have momentum Momentum depends on the mass of the object and the velocity with which it is traveling l Formula for calculating momentum p m v momentum = mass • velocity

Conservation of Momentum l l l The total momentum of any group of objects remains the same unless outside forces act on the objects. Ex. 2 billiard balls hit into one another. The total momentum of the 2 balls before they collide is the same as the total momentum of the two balls after they collide.

Momentum Calculations l What is the momentum of a 0. 3 kg bluejay flying at 17 m/sec? GIVEN: WORK: p=? v = 17 m/s m = 0. 3 kg p=m • v p = 0. 3 kg • 17 m/s p m v p = 5. 1 kg • m/sec