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**Describing and Measuring Motion**

Physical Science #1

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**Vocabulary Average Speed Instantaneous Speed**

International System of Units Meter Motion Reference Point Slope Speed Velocity

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Describing Motion Deciding if an object is moving is not as easy as you might think. An object is in motion if its distance from another object is changing.

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Reference Point A reference point is a place or objet used for comparison to determine if something is in motion. An object is in motion if it changes position relative to a reference point. Objects that are stationary – such as a tree, sign or building – make good reference points.

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**Relative Motion Are you moving right now?**

The answer to that question depends on your reference point. If you choose your chair, then no, you are not moving. If you choose the sun, then yes, you are moving with the Earth.

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Measuring Distance You can use units of measurement to describe motion precisely. Scientists use the same units of measurement, called the International System of Units (SI). The SI unit of length is the meter.

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**Measuring Distance Millimeter = 1000th of a meter**

Centimeter = 100th of a meter Decimeter = 10th of a meter Meter Decameter = 10 meters Hectameter = 100 meters Kilometer = 1000 meters

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Calculating Speed A measurement of distance can tell you how far an object travels. If you know the distance an object travels in a certain amount of time, you can calculate the speed of an object. Speed is a type of rate.

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**The Speed Equation Speed = Distance Time**

The speed equation consists of a unit of distance divided by a unit of time. If you measure distance in meters and time in seconds, your speed would be in meters per second (m/s).

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**Average Speed The speed of most moving objects is not constant.**

Although speed is not always constant, you can find the average speed of a trip by dividing the total distance traveled by the total time.

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Instantaneous Speed Instantaneous speed is the rate at which an object is moving at a given instant in time. How does the average speed compare to the instantaneous speed? ____________________________________________________________________

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Describing Velocity To describe an object’s motion completely, you need to know the direction of its motion. When you know both the speed and direction of an object’s motion, you know the velocity of an object.

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Graphing Motion You can show the motion of an object on a line graph in which you plot distance vs. time. Time is on the x axis Distance is on the y axis The steepness of a line on a graph is called the slope.

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Calculating Slope The slope tells you how fast one variable changes in relation to the other variable in the graph. The steeper the slope, the greater the speed. Slope is calculated by dividing the rise by the run. Slope = Rise Run

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Different Slopes Since moving objects do not travel at a constant speed, sometimes lines will have different slopes. The steepness in each segment tells you how fast the person went.

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**Homework Questions a. How do you know if an object is moving?**

a. What is speed? c. What is the difference between speed and velocity? 3. a. What does the slope of a distance vs time graph show you about the motion of an object?

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Calculation Homework This week at practice, Jamie swam a total of 1,500 meters, while Ellie swam 1.6 kilometers. Convert Ellie’s distance to meters. Who swam the greater distance? How many kilometers did Jamie swim?

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