Chapter 3. Kinematics in Two Dimensions

Transcription

1 Chapter 3 Kinematics in Two Dimensions

2 3.1 Trigonometry

3 3.1 Trigonometry sin! = h o h cos! = h a h tan! = h o h a

4 3.1 Trigonometry tan! = h o h a tan50! = h o 67.2m h o = tan50! ( 67.2m) = 80.0m

5 3.1 Trigonometry! = sin!1 " h o % $ ' # h &! = cos!1 " h $ a # h % ' & " h! = tan!1 o # $ h a % & '

6 3.1 Trigonometry

7 3.1 Trigonometry Pythagorean theorem:

8 3.2 Scalars and Vectors Directions of vectors! F 1 and! F 2 appear to be the same. Vector F! 1, (bold + arrow over it)! # magnitude = F has 2 parts: 1 (italics) " $# direction = up & to the right F = 4 lb 1! F 1 Vector F! 2, (bold + arrow over it)! # magnitude = F has 2 parts: 2 (italics) " $# direction = up & to the right F 2 = 8 lb!f2 Directions of vectors! F 1 and! F 2 appear to be the same.

9 3.2 Scalars and Vectors Displacement vector S!!# S = 2 km (magnitude), and has 2 parts: " $# 30 North of East (direction) S

10 3.2 Scalars and Vectors (Vector Addition and Subtraction) Often it is necessary to add one vector to another.

11 3.2 Scalars and Vectors (Vector Addition and Subtraction) Two displacement vectors 5 m 3 m Adding together Resultant 5 m 3 m 8 m

12 3.2 Scalars and Vectors (Vector Addition and Subtraction) Two displacement vectors! A = 6.00m, East Resultant B = 2.00m North Adding together

13 3.2 Scalars and Vectors (Vector Addition and Subtraction) Graphical Addition Two displacement vectors! A = 6.00m, East B = 2.00m North Graphical addition only qualitative YOU need quatitative addition!! R 6.00m 2.00m To do this addition of vectors requires trigonometry

14 3.2 Scalars and Vectors (Vector Addition and Subtraction) Apply Pythagorean Theroem R = 6.32m 2.00m! 6.00m

15 3.2 Scalars and Vectors (Vector Addition and Subtraction) Use trigonometry to determine the angle tangent (angle) = opposite side adjacent side R 2.00m! 6.00m

16 3.2 Scalars and Vectors (Vector Addition and Subtraction) When a vector is multiplied by -1, the magnitude of the vector remains the same, but the direction of the vector is reversed.

17 3.2 Scalars and Vectors (Vector Addition and Subtraction) Add vectors A and B!! A + B! A! B Now you are asked to find A B Instead of trying to do Vector Subtraction add to vector A the negative of the vector B! B Subtracting vector B from vector A! A!! B!! B!! A! B

18 3.2 Vector Addition and Subtraction (The Components of a Vector) x and y are called the x component vector and the y component vector of r.

19 3.2 Vector Addition and Subtraction + y (The Components of a Vector) + y A y A A A y!! A x +x A x +x The vector components of! A are two perpendicular vectors! A x and! A y that are parallel to the x and y axes, and add together vectorially so that! A =! A x +! A y.

20 3.2 Vector Addition and Subtraction (The Components of a Vector) It is often easier to work with the scalar components of the vectors, rather than the component vectors themselves. + y + y "i-hat" î "j-hat" ĵ î and ĵ are "unit vectors", magnitude is 1. "j-hat" Ayĵ vector A expressed in terms of! its scalar components: A = A x î + A y ĵ ĵ î "i-hat" Axî +x +x Ax and Ay are the scalar components of the vector, A.

21 3.2 Vector Addition and Subtraction (The Components of a Vector) Example A displacement vector has a magnitude of 175 m and points at an angle of 50.0 degrees relative to the x axis. Find the x and y components of this vector. Given magnitude r = 175 m r 50.0 x α 90 y vector x! has magnitude x vector y has magnitude y sin! = y r y = r sin! = 175 m x = r cos! = 175 m y-component of the vector! r ( ) = 134 m ( ) sin50.0! x-component of the vector! r ( ) = 112 m ( ) cos50.0!! r = xî + yĵ = ( 112 m)î + ( 134 m )ĵ

22 3.2 Vector Addition and Subtraction + y A θ 1 θ 2 B +x THIS IS A BIG PROBLEM What is the magnitude of C, and what angle θ does it make relative to x-axis? (using Components) Vector A has magnitude A and angle θ 1 Vector B has magnitude B and angle θ 2 What is the vector C = A + B? Graphically no PROBLEM + y C = A + B A θ 1 B θ 2 +x

