Physics 10 Chapter 3 1-D Motion

Transcription

1 Physics 10 Chapter 3 1-D Motion

2 Natural Motion Objects have a proper place Objects seek their natural place The heavier the object, the faster it falls.

3 Violent Motion Imposed motion A Moving object must have a force acting on it.

4 Galileo Challenged The Dogma Of Natural Motion The natural motion of a body is to remain in whatever state of motion it is in unless acted upon by net external forces.

5 Galileo Challenged The Dogma Of Natural Motion with Experiments

6 Galileo s Motion Studies What causes the ball to speed up? gravity What causes the ball to slow down? gravity Does the speed change? No!

7 Galileo s Inclined Planes If I have a long horizontal plane without friction, how far will the ball go to reach the same height?

8 Once the ball rolls down the incline, what keeps it moving?

9 Inertia The tendency of an object to resist a change in motion.

10 Inertia

11 Inertia

12 Inertia

13 Inertia Question A quick pull will break which string? A slow and steady pull will break which string?

14 The Earth Revolves around the sun at 30,000 m/sec!!! (~67,000 miles/hr) The Earth Rotates on its axis at 464 m/s (1000 miles/hr)! The Solar System moves through the Galaxy at 220,000 m/s ( 500,000 miles/hr) How come we can t feel these motions?

15 Cuz we are moving right along with it!

16 Mass is a Measure of Inertia Kilogram: KG

17 Mass & Weight Mass is how much stuff an object has inertia. Weight is due to the force of gravity and depends on the mass and the planet! F = ma W = mg g = 10 m/ s 2

18 Mass & Weight W = mg g = 10 m/ s 2 Calculate your weight in N. Calculate your mass in Kg. 1 lb = 4.45 N

19 Weight and Force W = mg g = 10 m/ s 2 The hanging weight has a mass of 1 kg. What is its weight in Newtons? a) 0.1 N b) 1 N c) 10 N d) 100 N What is the reading on each of the two scales? a) 1 N b) 5 N c) 10 N d) 10 N

20 Mass on the Moon Gravity on the surface of the moon is only 1/6 as strong as on the Earth. What is the mass of a 12-kg object on the moon? What is the weight on the moon?

21 Isaac Newton ( ) 1687 Published Principia Invented Calculus 3 Laws of Motion Universal Law of Gravity

22 Newton s First Law (Law of Inertia) An object will remain at rest or in a constant state of motion unless acted upon by net forces.

23 Newton s First Law (Law of Inertia) F = 0 An object will remain at rest or in a constant state of motion unless acted upon by net external forces.

24 The unit is a Newton (N) Push or a Pull

25 Contact Forces

26 Tension Forces

27 Support Forces

28 Friction Forces

29

30 Net Force Direction of applied forces matters!!! Opposite forces cancel!

31 F = Net Forces 0 Forces up equal the forces down. Forces sideways cancel too. T 1 T 2 W

32 F = Static Equilibrium 0 If the Net Force is zero, then the object remains at rest in static equilibrium. T 1 T 2 W

33 Dynamic Equilibrium F = 0 If the Net Force is zero, then the object moves at a constant speed in dynamic equilibrium. W

34 Newton s First Law If F = 0 => No Change in Motion Dynamic Equilibrium Static Equilibrium

35 Net Force Question The person weighs 250 N and the tension in each rope is 200N. What is the weight of the staging? a) 50 N b) 100 N c) 150 N d) 200 N

36 Two 2 N weights are pulling on the spring scale as shown. The reading on the scale is: a) 0 N b) 1 N c) 2 N d) 4 N

37 Galileo s Motion Studies gave us Definitions: Δx v = Δ t v0 + v v = 2 Δv a = Δ t f

38 Speed How fast How far is changing. The rate at which an object moves. distance

39 Velocity Velocity is a vector. It has both magnitude (speed) and direction. Instantaneous velocity is the velocity at any instant. distance

40 Speed & Velocity Average Speed = total distance traveled total time Δd = Δ t m = = s [ speed ], [ distance] m Speed: Scalar (magnitude) Velocity: Vector (magnitude AND direction) Unit is m/s s and v often interchanged..

