MOTION (Chapter 2) Student Learning Objectives 2/11/2016. Compare and contrast terms used to describe motion Analyze circular and parabolic motion

Transcription

1 MOTION (Chapter 2) Student Learning Objectives Compare and contrast terms used to describe motion Analyze circular and parabolic motion PHYSICS:THE MOST FUNDAMENTAL PHYSICAL SCIENCE Physics is concerned with the basic principles that describe how the universe works. Physics deals with matter, motion, force, and energy. Intro PHYSICS AREAS OF STUDY Classical mechanics Waves and sounds Thermodynamics Electromagnetism Quantum mechanics Atomic and nuclear physics Relativity Intro 1

2 MOTION Motion is everywhere walking, driving, flying, etc. This chapter focuses on definition/discussion of: speed, velocity, and acceleration. There are two basic kinds of motion: Straight line Circular Intro DEFINING MOTION Position the location of an object A reference point must be given in order to define the position of an object Motion an object is undergoing a continuous change in position Description of Motion the time rate of change of position A combination of length and time describes motion Section 2.1 HOW DO SPEED AND VELOCITY COMPARE? Average speed is the amount of distance covered in some amount of time. Time can only go forward atch?v=-lzq_w-til8 Velocity is speed in some direction. Up, Down Left, Right North, South, East, West, + s = d t 2

3 VECTORS Note that vectors may be both positive and negative. Section 2.2 INSTANTANEOUS SPEED Section 2.2 PRACTICE: IF AN OLYMPIC RUNNER COMPLETES THE 100 METER DASH IN 11 SECONDS, WHAT IS THE RUNNER'S AVERAGE SPEED? HOW FAST IS THIS IN MPH? Scalars are quantities that indicate size, indicate a numerical value. Vectors are quantities that indicate the size and direction. The magnitude of a vector is a scalar; it is the size of the vector. Example: 65 mph North 3

4 Acceleration is the change of velocity in some amount of time. a = v t Deceleration is negative acceleration. (opposite direction of motion) HOW IS ACCELERATION DEFINED? A car that maintains a constant speed in a straight line has zero acceleration. The distance traveled depends on how long the object has been accelerating. d = 1/2)at 2 PRACTICE 1) When driving through a curve at a constant rate of speed, is there acceleration? On the freeway, it takes 3 seconds to increase your speed from 15 mph (6.7 m/s) to 65 mph (29 m/s). 2)What is the average acceleration? 3)If the car continues to accelerate, at the same rate, for another 1.5 seconds, what will the final velocity be? 4) How far will the car have traveled during the 4.5 seconds of acceleration? 4

5 WHAT CAUSES VERTICAL ACCELERATION? Objects are accelerated downward because of gravity. The gravitational pull from the Earth is the same on all objects, so the downward acceleration is the same on all objects, 9.81 m/s 2. Any difference in the net acceleration is due to air resistance. 6FH5ho PRACTICE 1) I toss a ball upward and it returns to my hand. What is the ball s rate of acceleration if there is no air resistance? a) Just after the ball leaves my hand? b) At the top of the trajectory? c) Just before the ball hits the ground? 2) Professional cliff divers fall from a cliff into the sea. If the dive takes 2.7 seconds, what is the distance they fall? What would be the speed of the diver as the diver enters the water? An object in free fall is only affected by gravity (no air resistance). Air resistance provides acceleration upward. Surface area increases air resistance. Speed increases air resistance. Terminal velocity occurs when air resistance balances WHAT gravity. IS TERMINAL VELOCITY? 5

6 WHAT CAUSES CIRCULAR MOTION? Objects move in a straight line, unless an unbalanced force changes the motion. In circular motion, a centripetal force ( center-seeking force) causes a constant change in motion, a constant change in direction. a c = v 2 F c = ma c r ACCELERATION IN UNIFORM CIRCULAR MOTION Although an object in uniform circular motion has a constant speed, it is constantly changing directions and therefore its velocity is constantly changing directions. Since there is a change in direction there is a change in acceleration. What is the direction of this acceleration? It is at right angles to the velocity, and generally points toward the center of the circle. Supplied by friction of the tires of a car The car remains in a circular path as long as there is enough centripetal acceleration. CENTRIPETAL ( CENTER- SEEKING ) ACCELERATION 6

7 PRACTICE 1) What are some examples of centripetal motion? 2) An object is circling two times every second at the end of an 8 cm long string. a) Determine how to calculate the velocity, and calculate it. b) Calculate the centripetal acceleration. c) Calculate the amount of centripetal force on a 0.5 kg object. PROJECTILE MOTION An object thrown horizontally combines both straight-line and vertical motion each of which act independently. Neglecting air resistance, a horizontally projected object travels in a horizontal direction with a constant velocity while falling vertically due to gravity. An object thrown horizontally will fall at the same rate as an object that is dropped. 7

8 The velocity in the horizontal direction does not affect the velocity and acceleration in the vertical direction. Shoot the Monkey WHY DOES A PROJECTILE HAVE A PARABOLIC TRAJECTORY? The addition of the horizontal velocity and the vertical acceleration due to gravity results in a curved path. 4f61-bb2a-b6c07685fb02/science-nfl-football-projectilemotion-parabolas PROJECTED AT AN ANGLE (NOT HORIZONTAL) Combined Horz/Vert. Components Vertical = + Component Horizontal Component 8

9 A projectile's motion in x does not affect its motion in y! (and vice versa) Velocity in x is constant if there is no air resistance. Why? Velocity in y changes. Why? A projectile will fall a vertical distance (y) each second after reaching maximum height. y = 1/2)gt 2 The range (x) of a projectile relative to the horizontal plane depends on the object s horizontal velocity and time of flight. x = vt Under real-world conditions, air resistance causes the paths to be non-symmetric. Air resistance reduces the horizontal velocity. 9

10 PRACTICE 1) At what angle relative to the ground should a football be thrown to have the greatest horizontal range? 2) What are some examples of projectile motion that you have observed? MORE PRACTICE 3) A player throws a baseball parallel to the ground with a horizontal velocity of 90 mph (40 m/s). Assume no air resistance. a) What is the direction of the acceleration? b) Does the horizontal speed change? c) Does the vertical speed change? d) What is the horizontal speed the instant before the ball hits the ground? e) If there was no gravity, what would be the path of the ball? Why? f) What is the maximum horizontal range of the baseball if it is released from a height of 1.5 meters? 10