23 3.2 Vector Addition and Subtraction (using Components) THIS IS A BIG PROBLEM What is the magnitude of C, and what angle θ does it make relative to x-axis? The only way to solve this problem is to use vector components! + y + y B θ 2 A A y ĵ A θ 1 B x î +x A x î B B y ĵ +x

24 3.2 Vector Addition and Subtraction (using Components) Get the components of the vectors A and B. A : magnitude A and angle θ 1 B : magnitude B and angle θ 2 Ax = Acosθ 1 Ay = Asinθ 1 B x= Bcosθ 2 By = Bsinθ 2 + y + y B θ 2 A A y ĵ A θ 1 B x î +x A x î B B y ĵ +x

25 3.2 Vector Addition and Subtraction (using Components) Get components of vector C from components of A and B. Ax = Acosθ 1 Ay = Asinθ 1 B x= Bcosθ 2 By = Bsinθ 2 Cx = Ax + B x Cy = Ay + B y C = A + B B y ĵ C C y ĵ θ A y ĵ θ A x î B x î C x î

26 3.2 Vector Addition and Subtraction (using Components) What is the magnitude of C, and what angle θ does it make relative to x-axis? Cx = Ax + B x Cy = Ay + B y θ C C y ĵ PROBLEM IS SOLVED magnitude of C : C = Cx 2 + Cy 2 Angle θ : tanθ = C y Cx θ = tan 1 (Cy Cx ) ; C x î

27 3.2 Addition of Vectors by Means of Components + y Summary of adding two vectors together A θ 2 B Vector A has magnitude A and angle θ 1 Vector B has magnitude B and angle θ 2 What is the vector C = A + B? θ 1 +x 1) Determine components of vectors A and B : Ax, Ay and Bx, By 2) Add x-components to find Cx = Ax + Bx 3) Add y-components to find Cy = Ay + By 4) Determine the magnitude and angle of vector C magnitude C = Cx 2 + Cy 2 ; θ = tan 1 (Cy Cx )

28 3.2 Equations of Kinematics in Two Dimensions Except for time, motion in x and y directions are INDEPENDENT. + y Using only x thruster ax vx0 vx +x Δx Motion in x direction. ( ) t v x = v x0 + a x t Δx = 1 2 v x0 + v x Δx = v x0 t a x t 2 v 2 = v 2 + 2a Δx x x0 x

29 3.2 Equations of Kinematics in Two Dimensions Except for time, motion in x and y directions are INDEPENDENT. Using only y thruster + y Motion in y direction. v y = v y0 + a y t vy Δy = v y0 t a y t 2 Δy ay Δy = 1 2 ( v y0 + v ) t y vy0 +x v 2 = v 2 + 2a Δy y y0 y

30 3.2 Equations of Kinematics in Two Dimensions Reasoning Strategy 1. Make a drawing. 2. Decide which directions are to be called positive (+) and negative (-). 3. Write down the values that are given for any of the five kinematic variables associated with each direction.

31 3.2 Equations of Kinematics in Two Dimensions Example: A Moving Spacecraft In the x direction, the spacecraft has an initial velocity component of +22 m/s and an acceleration of +24 m/s 2. In the y direction, the analogous quantities are +14 m/s and an acceleration of +12 m/s 2. Find (a) Δ x and v x, (b) Δy and v y, and (c) the final velocity of the spacecraft at time 7.0 s. Want final values: + y!x,v x,!y,v y, and v, at time t = 7s. vy initial values v x0 = +22m/s v y0 = +14m/s vy0 vx0 a x = +24m/s a y = +12m/s 2 vx constant accelerations act throughout the motion +x

32 3.2 Equations of Kinematics in Two Dimensions Reasoning Strategy 1. Make a drawing. 2. Decide which directions are to be called positive (+) and negative (-). 3. Write down the values that are given for any of the five kinematic variables associated with each direction. 4. Verify that the information contains values for at least three of the kinematic variables. Do this for x and y. Select the appropriate equation. 5. When the motion is divided into segments, remember that the final velocity of one segment is the initial velocity for the next. 6. Keep in mind that there may be two possible answers to a kinematics problem.

