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# Sec.1 describing and measuring_motion

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Dear Students of grade 8, this presentation has been made for you to revise, and to copy what would you miss out of your class work. I hope to be useful for all of you.

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### Sec.1 describing and measuring_motion

1. 1. Force & Motion
2. 2. Describing and Measuring Motion 1-1
3. 3. Learning Objectives <ul><li>Explain when an object is in motion and how motion is relative to a reference point. </li></ul><ul><li>Calculate an object’s speed and velocity using SI units of distance. </li></ul><ul><li>Graph motion showing changes in distances as a function of time. </li></ul>
4. 4. Key Terms <ul><li>Motion </li></ul><ul><li>Reference point </li></ul><ul><li>International System of Unit (SI) </li></ul><ul><li>Meter </li></ul><ul><li>Speed </li></ul><ul><li>Velocity </li></ul><ul><li>Slope </li></ul>
5. 5. How Fast and How Far <ul><li>Find out how long it takes you to walk 5 meters at a normal pace. Record your time. </li></ul><ul><li>Now find out how far you can walk in 5 seconds if you walk at a normal pace. Record your distance. </li></ul><ul><li>Repeat Step 1 and 2, walking slower than your normal pace. Then repeat steps 1 and 2, walking faster than your normal pace. </li></ul>
6. 6. Question <ul><li>What is the relationship between the distance you walk, the time it takes you to walk, and your walking speed? </li></ul>
7. 7. Recognizing Relative Motion <ul><li>Deciding if an object is in motion isn’t as easy as it sounds. </li></ul><ul><li>An object is in motion when its distance from another object is changing. </li></ul><ul><li>At the same time that you think you are sitting still, you are actually moving about 30kilometers every second. (18.64 mile/second) </li></ul><ul><li>At that speed, you could travel from New York City to Los Angeles in about 2 minutes! </li></ul>
8. 8. <ul><li>Whether an object is moving or not depends on your point of view. </li></ul><ul><li>If you compare the books on a desk to the floor beneath them, they are not moving. </li></ul><ul><li>But if you compare them to the sun, the books are moving quite rapidly. </li></ul><ul><li>Earth and the sun are different reference points. </li></ul>
9. 9. What is A reference point? <ul><li>A reference point is a place or object used for comparison to determine if something is in motion . </li></ul>
10. 10. <ul><li>An object is in motion if it changes position relative to a reference point. </li></ul><ul><li>The reference point is stationary . </li></ul>
11. 11. Describing Distance <ul><li>Scientists all over the world use the same system of units so that they can communicate information clearly. </li></ul><ul><li>This system of measurement is called the International System of Units. </li></ul><ul><li>SI is a system based on the number ten. </li></ul><ul><li>The basic SI unit of length is the meter (m). </li></ul><ul><li>A meter is a little longer than a yard. </li></ul>
12. 12. What do scientists use to measure the length of an object smaller than a yard? <ul><li>A centimeter – one hundredth of a meter, so there are 100 centimeters in a meter </li></ul><ul><li>A millimeter – There are 1,000 millimeters in a meter </li></ul>
13. 13. How do scientists measure long distances? <ul><li>The kilometer – There are 1,000 meters in a kilometer </li></ul>
14. 14. How do scientists calculate speed? <ul><li>Speed – the distance the object travels in one unit of time </li></ul><ul><ul><li>Rate – tells you the amount of something that occurs or changes in one unit of time </li></ul></ul><ul><ul><li>Speed = distance </li></ul></ul><ul><ul><li>time </li></ul></ul>
15. 15. Constant Speed <ul><li>If the speed of an object does not change, the object is traveling at a constant speed. </li></ul><ul><li>When an object travels at a constant speed, its speed is the same at all times during its motion. </li></ul>
