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Chapter 7: Using Vectors: Motion and Force

Chapter 7: Using Vectors: Motion and Force. 7.1 Vectors and Direction 7.2 Projectile Motion and the Velocity Vector 7.3 Forces in Two Dimensions. Chapter 7 Objectives. Add and subtract displacement vectors to describe changes in position.

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Chapter 7: Using Vectors: Motion and Force

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  1. Chapter 7: Using Vectors: Motion and Force 7.1 Vectors and Direction 7.2 Projectile Motion and the Velocity Vector 7.3 Forces in Two Dimensions

  2. Chapter 7 Objectives Add and subtract displacement vectors to describe changes in position. Calculate the x and y components of a displacement, velocity, and force vector. Write a velocity vector in polar and x-y coordinates. Calculate the range of a projectile given the initial velocity vector. Use force vectors to solve two-dimensional equilibrium problems with up to three forces. Calculate the acceleration on an inclined plane when given the angle of incline.

  3. Chapter 7 Vocabulary • Cartesian coordinates • component • cosine • displacement • inclined plane • magnitude • parabola • polar coordinates • projectile • Pythagorean theorem • range • resolution • resultant • right triangle • scalar • scale • sine • tangent • trajectory • velocity vector • x-component • y-component

  4. Inv 7.1 Vectors and Direction Investigation Key Question: How do you give directions in physics?

  5. 7.1 Vectors and Direction A scalaris a quantity that can be completely described by one value: the magnitude. You can think of magnitude as size or amount, including units.

  6. 7.1 Vectors and Direction A vector is a quantity that includes both magnitude and direction. Vectors require more than one number. The information “1 kilometer, 40 degrees east of north” is an example of a vector.

  7. 7.1 Vectors and Direction In drawing a vector as an arrow you must choose a scale. If you walk five meters east, your displacement can be represented by a 5 cm arrow pointing to the east.

  8. 7.1 Vectors and Direction Suppose you walk 5 meters east, turn, go 8 meters north, then turn and go 3 meters west. Your position is now 8 meters north and 2 meters east of where you started. The diagonal vector that connects the starting position with the final position is called the resultant.

  9. 7.1 Vectors and Direction The resultant is the sum of two or more vectors added together. You could have walked a shorter distance by going 2 m east and 8 m north, and still ended up in the same place. The resultant shows the most direct line between the starting position and the final position.

  10. 7.1 Representing vectors with components • Every displacement vector in two dimensions can be represented by its two perpendicular component vectors. • The process of describing a vector in terms of two perpendicular directions is called resolution.

  11. 7.1 Representing vectors with components • Cartesian coordinates are also known as x-y coordinates. • The vector in the east-west direction is called the x-component. • The vector in the north-south direction is called the y-component. • The degrees on a compass are an example of a polar coordinates system. • Vectors in polar coordinates are usually converted first to Cartesian coordinates.

  12. 7.1 Adding Vectors Writing vectors in components make it easy to add them.

  13. 7.1 Subtracting Vectors • To subtract one vector from another vector, you subtract the components.

  14. Calculating the resultant vectorby adding components • You are asked for the resultant vector. • You are given 3 displacement vectors. • Sketch, then add the displacement vectors by components. • Add the x and y coordinates for each vector: • X1= (-2, 0) m + X2 = (0, 3) m + X3= (6, 0) m • = (-2 + 0 + 6, 0 + 3 + 0) m = (4, 3) m • The final displacement is 4 meters east and 3 meters north from where the ant started. An ant walks 2 meters West, 3 meters North, and 6 meters East. What is the displacement of the ant?

  15. 7.1 Calculating Vector Components Finding components graphically makes use of a protractor. Draw a displacement vector as an arrow of appropriate length at the specified angle. Mark the angle and use a ruler to draw the arrow.

  16. 7.1 Finding components mathematically • Finding components using trigonometry is quicker and more accurate than the graphical method. • The triangle is a right triangle since the sides are parallel to the x- and y-axes. • The ratios of the sides of a right triangle are determined by the angle and are called sine and cosine.

  17. 7.1 Finding the Magnitude of a Vector When you know the x- and y- components of a vector, and the vectors form a right triangle, you can find the magnitude using the Pythagorean theorem.

  18. Finding two vectors… Robots are programmed to move with vectors. A robot must be told exactly how far to go and in which direction for every step of a trip. A trip of many steps is communicated to the robot as series of vectors. A mail-delivery robot needs to get from where it is to the mail bin on the map. Find a sequence of two displacement vectors that will allow the robot to avoid hitting the desk in the middle? • You are asked to find two displacement vectors. • You are given the starting (1, 1) and final positions (5,5) • Add components (5, 5) m – (1, 1) m = (4, 4) m. • Use right triangle to find vector coordinates x1 = (0, 4) m, x2 = (4, 0) m • Check the resultant: (4, 0) m + (0, 4) m = (4, 4) m

  19. Chapter 7: Using Vectors: Motion and Force 7.1 Vectors and Direction 7.2 Projectile Motion and the Velocity Vector 7.3 Forces in Two Dimensions

  20. Inv 7.2 Projectile Motion Investigation Key Question: How can you predict the range of a launched marble?

  21. 7.2 Projectile Motion and the Velocity Vector Any object that is moving through the air affected only by gravity is called a projectile. The path a projectile follows is called its trajectory.

  22. 7.2 Projectile Motion and the Velocity Vector The trajectory of a thrown basketball follows a special type of arch-shaped curve called a parabola. The distance a projectile travels horizontally is called its range.

  23. 7.2 The velocity vector The velocity vector (v) is a way to precisely describe the speed and direction of motion. There are two ways to represent velocity. Both tell how fast and in what direction the ball travels.

