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Physics

Physics. Describing Motion: Velocity. Introduction. Everything around us is in motion. People walk, run, ride bikes, or drive. The earth rotates and orbits the sun. The sun is also moving around the Milky Way Galaxy which is also moving through space.

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Physics

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  1. Physics Describing Motion: Velocity

  2. Introduction • Everything around us is in motion. People walk, run, ride bikes, or drive. The earth rotates and orbits the sun. • The sun is also moving around the Milky Way Galaxy which is also moving through space. • Even the particles of atoms are moving. • Since motion is common to everything in the universe, it is essential to study how it works.

  3. Three Ways to Describe Motion • Language—Common words can describe motion in sentences. • Mathematical equations. • Graphs show how quantities involving motion change over time.

  4. Frame of Reference: Motion is relative to where you are in relation to the object in motion Reference Point: A point to begin in making a measurement (the zero point) Position: the separation between the object and the reference point Distance: how far something has moved Position and DistanceKey Terms

  5. Scalar: a quantity having only magnitude. Ex: distance Vector: a quantity having both magnitude and direction. Ex: displacement or placement w Scalar VS Vector

  6. Examples of Scalars and Vectors

  7. Average Velocity • Key Terms: • Instantaneous position: the position of an object at any given time, t • Displacement: The change in position of an object • Average Velocity is the ratio of the change in displacement and the change in time

  8. Example Problem • In the 1988 Summer Olympics, Florence Griffith-Joyner won the 100-m race in 10.54 s. Assuming the length of the race is measured to 0.1 m, find her average velocity in m/s and km/h. • Dd =+100.0 m 100.0 m/10.54 s • Dt = 10.54 s = 9.488 m/s or 9.5 m/s 9.488 m/s(3600s/h)/(1000 m/km) = 34.2 km/h

  9. Position-Time Graphs • A graph that shows how position depends on time is called a position-time graph. • Time is the independent variable and is graphed on the x-axis. • Position is the dependent variable and is graphed on the y-axis. • The slope of the graph is the average velocity.

  10. Constant Velocity=Uniform Velocity • Constant velocity—the average velocity of an object is the same for all time intervals • For a position-time graph, constant velocity is a straight line equal to the slope.

  11. Slope Review • Slope is defined as rise over run. • The rise refers to the numbers of the vertical axis; the run refers to the numbers of the x-axis. • The slope, m, in the equation: Y = mx + b, is found by dividing the change in y by the change in x. • The slope of a position-time graph is velocity.

  12. Positive and Negative Velocities • Positions can be positive or negative. Positive is to the right of a reference point and negative is to the left of a reference point. • Time intervals, however, are always positive. • Velocity can be either positive or negative depending on if the position is positive or negative.

  13. Instantaneous Velocity • To find the instantaneous velocity, draw a straight line tangent to that point. The slope of the tangent line is the instantaneous velocity.

  14. Velocity-time Graph • A velocity-time graph is used to describe motion with either constant or changing velocity. • For constant velocity, the graphed line is parallel to the x-axis.

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