Motion: Kinematics

1. Motion: Kinematics Chapter 2

2. Scalar versus Vector Quantities Scalar Quantities • Magnitude (size) • “55 mph” Vector Quantities • Magnitude (size) • Direction • “55 mph, North”

3. Direction Matters Example One Dx = +2 m Example Two Dx = -3 m 1 m 3 m 1 m 4 m

4. Speed Two Possible Situations for Everything: • Object moves at a constant speed. • Object is being “speeded up” or “slowed down” by some force (acceleration) “What about an object that is just sitting there?”

5. Speed Average Speed = distance (in meters) time (in seconds) vave = Dx Dt

6. Speed: Example One During a 3.00 second time interval, a runner’s position changes from x1 = 50.0 m to x2=30.5 m. What was her average speed? (Ans: -6.50 m/s)

7. Speed: Example Two A bicycle has an average speed of 5miles/hr. How far did it go in two hours?

8. Speed: Example Three How long would it take a car traveling 72 km/hr to travel 60 km?

9. Speed: Example Four A car is driven at an average speed of 100.0 km/hr for two hours, then driven at an average speed of 50.0 km/hr for the next hour. What was the average speed for the three-hour trip? (Ans: 83.3 km/hr)

10. Since the speed changed, we have to address each part of the trip separately. 1. To find the distance for the first two hours, we use: x=vt x= 100km/hr X 2 hr = 200 km 2. To find the distance for the last hour, we use the same process: x= 50km/hr X 1 hr = 50 km

11. To find the average velocity, we will divide the total distance by the time: v=x/t = 250 km /3 hr = 83.3 km/hr

12. Velocity: Example 5 A plane flies 300.0 miles east in 2.00 hours. The plane then turns around and flies west for 100.0 miles in 1.00 hour. • Calculate total distance, total time, and average speed. (133 mph) • Calculate the average velocity. (67.0 mph)

13. Average vs. Instantaneous Speed Average Speed averaged for an entire trip Instantaneous Speed Speed at an infinitely short interval

14. Constant velocity Changing velocity

15. Acceleration Acceleration – A change in velocity • Speed (speeding up or slowing down) • Direction (turning a bicycle) Speeding up or slowing down

16. Acceleration • Formula a = Dv = vfinal – vinitial Dt time • Units = m/s2 • Acceleration can be negative (slowing down, decelerating)

17. Acceleration: Example 1 What is the acceleration of a bicycle that moves from rest to 5 m/s in 3 seconds?

18. Acceleration: Example 2 Superman is flying through space and slows down from 24,000 m/s to 20,000 m/s in 5 seconds. What is his acceleration? What is unusual about this number?

19. Acceleration: Example 3 A car starts from a complete stop at a stoplight and accelerates at 5 m/s2 for 10 seconds. What is its’ final velocity? (Ans = 50 m/s)

20. Acceleration: Example 4 A car moving at 30 m/s decelerates at –3.0 m/s2. How long will it take for it to come to a complete stop? (ANS: t = 10 seconds)

21. Kinematic Equations (Constant acceleration only) a = v – vo t x = xo + vot + ½ at2 a = v2 - vo2 2x v = v + vo 2 Time-independent

22. Kinematics: Example 1 An airplane takes off at 27.8 m/s and can accelerate at 2.00 m/s2. How long must the runway be if the plane is to take off safely? (Ans: 193 m)

23. Kinematics: Example 2 How long does it take a car to cross a 30.0 m intersection, from rest, if it accelerates at 2.00 m/s2? (Ans: 5.48 s)

24. Warm Up • Determine the final velocity and time for the Hot Wheels car as it goes down the track. • Calculate acceleration. • Calculate how far the car would travel on the level in 10 seconds. • Would it really travel this far?

