Fast and SlowWords like fast and slow describe the speed of something. Speed is a way to tell how much an object’s position has changed over time.Objects that are fast, such as airplanes and rockets, change their position quickly. Objects that are slow, such as merry-go-rounds, change their position slowly.The best way to understand speed is to use measurements. First, you need to know the distance, or how far the object has traveled. Second, you need to know the time it took to travel the distance. Then you can ?nd the speed by dividing the distance by the time. For example, Chicago, Illinois, and St. Louis, Missouri, are about 480 kilometers apart. If it takes you six hours to drive between the two cities, you travel at about 80 kilometers per hour.A merry-go-round goes at a slow speed.4A roller coaster’s speed changes as it goes up and down the track.Here’s something else to keep in mind: Not all objects move at the same speed all the time. Some machines, like elevators or fans, move at a steady speed. Others don’t.
For instance, when we say a car is traveling at a speed of 80 kilometers (50 mi) per hour, we are taking an average. Sometimes the driver might be traveling at 85 kilometers (53 mi) per hour. At other times, when the car slows down for tolls or exits, the car could be traveling between 15 and 25 kilometers (9 and 16 mi) per hour.
COMPARE AND CONTRAST How can you contrast the steadiness in speed of a slowly moving motorboat with that of a moving rowboat?
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6Describing SpeedWhen we think about how fast an object is traveling, we don’t usually think only about its speed. We also think of the direction it’s going. For example, if you saw a car driving on a highway with a tire going ?at, you might call the police for help and tell them that there’s a “white station wagon going 55 miles an hour, heading north.” You have just given information about the car’s velocity. Velocity is a
measurement of an object’s speed and direction. The car’s velocity is 55 miles per hour, north.To describe directions, we use words that tell us about location—right, left, up, down, north, south, east, or west. If two objects are traveling in opposite directions, they may have the same speed, but they will have different velocities.You read earlier that a car doesn’t always travel at the same speed. Sometimes it speeds up, sometimes it slows down, and sometimes it stops. A car has to slow down and stop for this sign! Stopping is also acceleration.A change in direction is a change in velocity. Cars also sometimes turn and change their direction. They can go backward when the driver puts the car into the reverse gear. These changes are called acceleration, or a change in the speed or direction of an object’s motion. Sometimes people think acceleration only has to do with speeding up. It also has to do with slowing down. Acceleration is any change in velocity.
CAUSE AND EFFECT What will happen to the velocity of a car that makes a right turn at a street corner?
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8Feeling the ForceWhen you were younger, an older person probably helped you ride a playground swing by pushing you. Any time something is pulled or pushed, that is force. When you were being pushed, the person was using force to move your body. Did the person also pull you way back to give your swinging a good start? Force was used then, too. When the older person used force, you moved. Unless you used your own force to move the swing, if the other person didn’t push you, you would have stayed in one place. Force changes motion.Any change of speed or direction needs a force. Force causes acceleration.The direction of the force changes the direction in which an object moves. If the older person pushed you to the side instead of forward, you would move to the side.This swing doesn’t go anywhere unless force is used.
move it
