Motion

INTRODUCTION It’s 2:26 and school is over! You hurry out of class and into the bright daylight. A light breeze is blowing. A few clouds drift lazily in the sky, a few leaves float down from the trees. Several birds fly over you head, and you see cars speeding by on the street. In all these situations there are all types of motion taking place, from simple to complex.

INTRODUCTION Deciding if an object is in motion isn’t as easy as it sounds. For example, as you read this text are you moving? Other than your eyes moving and blinking, your chest rising and falling, you would probably say that you are not moving. An object is in motion when the distance between two objects is changing. Since the distance between you and other things is not changing, you conclude that you are not moving.

INTRODUCTION At the same time that you think you are not moving, you are actually traveling about 30 kilometers every second. At that speed, you could travel from Las Vegas to Los Angeles in about 10 minutes!

INTRODUCTION Whether an object is moving or not depends on your point of view. If you compare books setting on a table to the floor underneath they do not appear to be moving.

INTRODUCTION But if you compare them to the sun, they are moving quite rapidly. Earth and the sun are different reference points. An object is in motion if it changes position relative to a reference point.

INTRODUCTION When you ride in a car and you look out the window, you can tell you are moving by comparing yourself to the objects that are stationary, or not moving. If you look at an object in the car, it does not appear to be moving at all, because it is moving with you.

INTRODUCTION If you have ever been in a bus that is side by side with another school bus you may suddenly feel like you are moving backwards. But as you look out the other window you realize that the bus next to you is moving, not you. That is why it is important to choose your point of reference carefully.

INTRODUCTION An object that is stationary in one frame of reference may be moving in another frame of reference. Any frame of reference can be chosen to describe a given movement, but the most common frame of reference is the Earth.

INTRODUCTION Scientists use SI units to describe the distance an object travels. If you know the distance and object travels in a certain amount of time, then you know the speed of the object. The speed of an object is the distance the object travels in one unit of time.

INTRODUCTION This relationship can be written as follows: DISTANCE TIME If you measure speed in meters and time in seconds you express speed in meters per second. m/s SPEED =

INTRODUCTION Constant speed is when an object travels at the same speed for a period of time, like a car that is set on cruise control and travels at the same speed of 65 miles per hour for several hours without changing.

INTRODUCTION Constant speed can be calculated if you know the speed the object traveled and the rate that the object was moving. Speed is a type of rate. A rate tells you the amount of something that occurs or the changes in one unit of time.

INTRODUCTION Most objects do not move at a constant speed for very long. The car will most likely change speeds several times during a trip. Since you cannot describe the motion of the car during the trip in terms of constant speed you will describe the motion in terms of the car’s average speed.

INTRODUCTION To find the average speed the car traveled during the whole trip, you would divide the distance the car traveled by the amount of time it took to complete the entire trip. Total distance divided by total time gives you the average speed.

OBJECTIVES Explain when an object is in motion. Explain how motion is relative to a reference point. Calculate an object’s speed.

VOCABULARY Point of view – position from which something is observed Reference point – a point to which an object is compared. Stationary – not moving

VOCABULARY Rate - ratio between two measurements with different units. Constant speed – speed of the object does not change. Average speed – the rate at which an object covers a distance, over a unit of time.

IN QUESTION Why don’t we get closer to or farther away from other states when our planet is turning so fast? The other parts of the planet are moving at the same rate as we are.

NOTES Motion happens when the distance between two objects changes. To determine motion you have to have a reference point. Reference point should be stationary. Most common reference point is Earth.

NOTES Speed is a ratio of distance to time. Speed = Distance/Time S D T x

EXAMPLE 1 Constant speed: A car travels 5,000 meters in 50 seconds at a constant speed. At what rate did the car travel? 5,000/50 = 100 m/s

EXAMPLE 2 Average speed: A car traveled 30 km in 20 min, 60 km in 30 min, and 10 km in 10 minutes. What was the average speed? 30 km + 60 km + 10 km = 100 km 20 min + 30 min + 10 min = 60 min 100 km / 60 min = 1.66 km/min

OUT QUESTION How do you know when an object is in motion? The distance between the object and a stationary reference point is increasing or decreasing in distance.