Physics. Chapter 3 Linear Motion

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1 Physics Chapter 3 Linear Motion

2 Motion is Relative How fast are you moving? We can only speak of how fast in relation to some other thing. Unless otherwise specified, we will assume motion relative to the surface of the Earth. 2

3 Distance Distance How far something moves Distance is a scalar quantity. Units Meters (Kilometers) d 3

4 Displacement Displacement Straight-line distance for how far Displacement is a vector quantity Units Meters (Kilometers) For straight line motion, distance and displacement are the same. Symbol lowercase x, especially for horizontal motion. Sometimes, y or h are used for vertical motion. 4

5 Speed and Velocity Speed How fast something moves. Speed is a scalar quantity Units miles/hour; meters/second; etc. Velocity Speed with direction Velocity is a vector quantity Example: 25 miles per hour East Symbol lowercase v When velocity changes at a steady rate, we can find the average velocity. 5

6 Try This... A biker begins from rest and starts downhill. His velocity changes steadily until at the bottom of the hill his velocity is 6 m/s. What is his average velocity? If the trip took him 5 s, how far did he travel? 6

7 Acceleration Acceleration The rate at which velocity changes. Acceleration is a vector quantity Units meters per second per second (m/s 2 ) Symbol lowercase a Suppose it takes 4 seconds for a biker traveling at 4 m/s steadily to speed up until he s traveling at 12 m/s. What is his acceleration? 7

8 Acceleration The term acceleration applies to decreases as well as to increases in velocity. Sometimes called, deceleration When you lurch forward as brakes are applied in car. Indicated by a negative value. Can an object be accelerating if it maintains constant speed? YES! Remember Velocity is a vector. It has magnitude (speed) and direction. Change in direction only is still acceleration! A racecar on a circular track is accelerating even if his speed is a constant 100 miles per hour. 8

9 Check Questions 1. One plane travels due north at 300 km/h while another travels due south at 300 km/h. Are their speeds the same? Are their velocities the same? 2. Can a car with a velocity toward the north simultaneously have an acceleration toward the south? 3. Cite an example of something that undergoes acceleration while moving at a constant speed. Can you also give an example of something that accelerates while traveling at constant velocity? 9

10 For Constant Acceleration We can use these very basic mathematical relationships and by substituting and solving, can derive some very useful formulas. 10

11 Check Question A biker travels downhill, starting from rest. He accelerates smoothly. When 4 seconds later he encounters a speed bump, he s going 12m/s. What is the distance between his starting point and the speed bump? 11

12 Free Fall Free fall is when a falling object is free of all restraints no friction, no air drag, but rather under the influence of gravity alone. During free fall, the boulder will experience the exact same pickup of speed during each second. We say it falls with constant acceleration. 12

13 Free Fall 13

14 Free Fall 14

15 Free Fall 15

16 Free Fall 16

17 Free Fall 17

18 Free Fall - Speed 18

19 Free Fall - Distance x = 5 m x = 20 m x = 45 m x = 80 m 19

20 Free Fall - Distance 20

21 Check Question Back to the simpler case of falling from rest. After 5 seconds, what will be the boulder s Speed? Distance? Acceleration? 21

22 Kinematic Equations p. 79 in Barron s. 22

23 Check Questions One second after being thrown straight down, an object is falling with a speed of 20 m/s. How fast will it be falling 2 seconds later? If an object is thrown straight upward with an initial speed of 8 m/s and takes 3 seconds to strike the ground, from what height was the object thrown? 23

24 Position vs Time Graphs The slope of a position vs time graph gives the velocity. 24

25 Velocity vs. Time Graphs The slope of a velocity vs time graph gives acceleration. The area between the graph and the time axis gives the displacement. 25

A. VOCABULARY REVIEWS On the line, write the term that correctly completes each statement. Use each term once.

PART III. KINEMATICS A. VOCABULARY REVIEWS On the line, write the term that correctly completes each statement. Use each term once. 1. rise (Δy) The vertical separation of any two points on a curve is