16. 16. What is average speed? <ul><li>Most objects do not move at constant speeds for very long </li></ul><ul><li>To find average speed divide the total distance by the total time </li></ul><ul><li>Car trip </li></ul>
17. 17. Average Speed Average Speed = distance time
18. 18. Numerator Denominator 5 12
19. 19. Example; A cyclist travels 32 kilometers during the first 2 hours. Then the cyclist travels 13 kilometers during the next hour. The average speed of the cyclist is the total distance by the total time. <ul><li>Instantaneous Speed </li></ul><ul><li>is the rate at which an object is moving at a given instant in time. </li></ul><ul><li>Example; Calculating the average speed of a cyclist during a race. </li></ul>Total distance = 32 km + 13 km = 45 km Total time = 2 h + 1 h = 3 h Average speed = 45 km / 3 h = 15 km/h
20. 20. Describing Velocity <ul><li>When you know both the speed and direction of an object’s motion, you know the velocity of the object. </li></ul><ul><li>Speed in a given direction is called velocity. </li></ul><ul><li>Example: A weather forecaster may give the speed of the storm as 25 km/h, but you don’t know its velocity unless you know that the storm is moving 25 km/h eastward. </li></ul>
21. 21. Velocity speed of an object in a certain direction. seconds 0 2 1 3
22. 22. Speed=? Velocity=? 16 meters 12 meters 20 meters Total time= 4 seconds A B
23. 23. Graphing Motion <ul><li>To show the motion of an object on a line graph in which you plot distance against time. </li></ul><ul><li>Time is shown on the horizontal, or x-axis . </li></ul><ul><li>It is represented by the first coordinate of the point –x. </li></ul><ul><li>Distance is shown on the vertical, or y-axis . </li></ul><ul><li>It is represented by the second coordinate of the point –y. </li></ul><ul><li>A point (x,y) on the graph represents the location of an object at a particular time. </li></ul>
24. 25. Slope of a line <ul><li>The steepness, or slant, of a line on a graph is called its slope. </li></ul><ul><li>The slope tells you how fast one variable changes in relation to the other variable. </li></ul><ul><li>Slope tells you the rate of change. </li></ul>
25. 26. <ul><li>Speed is the rate of change of distance in relation to time, the slope of a distance-time graph represents speed. </li></ul><ul><li>The faster the motion, the steeper the slope , because the object moves a greater distance in a given amount of time. </li></ul><ul><li>A distance-time graph with a constant slope represents motion at a constant speed. </li></ul>
26. 27. Calculating Slope <ul><li>The slope of a line is its rise divided by its run. </li></ul><ul><li>First, choose 2 points on the line. </li></ul><ul><li>The rise is the vertical difference between the two points. </li></ul><ul><li>The run is the horizontal difference between the 2 points. </li></ul><ul><li>Slope= Rise/Run </li></ul>
27. 28. <ul><li>Interpreting Graphs: </li></ul><ul><ul><li>What variable is on the horizontal axis? The vertical axis? </li></ul></ul><ul><ul><li>Answer; ……………….. </li></ul></ul><ul><li>Drawing Conclusions: </li></ul><ul><ul><li>Suppose another object of the same size but with a greater mass was dropped instead. How would the speed values change? </li></ul></ul><ul><ul><li>Answer; ……………….. </li></ul></ul>
28. 29. Slope means the Speed <ul><li>The general form of the equation is y=kx, where the constant k represent the slope of a linear graph. </li></ul>
29. 30. Section Review Questions <ul><li>Why do you need a reference point to know if an object is moving? </li></ul><ul><li>What is the difference between an object’s speed and an object’s velocity? </li></ul><ul><li>A bamboo plant grows 15 cm in 4 hours. At what average speed does the plant grow? </li></ul>
30. 31. <ul><li>4. The distance traveled by 2 crawling babies is shown in the table. Graph the information and find the slope of each line. </li></ul><ul><li>What is each baby’s speed? </li></ul><ul><li>Write the equation of each baby’s line. </li></ul>1.6 2 4 1.2 1.5 3 0.8 1 2 0.4 0.5 1 0 0 0 Baby Scott Distances (m) Baby Sarah Distance) Time (s)