Describing and Measuring Motion Chapter 1 Section 1

Examples of Motion • Examples of motion are drifting, fluttering, flying, and chasing. • There are simple motions and complicated motions. • Motions may be over in a moment and motions may be continuous.

Recognizing Motion • Deciding if an object is in motion isn’t as easy as it sounds. • Think about this, as you read this, are you in motion? • An object is in motion when its distance from another object is changing.

Are you in Motion? • Even if you are sitting still, you are in motion. • You are actually moving about 30 kilometers every second. • At that speed, you could travel from New York City to Los Angeles in about 2 minutes.

The Earth is in Constant Motion • You are moving because you are on the planet Earth, which is orbiting the sun. • Earth moves about 30 kilometers every second, so you and everything else on Earth are moving at that speed!

Reference Point • Whether an object is moving or not depends on your point of view. If you compare a book to the floor beneath it, they are not moving. • If you compare them to the sun, the books are moving quite rapidly. • Earth and the sun are different reference points!

Reference Point and Motion • A reference point is a place or object used for comparison to determine if something is in motion. • An object is in motion, if it changes position relative to a reference point. • You must assume that the reference point is not moving.

Tricky Reference Point • Have you ever been on a stopped school bus next to another school bus, and looked out the window? • Suddenly, you think your bus is moving backward. • When you look out the other window you realize that it was the other bus moving forward. • Your bus seemed to be moving backward because you used the moving bus as your reference point. • You assumed your reference point was stationary but it wasn’t.

Describing Distance • To describe motion further, you need to use units of measurement. • The system measurement is called the International System of Units or in French, Systeme International (SI) • The SI unit for length is meter (m).

Meter • A meter is little longer than a yard. • To measure small objects, like the width of your fingernail, we use centimeters. • The prefix centi means 1/100 th. • In order to measure even smaller objects such as a small black ant, we would use millimeters. • Milli means 1/1000 th. • To measure long distances, scientists use kilometers. • The prefix kilo means 1000.

Calculating Speed • Scientists use the SI unit to describe distance an object travels. • A car might travel 90 kilometers per hour and an ant might travel 2 centimeters per second. • If you know the distance an object travels in a certain amount of time, you know the speed of the object. • Speed is the distance an object travels divided by the amount of time it takes to travel that distance. • Speed = Distance Time

Constant Speed • A ship traveling across the ocean may move at the same speed for several hours. • A horse may canter across a field at a steady pace for several minutes. • If so both the ship and horse, travel at a constant speed. • If the speed of an object does not change, the object is traveling at a constant speed.

Calculating Constant Speed • If the horse travels at a constant speed of 21 meters for 3 seconds then you can calculate the horse’s speed per second. • Speed = 21 m = 7 m/s 3 s

Average Speed • Most objects do not move at a constant speed for very long. • Look at page 21 Figure 5. • The cyclists in the picture change their speed many times during the race. • You can, however, find the average speed on the cyclists.

Calculating Average Speed • To find average speed, you would divide the total distance traveled by the total time. • For example: Suppose the cyclists travel 30 kilometers during the first 2 hours and 16 kilometers during the last hour. Add the total distance and divide by the total hours. • 30 + 16 = 36 kilometers • 2 + 1 = 3 hours • 36/3 = 12 kilometers/hour or 12 km/h

Describing Velocity • Knowing and understanding velocity helps meteorologists predict storms. • For example, if a weather forecaster announces that a severe storm is traveling 25 km/h, would you prepare for the storm? • You must know where the storm is and which direction it is heading. • Storms travel west to east so if you live east of the storm, you will need to take cover.

Velocity • Velocity is speed in a given direction. • You must know both the speed and direction of an object’s motion in order to know it’s velocity. • The weather forecaster must state the storm’s velocity by saying it is traveling 25 km/h eastward. • Air traffic controllers must also keep track of aircraft’s velocities because one error in speed or direction cause a major collision.

Graphing Velocity • To show the motion of an object, you would use a line graph. • You would plot distance against time. • The steeper the slope of the line, the faster the object is moving. • Lets practice graphing the distance of two crawling babies. • Open your textbook to page 25 and look at number 4.