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  • Phy Sci/Motion and Forces

    Introductory Video


    Think about where you are sitting right now. Are you facing front? Are you 3 feet from the door? Are you on the right of the window? Position is the location of an object. Every object has a position. The position of your ears are on the side of your head. How would you describe the position of your classroom window or doorway? These positions do not change. 

    Sometimes an object's position does change. This is called motion. Motion is a change in the position of an object. There are many kinds of motion. An escalator goes up or down, a swing swings back and forth. Things may move fast or slow. But whenever something moves, its position changes. 



    Below are two pictures of a boat race. In these pictures you cannot see the movement, but how can you tell the boats have moved?


    If you notice, the houses in the background have changed. You used the houses as your frame of reference.

    Assume that a school bus passes by as you stand on the sidewalk. It’s obvious to you that the bus is moving. It is moving relative to you and the trees across the street. But what about to the children inside the bus? They aren’t moving relative to each other. If they look only at the other children sitting near them, they will not appear to be moving. They may only be able to tell that the bus is moving by looking out the window and seeing you and the trees whizzing by.

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    This example shows that how we perceive motion depends on our frame of reference. Frame of reference refers to something that is not moving with respect to an observer that can be used to detect motion. For the children on the bus, if they use other children riding the bus as their frame of reference, they do not appear to be moving. But if they use objects outside the bus as their frame of reference, they can tell they are moving. What is your frame of reference if you are standing on the sidewalk and see the bus go by? How can you tell the bus is moving? 

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    You know from experience what motion is, so it may seem like a straightforward concept. Motion can also be defined simply, as a change in position. But if you think about examples of motion in more depth, you’ll find that the idea of motion is not quite as simple and straightforward as it seems. 

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    Running events in track include 100-meter sprints and 2000-meter races. Races are named for their distance. Distance is the length of the route between two points. The length of the route in a race is the distance between the starting and finishing lines. In a 100-meter sprint, for example, the distance is 100 meters. 

    Maps can often be used to measure distance. Look at the map in Figure below. Find Mia’s house and the school. You can use the map key to directly measure the distance between these two points. The distance is 2 kilometers. Measure it yourself to see if you agree. 

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    Things don’t always move in straight lines like the route from Mia’s house to the school. Sometimes they change direction as they move. For example, the route from Mia’s house to the post office changes from west to north at the school (see Figure above). To find the total distance of a route that changes direction, you must add up the distances traveled in each direction. From Mia’s house to the school, for example, the distance is 2 kilometers. From the school to the post office, the distance is 1 kilometer. Therefore, the total distance from Mia’s house to the post office is 3 kilometers. 

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    You Try It!

    Problem: What is the distance from the post office to the park in Figure above

    Direction is just as important as distance in describing motion. For example, if Mia told a friend how to reach the post office from her house, she couldn’t just say, "go 3 kilometers." The friend might end up at the park instead of the post office. Mia would have to be more specific. She could say, "go west for 2 kilometers and then go north for 1 kilometer."  

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    How fast does your dog run? If your dog runs faster than you, his speed is greater. Speed tells you how the position of an object changes during a certain amount of time. Most of the time we use words like "fast" or "slow" to describe speed. If you are fast, you change your position quickly. But we can be more exact if we use numbers or units such as 15 miles per hour or 20 meters per second. To find speed you need to know two things, distance traveled and time it took to travel that distance. The formula for finding speed is distance divided by time. S = D/T.

    For example, if a runner ran 100 meters in 10 seconds his speed would be 100/10 or 10 meters per second.

    Technology helps us figure speed. For example, a police officer uses a radar gun to measure speed instead of calculating that himself.

    Speed tells you only how quickly or slowly something is moving, it does not tell you the direction of the motion.

    Speed Activity: Mark off a distance of at least 20 m. Walk that distance twice measuring the time with a stopwatch. Cover the distance first in 20 seconds and then in 25 seconds. How do you determine how quickly you need to walk?


     Speed and Direction Song


    When you tell both the speed and direction of an object, you are telling its velocity. When describing the direction part of velocity you could use words like "up" or "down" or "left" or "right". Speed could be 25 m/h but velocity would be 25 m/h west.

    When you make a change in velocity you use acceleration. Accerleration is speeding up, slowing down, or changing direction. When a car speeds up after a stop light turns green, that car is accelerating. Also, when the car slows down to stop for a red light that is acceleration as well. If a car turns a corner, that is acceleration because it is changing direction.



    To make acceleration happen, you must use a force. Forces are pushes and pulls and you measure force in newtons (N). Forces change motion. With more force, comes more acceleration. The harder you push a box across the floor, the faster it moves.

    What if there is more than one force acting on an object? Then the forces either work together or against each other.

    If 2 people are pushing on the same box, that means more force is exerted on the box causing it to move faster (more acceleration).

    But if 2 people are pushing against each other the forces cancel each other out and the one with the larger force moves the object.

    In this picture, one force is greater than the other so the book would move 2 N to the left.


    The size of the force affects the accerleration. If you lightly tap a baseball with a bat, it will not leave the in-field or even get past the pitcher. But if you really hit that ball, it will go to the out-field or maybe even out of the ballpark!


     Video about Forces



    Try this. Place your hand on your throat and hum. What do you feel? You probably feel slight movements or vibrations. A quick back-and-forth movement is a vibration. When something vibrates, the air around it vibrates too. The vibrations then move through the air. They travel out in all directions and when they finally reach your ear, you hear them as sounds.



    You can hear sounds through the air, but you can also hear through other materials. You can hear sound through water or walls. Vibrations can travel through liquids, solids and gas mixtures like air.

    The vibrations that carry the sound are called sound waves. When those waves reach your ear they make your eardrum vibrate which sends a signal to your brain so you can hear it. Sounds can be soft or loud, high or low but they are all carried by sound waves.

    What is the difference between a German Shepard's bark and a Chihuahua's? Both bark but one sounds very deep while the other is rather high. Pitch is how high or how low a sound is. The two dogs both bark but in different pitch. What makes that happen?


    A small dog has smaller vocal cords which will vibrate faster and closer together. They make sound waves that are close together. Big objects vibrate slowly and produce waves that are farther apart. When the sound waves reach your ears, the close together waves sound higher than the ones that are farther apart. What other examples can you think of that are sounds in different pitch?

     Bill Nye video about sound