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8.1 The Language of Motion Some common words used to describe motion include: Distance Time Speed Position How would you describe the motion of the soccer ball before and after it is kicked? What key words did you use when describing this situation?

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**Direction Makes a Difference**

Two main types of quantities: Scalars: Describe magnitude but not direction. (Magnitude is the size of a measurement) Example: Johnny walked 25 km Vectors: Describe magnitude and direction. Example: Johnny walked 25 km North Every time you use a map or give directions, you are using vectors.

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Vectors vs. Scalars You can always tell if a quantity is a vector because there will be an arrow drawn above it. Example: A scalar has no arrow.

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**Distance vs. Displacement**

Distance (d) is a scalar that tells you how far something has travelled. Example: Johnny ran a distance of 400 m Displacement (d) is a vector that describes your position relative to where you started. Example: Johnny ran 400 m North of his home. We measure both distance and displacement in metres (m).

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Example A car leaves home and drives 10 km to the store and then returns home. The car has driven a total distance of 20 km but its final displacement is 0 km.

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**Time Interval ti = initial time**

Time interval or change in time is calculated by: Where: Δt = change in time (the Δ symbol is the greek letter delta. It means “change”. ti = initial time tf = final time

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Example The time interval to move from the fire hydrant to the sign is calculated by:

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**Displacement Displacement or change in position is calculated by:**

Where: = change in position or displacement = initial position = final position

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**Displacement and Distance**

Between 2 s and 5 s, the skateboarder’s: displacement is 5 m [E] and distance travelled is 5 m.

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**Watch for Signs Turn to page 349 for common sign conventions**

Copy Figure 8.8 into notes Turn to page 352 in textbook and do Activity 8-1B

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Uniform Motion Uniform motion is a term that describes objects that do not speed up, slow down, or change direction. In other words, they travel at constant velocities (we will discuss velocity more later)

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Example The position of the ball in this photo is shown at equal time intervals. How would you determine if this motion is uniform motion? What would the picture look like if the ball was NOT in uniform motion?

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**Graphing Uniform Motion**

Motion of an object can be analyzed by drawing a position-time graph. A position-time graph plots position data on the vertical axis (y-axis) and time data on the horizontal axis (x-axis).

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**Uniform motion is represented by a straight line on a position-time graph.**

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**Positive Slope Slants up to the right.**

Indicates an object travelling in the positive direction (ie: North, East, to the right, up, etc.)

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Zero Slope Horizontal line. Indicates that the object is stationary.

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**Negative Slope Slants down to the right.**

Indicates an object travelling in the negative direction (ie: South, West, to the left, down, etc.)

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