C. Acceleration

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

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

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

4. vf - vi t a 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 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

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

6. vf - vi t a 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: 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

7. Distance-Time Graph A B E. Graphing Motion • slope = • steeper slope = • straight line = • flat line = speed faster speed constant speed no motion

8. Distance-Time Graph A B E. Graphing Motion • 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

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

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

11. Speed-Time Graph E. Graphing Motion 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

12. III. Defining Force Force Newton’s First Law Friction

13. A. Force • Force • a push or pull that one body exerts on another • What forces are being exerted on the football? Fkick Fgrav

14. A. Force • Balanced Forces • forces acting on an object that are opposite in direction and equal in size • no change in velocity

15. N N A. Force • Net Force • unbalanced forces that are not opposite and equal • velocity changes (object accelerates) Fnet Ffriction Fpull W

16. B. Newton’s First Law • Newton’s 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.

17. B. Newton’s First Law • Newton’s First Law of Motion • “Law of Inertia” • Inertia • tendency of an object to resist any change in its motion • increases as mass increases

18. ConcepTest 1 TRUE or FALSE? The object shown in the diagram must be at rest since there is no net force acting on it. FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration.

19. ConcepTest 2 You are a passenger in a car and not wearing your seat belt. Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door. Which is the correct analysis of the situation? ...

20. ConcepTest 2 1. Before and after the collision, there is a rightward force pushing you into the door. 2. Starting at the time of collision, the door exerts a leftward force on you. 3. both of the above 4. neither of the above 2. Starting at the time of collision, the door exerts a leftward force on you.

21. C. Friction • Friction • force that opposes motion between 2 surfaces • depends on the: • types of surfaces • force between the surfaces

22. C. Friction • Friction is greater... • between rough surfaces • when there’s a greater force between the surfaces (e.g. more weight) • Pros and Cons?

23. Ch. 3 & 4Motion & Forces IV. Force & Acceleration Newton’s Second Law Gravity Air Resistance Calculations

24. A. Newton’s Second Law • Newton’s Second Law of Motion • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. F = ma

25. F a m A. Newton’s Second Law F m F = ma F: force (N) m: mass (kg) a: accel (m/s2) 1 N = 1 kg ·m/s2

26. B. Gravity • Gravity • force of attraction between any two objects in the universe • increases as... • mass increases • distance decreases

27. more mass less distance B. Gravity • Who experiences more gravity - the astronaut or the politician? • Which exerts more gravity - the Earth or the moon?

28. B. Gravity • Weight • the force of gravity on an object W = mg W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) MASS always the same (kg) WEIGHT depends on gravity (N)

29. B. Gravity • Would you weigh more on Earth or Jupiter? • Jupiter because... • greater mass • greater gravity • greater weight

30. elephant feather B. Gravity • Accel. due to gravity (g) • In the absence of air resistance, all falling objects have the same acceleration! • On Earth: g = 9.8 m/s2

31. C. Air Resistance • Air Resistance • a.k.a. “fluid friction” or “drag” • force that air exerts on a moving object to oppose its motion • depends on: • speed • surface area • shape • density of fluid

32. Fair Fgrav C. Air Resistance • Terminal Velocity • maximum velocity reached by a falling object • reached when…Fgrav = Fair • no net force  no acceleration  constant velocity

33. C. Air Resistance • Terminal Velocity • increasing speed  increasing air resistance until… Fair = Fgrav Animation from “Multimedia Physics Studios.”

34. C. Air Resistance • Falling with air resistance • heavier objects fall faster because they accelerate to higher speeds before reaching terminal velocity Fgrav = Fair • larger Fgrav  need larger Fair  need higher speed Animation from “Multimedia Physics Studios.”

35. F a m D. Calculations • What force would be required to accelerate a 40 kg mass by 4 m/s2? GIVEN: F = ? m = 40 kg a = 4 m/s2 WORK: F = ma F = (40 kg)(4 m/s2) F = 160 N

36. F a m D. Calculations • A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration? GIVEN: m = 4.0 kg F = 30 N a = ? WORK: a = F ÷ m a = (30 N) ÷ (4.0 kg) a = 7.5 m/s2

37. F a m D. Calculations • Mrs. J. weighs 557 N. What is her mass? GIVEN: F(W) = 557 N m = ? a(g) = 9.8 m/s2 WORK: m = F ÷ a m = (557 N) ÷ (9.8 m/s2) m = 56.8 kg

38. ConcepTest • Is the following statement true or false? • An astronaut has less mass on the moon since the moon exerts a weaker gravitational force. • False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.