2. Chapter 12 What is Motion? Describing Motion  Frame of Reference  Motion  Speed & Velocity  Acceleration  Momentum

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

4. Motion Motion  Change in position in relation to a point of reference. Reference point Motion

5. 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.

6. Speed Speed  rate at which an object moves  distance traveled per unit time s d t

7. Speed Instantaneous Speed  speed at a given instant ex - the car is going 35 mph right now Average Speed  The average speed of the entire journey

8. 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 storm’s direction!

9. Velocity Velocity  speed in a given direction  Velocity can change even when the speed is constant!

10. 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: d = 100 m t = 20 s s = ? WORK : s = d ÷ t s = (100 m) ÷ (20 s) s = 5 m/s You skate faster! v d t

11. 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: v = 330 m/s d = 1km = 1000m t = ? WORK : t = d ÷ v t = (1000 m) ÷ (330 m/s) t = 3.03 s v d t

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

13. Graphing Speed & Velocity Who started out faster?  A (steeper slope) Who had a constant speed? A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 Distance-Time Graph A B

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

15. Acceleration Acceleration  the rate of change of velocity  change in speed or direction a: acceleration v f : final velocity v i : initial velocity t: time a v f - v i t

16. Acceleration Positive acceleration  “speeding up” Negative acceleration  “slowing down”

17. Acceleration Calculations 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: v i = 10 m/s t = 3 s v f = 32 m/s a = ? WORK : a = ( v f - v i ) ÷ t a = (32m/s - 10m/s) ÷ (3s) a = 22 m/s ÷ 3 s a = 7.3 m/s 2 a v f - v i t

18. 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: t = ? v i = 30 m/s v f = 0 m/s a = -3 m/s 2 WORK : t = ( v f - v i ) ÷ a t = (0m/s-30m/s)÷(-3m/s 2 ) t = -30 m/s ÷ -3m/s 2 t = 10 s a v f - v i t

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

20. Graphing Acceleration 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

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

22. Conservation of Momentum 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.

23. Momentum Calculations What is the momentum of a 0.3 kg bluejay flying at 17 m/sec? GIVEN: p = ? v = 17 m/s m = 0.3 kg WORK : p = m x v p = (0.3 kg)x(17 m/s) p = 5.1 kg x m/sec m p v