33 3.2 Equations of Kinematics in Two Dimensions Example: A Moving Spacecraft: x direction motion! x a x v x v x0 t? m/s 2? +22 m/s 7.0 s Δx = v x0 t a x t 2 = ( 22m s) ( 7.0 s) ( 24m )( s2 7.0 s) 2 = +740 m v x = v x0 + a x t = ( 22m s) + ( 24m s 2 )( 7.0 s) = +190m s

34 3.2 Equations of Kinematics in Two Dimensions Example: A Moving Spacecraft: y direction motion! y a y v y v y0 t? m/s 2? +14 m/s 7.0 s Δy = v y0 t a y t 2 = ( 14m s) ( 7.0 s) ( 12m )( s2 7.0 s) 2 = +390 m v y = v y0 + a y t = ( 14m s) + ( 12m s 2 )( 7.0 s) = +98m s

35 3.2 Equations of Kinematics in Two Dimensions Can also find final speed and direction (angle) at t = 7s.! v = v x 2 + v y 2 = ( 190m s) 2 + ( 98m s) 2 = 210m s

36 3.3 Projectile Motion Under the influence of gravity alone, an object near the surface of the Earth will accelerate downwards at 9.81m/s 2. ay = 9.81m s 2 ax = 0 Great simplification for projectiles! vx = vx0 = constant

37 Pop and Drop Demonstration. v x0 v x = v x0

38 3.3 Projectile Motion Example: A Falling Care Package The airplane is moving horizontally with a constant velocity of +115 m/s at an altitude of 1050m. Determine the time required for the care package to hit the ground. Time to hit the ground depends ONLY on vertical motion v y0 = 0 a y = 9.81 m/s 2 y = 1050 m

39 3.3 Projectile Motion! y a y v y v y0 t m m/s 2 0 m/s? Δy = vy0t a yt 2 Δy = 1 2 a yt 2 t = 2Δy ay = 2 ( 1050 m ) 9.81m s 2 = 14.6 s

40 3.3 Projectile Motion Example: The Velocity of the Care Package What are the magnitude and direction of the final velocity of the care package? v y0 = 0 a y = 9.81 m/s 2 Δy = 1050 m t = 14.6 s v x0 = +115 m/s a x = 0 v x = v 0x = +115 m/s x-component does not change

41 3.3 Projectile Motion! y a y v y v y0 t 1050 m 9.81 m/s 2? 0 m/s 14.6 s v y = v oy + a y t = 0 + ( 9.81m s 2 )( 14.6 s) = 143m s y-component of final velocity. v x = v ox = +115m s v = v x 2 + v y 2 = 184 m/s # v! = tan "1 y % $ v x & ( ' = # "143& tan"1 $ % +115' ( = "51

42 3.3 Projectile Motion Conceptual Example: I Shot a Bullet into the Air... Suppose you are driving a convertible with the top down. The car is moving to the right at constant velocity. You point a rifle straight up into the air and fire it. In the absence of air resistance, where would the bullet land behind you, ahead of you, or in the barrel of the rifle? Ballistic Cart Demonstration

43 3.3 Projectile Motion Example: The Height of a Kickoff A placekicker kicks a football at and angle of 40.0 degrees and the initial speed of the ball is 22 m/s. Ignoring air resistance, determine the maximum height that the ball attains. vy0 v 0 = 22m s! = 40! maximum height and hang time depend only on the y-component of initial velocity vx0

44 3.3 Projectile Motion vy0 v 0 = 22m s v 0 v 0 y vx0! = 40! v 0x v y0 = v 0 sinθ = ( 22m s) sin40 = 14m s v x0 = v 0 cosθ = ( 22m s) cos40 = 17m s

45 3.3 Projectile Motion vy0 vx0! y a y v y v y0 t? 9.80 m/s m/s v 2 y = v 2 0 y + 2a y!y!y = v 2 2 " v y 0 y 2a y ( ) 2!y = 0 " 14m s 2 "9.8m s 2 ( ) = +10 m

46 3.3 Projectile Motion Example: The Time of Flight of a Kickoff What is the time of flight between kickoff and landing? vy0 vx0! y a y v y v y0 t m/s 2 14 m/s?

47 3.3 Projectile Motion! y a y v y v y0 t m/s 2 14 m/s? Δy = v y0 t a y t 2 0 = ( 14m s)t ( 9.81m s2 )t 2 0 = 2( 14m s)+ ( 9.81m s 2 )t

48 3.3 Projectile Motion Example: The Range of a Kickoff Calculate the range R of the projectile. Range depends on the hang time and x-component of initial velocity vy0 vx0 Δx = vx0t a xt 2 = vx0t = ( 17m s) ( 2.9 s) = +49 m

49 3.3 Projectile Motion Conceptual Example: Two Ways to Throw a Stone From the top of a cliff, a person throws two stones. The stones have identical initial speeds, but stone 1 is thrown downward at some angle above the horizontal and stone 2 is thrown at the same angle below the horizontal. Neglecting air resistance, which stone, if either, strikes the water with greater velocity?

50 Shoot the Monkey Demonstration Fire and drop at the same time Δy B = v y0 t 1 2 gt 2!y M = h " 1 2 gt 2 h! v x0 v y0 Hit height:!y B =!y M! Hit time: t = Δx v x0 x v 0 y t = h v y0 v x0 x = h Shoot at the Monkey! v y0 v x0 = h x = tanθ