  24. Draw the velocity vector v = (5, 5) m/sec and calculate the magnitude of the velocity (the speed), using the Pythagorean theorem. Drawing a velocity vectorto calculate speed • You are asked to sketch a velocity vector and calculate its speed. • You are given the x-y component form of the velocity. • Set a scale of 1 cm = 1 m/s. Draw the sketch. Measure the resulting line segment or use the Pythagorean theorem: a2 + b2 = c2 • Solve: v2 = (5 m/s)2 + (5 m/s)2 = 50 m2 /s2 • v = 0 m2 /s2 = 7.07 m/s

  25. 7.2 The components of the velocity vector Suppose a car is driving 20 meters per second. The direction of the vector is 127 degrees. The polar representation of the velocity is v = (20 m/sec, 127°).

  26. Calculating the componentsof a velocity vector • You are asked to calculate the components of the velocity vector. • You are given the initial speed and angle. • Draw a diagram to scale or use vx= v cos θ and vy= v sin θ. • Solve: • vx = (10 m/s)(cos 30o) = (10 m/s)(0.87) = 8.7 m/s • vy= (10 m/s)(sin 30o) = (10 m/s)(0.5) = 5 m/s A soccer ball is kicked at a speed of 10 m/s and an angle of 30 degrees. Find the horizontal and vertical components of the ball’s initial velocity.

  27. 7.2 Adding velocity vectors Sometimes the total velocity of an object is a combination of velocities. • One example is the motion of a boat on a river. • The boat moves with a certain velocity relative to the water. • The water is also moving with another velocity relative to the land.

  28. 7.2 Adding Velocity Components • Velocity vectors are added by components, just like displacement vectors. • To calculate a resultant velocity, add the x components and the y components separately.

  29. Calculating the componentsof a velocity vector • You are asked to calculate the resultant velocity vector. • You are given the plane’s velocity and the wind velocity • Draw diagrams, use Pythagorean theorem. • Solve and add the components to get the resultant velocity : • Plane: vx= 100 cos 30o = 86.6 m/s, vy= 100 sin 30o = 50 m/s • Wind: vx= 40 cos 45o = 28.3 m/s, vy= - 40 sin 45o = -28.3 m/s • v = (86.6 + 28.3, 50 – 28.3) = (114.9, 21.7) m/s or (115, 22) m/s An airplane is moving at a velocity of 100 m/s in a direction 30 degrees northeast relative to the air. The wind is blowing 40 m/s in a direction 45 degrees southeast relative to the ground. Find the resultant velocity of the airplane relative to the ground.

  30. 7.2 Projectile motion When we drop a ball from a height we know that its speed increases as it falls. The increase in speed is due to the acceleration gravity, g = 9.8 m/sec2. Vx Vy y x

  31. 7.2 Horizontal motion The ball’s horizontal velocity remains constant while it falls because gravity does not exert any horizontal force. Since there is no force, the horizontal acceleration is zero (ax = 0). The ball will keep moving to the right at 5 m/sec.

  32. 7.2 Horizontal motion The horizontal distance a projectile moves can be calculated according to the formula:

  33. 7.2 Vertical motion The vertical speed (vy) of the ball will increase by 9.8 m/sec after each second. After one second has passed, vyof the ball will be 9.8 m/sec. After 2 seconds have passed, vywill be 19.6 m/sec and so on.

  34. Analyzing a projectile A stunt driver steers a car off a cliff at a speed of 20 meters per second. He lands in the lake below two seconds later. Find the height of the cliff and the horizontal distance the car travels. • You are asked for the vertical and horizontal distances. • You know the initial speed and the time. • Use relationships: y = voyt – ½ gt2 and x = vox t • The car goes off the cliff horizontally, so assume voy= 0. Solve: • y = – (1/2)(9.8 m/s2)(2 s)2y = –19.6 m. (negative means the car is below its starting point) • Use x = voxt, to find the horizontal distance: x = (20 m/s)(2 s) x = 40 m.

  35. 7.2 Projectiles launched at an angle A soccer ball kicked off the ground is also a projectile, but it starts with an initial velocity that has both vertical and horizontal components. *The launch angle determines how the initial velocity divides between vertical (y) and horizontal (x) directions.

  36. 7.2 Steep Angle A ball launched at a steep angle will have a large vertical velocity component and a small horizontal velocity.

  37. 7.2 Shallow Angle A ball launched at a low angle will have a large horizontal velocity component and a small vertical one.

  38. 7.2 Projectiles Launched at an Angle The initial velocity components of an object launched at a velocity vo and angle θ are found by breaking the velocity into x and y components.

  39. 7.2 Range of a Projectile The range, or horizontal distance, traveled by a projectile depends on the launch speed and the launch angle.

  40. 7.2 Range of a Projectile The range of a projectile is calculated from the horizontal velocity and the time of flight.

  41. 7.2 Range of a Projectile A projectile travels farthest when launched at 45 degrees.

  42. 7.2 Range of a Projectile The vertical velocity is responsible for giving the projectile its "hang" time.

  43. 7.2 "Hang Time" You can easily calculate your own hang time. Run toward a doorway and jump as high as you can, touching the wall or door frame. Have someone watch to see exactly how high you reach. Measure this distance with a meter stick. The vertical distance formula can be rearranged to solve for time:

  44. Chapter 7: Using Vectors: Motion and Force 7.1 Vectors and Direction 7.2 Projectile Motion and the Velocity Vector 7.3 Forces in Two Dimensions

  45. Inv 7.3 Forces in Two Dimensions Investigation Key Question: How do forces balance in two dimensions?

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