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Motion

Motion. Motion. Motion. Section 2.1 – Describing Motion. Motion and Position. A reference point is needed to determine the position of an object. Motion occurs when an object changes position relative to its reference point. Motion depends on its point of reference.

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Motion

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  1. Motion Motion Motion

  2. Section 2.1 – Describing Motion

  3. Motion and Position • A reference point is needed to determine the position of an object. • Motion occurs when an object changes position relative to its reference point. • Motion depends on its point of reference. • Question – are you moving right now?

  4. Distance and Displacement • Distance – how far an object has moved Displacement – the distance and direction of an object’s change in position from the starting point The SI unit for distance and displacement is the meter (m).

  5. Distance and Displacement Example: A runner runs from the start line 50 m north plus 30 m in the opposite direction. What is the distance? What is the displacement? 50 m N 30 m S START

  6. Speed Speed – the distance an object travels per unit of time Average speed – total distance traveled divided by the total time of travel Instantaneous speed – speed at a given point in time Remember to use the metric system! m/s (meters/second) or kilometers/hour km/h

  7. Velocity • Velocity – includes the speed of an object and the direction • of its motion • Because velocity depends on direction as well as speed, • the velocity of an object can change even when its speed • does not! Car is driving 30 km/hr but it’s velocity is constantly changing!

  8. Equation to Find Speed Speed (in meters per second) = distance (in meters) / time (in seconds) s = d/t Units – meters/second (m/s) or kilometers/hour (km/h)

  9. Graphing Motion • Motion can be shown on a distance vs. time graph • Time on x-axis • Distance on y-axis • On a distance vs. time graph the slope describes the speed • Steeper slope = faster speed • Horizontal line = zero speed = object at rest

  10. Section 2.2 - Acceleration

  11. What Is Acceleration? • Acceleration is the rate of change of velocity. • When the velocity of an object changes, the object is accelerating. • A change in velocity can be either a change in how fast something is moving or a change in the direction it is moving. • So acceleration occurs when an object changes its speed, its direction, or both.

  12. Speeding Up and Slowing Down • When you think of acceleration, you probably think of something speeding up. However, an object that is slowing down also is accelerating. • Acceleration also has direction, just as velocity does.

  13. Speeding Up… • If the acceleration is in the same direction as the velocity, the speed increases and the acceleration is positive.

  14. Slowing Down… • If the speed decreases, the acceleration is in the opposite direction from the velocity, and the acceleration is negative.

  15. Changing Direction… • Any time a moving object changes direction, its velocity changes and it is accelerating. • The speed of the horses in this carousel is constant, but the horses are accelerating because their direction is changing constantly.

  16. Calculating Acceleration Vf = final velocity Vi = initial velocity t = time Units = m/s2

  17. Example with Positive Acceleration • Suppose a jet airliner starts at rest at the end of a runway and reaches a speed of 80 m/s in 20 s.

  18. Example with Positive Acceleration • The airliner is speeding up, so the final speed is greater than the initial speed and the acceleration is positive.

  19. Example with Negative Acceleration • Imagine that a skateboarder is moving in a straight line at a constant speed of 3 m/s and comes to a stop in 2 s.

  20. Example with Negative Acceleration • The skateboarder is slowing down, so the final speed is less than the initial speed and the acceleration is negative.

  21. Section 2.3 – Motion and Forces

  22. Force • Force – a push or pull • Sometimes it is obvious a force is being applied and • sometimes it is not • Forces cause the motion of an object to change • Forces are measured in Newtons (N)

  23. Net Force Net force – the sum of the forces acting upon an object

  24. Balanced Forces Balanced forces - occurs when the forces acting upon an object are equal in size but opposite in direction

  25. Unbalanced Forces Unbalanced forces – when unequal forces act on the same object; the object moves in the direction of the larger force

  26. Inertia and Mass • Inertia - tendency of an object to resist any change in its motion • The greater the mass of an object, the greater its inertia. • Think about playing ping pong with a bowling bowl… • what’s the problem? • Newton’s First Law of Motion (aka Law of Inertia) • An object moving at a constant velocity keeps moving at • that velocity unless an unbalanced net force acts upon it. • How does this apply to a car crash? • Why can a seat belt prevent serious injury?

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