41 Sense of Speed 1 m/ s 2.25 mi/ hr 1 m / s = 3.6 km / hr = 2.24 mi / hr 10 m / s = 36 km / hr = 22.4 mi / hr 20 m/ s = 72 km/ hr = 44.8 mi/ hr 30 m/ s = 108 km/ hr = 67.2 mi/ hr 1 km / hr 0.62 mi / hr

42 Speed Question What is its average speed? (in m/s) Average Speed = total distance traveled total time

43 Distance from Speed Average Speed = total distance traveled total time Average Speed x total time= total distance traveled d = vt If you travel an average of 80 km/hr for 15 minutes, how far have you traveled in that time?

44 Acceleration How fast How fast is changing. The rate at which the speed is changing. change in velocity Δv a = = change in time Δ t Speeding up Slowing down Speeding up or slowing down?

45 Acceleration is in the direction of the net Force but not necessarily in the direction of velocity. Velocity is always in the direction of the motion!

46 Velocity from Acceleration a change in velocity Δv = = change in time Δ t Δ v= aδt v = at If you start from rest and accelerate at a rate of 5m/s every second (5 m/s 2 ) for 10 seconds, what will be your final speed? 2 v = at = 5 m/ s x10s = 50 m/ s

47 Quicky Question An automobile enters a freeway on-ramp at 15.0m/s and accelerates uniformly up to 25.0 m/s in a time of 10.0s. a) What is the automobile s average velocity? Which equation? v ave = v v f 15 m/ s+ 25 m/ s = = 2 20 m/ s

48 Quicky Question An automobile enters a freeway on-ramp at 15.0m/s and accelerates uniformly up to 25.0 m/s in a time of 10.0s. b) What is the distance traveled in this amount of time? Which equation? d = vt = (20 m/ s)(10 s) = 200m d = 200m

49 Quicky Question An automobile enters a freeway on-ramp at 15.0m/s and accelerates uniformly up to 25.0 m/s in a time of 10.0s. c) What is the automobile s acceleration? Which equation? a Δv = = Δt v f Δt v i 25 m/ s 15 m/ s = = 1 m/ s 10s 2

50 Quizzy Questions A car moves with an average speed of 40 km/h for 15 minutes. How far does it travel? a) 60 km b) 10 km c) 30 km d) 30 km/h A car can go from rest to 90 km/hr in 10 s. What is the acceleration of the car? a) 60 km/hr/s b) 9km/hr/s c) 30 km/hr/s d) 3 km/h/s

51 Hill Question a) b) c) On which of these hills does the ball roll down with increasing speed and decreasing acceleration? a) b) c)

52 Tracks Question Two balls are released simultaneously from rest at the left end of equal length tracks A and B as shown. Which ball reaches the end of its track first?

53 Free Fall In the absence of air resistance, all objects fall due to the force of gravity with an acceleration g : 2 2 g = 9.8 m/ s ~ 10 m/ s

54 Free Fall In a vacuum, all objects fall with the same acceleration due to gravity: 10 m/s 2.

55 Falling from Rest v 0 = 0 Estimate : v = v + gt f a= g ~10 m/ s 0 1 Δ y = v0t + gt Take down as +y: + v= 10 m/ s Δ y = 5m v= 20 m/ s Δ y = 20m v= at = 10t 1 Δ y = at = 5t 2 How FAR is not How FAST! 2 2 v Δy! v= 30 m/ s Δ y = 45m v= 40 m/ s Δ y = 80m v= 50 m/ s Δ y = 125m

56 What Goes Up Must Come Down Someone standing at the edge of a cliff throws one ball straight up and one straight down at the same speed. Ignoring air resistance, which ball strikes the ground with the greatest speed?

57 Free Fall: Throwing UP What is the speed at the top of the path?(assume g = 10m/s 2 ) +y ZERO! What is the acceleration at the top? g=10m/s 2 What is the speed at the same height on the way down? -30 m/s

58 Free Fall Symmetry Which ball has the greatest speed when it hits the ground? SAME! The gravitational field is symmetric (in the absence of air resistance.)

59 Symmetry of The G-Field

60 Quizzy Questions If you throw a ball straight up in the air at 30 m/s, how fast is it going 1 second later? 10 m/s b) 5 m/s c) 20 m/s d) 0 m/s What is the acceleration of a falling object that has reached its terminal velocity? a) 10 m/s 2 b) 5 m/s 2 c) zero d) unknown

61 Free Fall Summary For an object without an initial velocity and without air resistance. a = g = 10 m/ s 2 v = 10t d = 5t 2 If there is an initial velocity, then add it! vt () = v+ 10t dt () = vt+ 5t 2 i Do Aunt Minnie problems in the Practice Book. i