25. Kinematics: Example 3 A pitcher throws a baseball at 44 m/s. What is the acceleration if the ball travels through 3.5 m from the start of the pitch to the release? (Ans: 280 m/s2)

26. Kinematics: Example 4 You wish to design an airbag that can protect a driver from a head-on collision at 100 km/h. Assume the car crumples on impact a distance of 1.00 m. • What is the driver’s acceleration? • How fast does the airbag need to act? (time)

27. Kinematics: Example 5 A driver is moving at 28 m/s. It takes him 0.50 seconds to react and slam on the brakes. At that time, the brakes provide an acceleration of –6.0 m/s2. Calculate the total stopping distance. (Ans: 79 m)

28. Warm Up A horse starts from rest and can reach a speed of 4.5 m/s in 5 seconds. • Calculate the average speed of the horse. (2.25 m/s) • Calculate the acceleration of the horse. (0.9 m/s2) • Calculate the distance the horse will travel in 35 seconds, assuming the horse keeps accelerating. (551 m)

29. Aristotle versus Galileo. In the steel cage.

30. Falling Objects Aristotle (Greece, ~400 B.C.) Heavier object fall faster than lighter ones Galileo (1600’s, Renaissance) All objects fall at the same rate (near the surface of the Earth, in a vacuum)

32. Graph Galileo’s Numbers Galileo found that an object is at the following heights over time (from the Leaning Tower) Time(s) Distance(m) 0 56 0.5 55 1.0 51 1.5 45 2.0 36 2.5 25 3.0 12

33. Graph the Distance versus the Time Distance vs. Time Distance (ft) Time (seconds)

35. The displacement gets greater as it falls

36. Acceleration due to Gravity • Can act at a distance, is constant near the surface of the earth • g = 9.80 m/s2 • Usually set the top of the drop at zero yo = 0

37. Gravity Why don’t people fall off the earth at the South Pole?

38. Gravity Do things accelerate forever? • Not in an atmosphere. • Air drag causes “terminal velocity” • Otherwise raindrops would hit us with deadly speed. • Terminal velocity • Fastest a human can drop in air • 120 miles/hr (about 60 m/s).

39. Gravity: Freefall • Freefall – falling only under the acceleration of gravity • Object was dropped, not thrown down y = yo + vot + ½ at2 Simplifies to: y = ½ at2

40. Gravity A rock is dropped from a cliff, calculate how far it falls during the following times. Also, draw a sketch of how it falls. Time Distance 1s 2s 3s 4s 5s

41. Gravity A rock is dropped from a cliff, calculate how far it falls during the following times. Also, draw a sketch of how it falls. Time Distance 1s 4.9 m 2s 19.6 m 3s 44.1 m 4s 78.4 m 5s 123 m

42. 0 s 0 m 1 s 4.9 m 2 s 19.6 m 3 s 44.1 m 4 s 78.4 m

43. Gravity: Example One (A) A rock is dropped into a well and hits the water in 2.45 seconds. How deep is the well? (Ans: 29.4 m)

44. Gravity: Example One (B) At what speed did the rock hit the bottom of the well? (Ans: 24.1 m/s)

45. Gravity: Example Two A ball is thrown off a cliff with a initial velocity of 3.00 m/s. Calculate its position and speed after 1.00 s and 2.00 s. (Ans: 7.90 m, 12.8 m/s and 25.6 m, 22.6 m/s)

46. A person throws a ball into the air with an initial velocity of 15.0 m/s. • How high will the ball go? (Hint: use –9.80 m/s2) (Ans: 11.5 m) • A person throws a ball into the air with an initial velocity of 15.0 m/s. How long is it in the air? (Ans: 3.06 s) • What is the ball’s velocity when it returns to the boy’s hand? (Ans: -15.0 m/s) • Does the ball have zero acceleration at its highest point?

47. Gravity: Example Three (D) What time will the ball pass a point 8.00 M above the person’s hand? Ans: 0.69 s and 2.37 s (you need to use the quadratic formula)

48. The Lava Bomb A volcano shots a blob of lava upward at 23.2 m/s. • Calculate the maximum height. (27.5 m) • Calculate the time to reach the maximum height. (2.37 s) • Calculate the total time in the air. (4.73 s)

49. Arrow example An arrow is shot into the air and reaches a maximum height of 30.0 m. • Calculate the initial speed of the arrow. (24.2 m/s) • Calculate the total time the arrow spent in the air. (4.94 s) • Calculate the time(s) when the arrow is at 15.0 m. (0.73 s and 4.21 s)

50. A person stands on a cliff that is 15.0 m from the ground. He throws a rock at 10.0 m/s straight up, but it falls down off the cliff. • Calculate the maximum height. (20.1 m) • Calculate the time up. (1.02 s) • Calculate the time down (2.03 s) • Calculate the speed at which it will hit the ground. (19.8 m/s)