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Relative Motion

Relative Motion. By: Charlene Duffett & Monica White. Definitions. Relative Motion: Motion of one object with respect to another. Reference Frame: A frame of reference is a place from with motion is observed. The description of any motion depends on the frame of reference of the observer. .

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Relative Motion

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  1. Relative Motion By: Charlene Duffett & Monica White

  2. Definitions • Relative Motion: Motion of one object with respect to another. • Reference Frame: A frame of reference is a place from with motion is observed. The description of any motion depends on the frame of reference of the observer.

  3. Reference Frames and Relative Motion Different reference frames give different relative motions. Example: If you are going 100 km/h, someone standing on the side of the road would see it differently than someone following you going at a different speed.

  4. Relative Displacement • There are two methods to calculating relative displacement: • In your head which is used for simple problems where the objects are moving in the same direction. • Vector algebra method which is used for more complex problems.

  5. Using Vector Algebra Method • The variables used in the formula for these problems are: • odm The displacement of the object with respect to the medium • mde The displacement of the medium with respect to the earth • ode The displacement of the object with respect to the earth

  6. Continued • When setting up your formula you want the variable that you aren’t looking for to cancel out in the middle. You also want the variable of what you are looking for to be on the outside of each variable. • Example: ode=odm + mde

  7. Example #1 • An ant travels 20 cm to the right on a plate relative to the plate as a disgusted picnicker pushes the plate a distance of 60 cm to the left relative to the earth. What is the displacement of the ant with respect to the earth?

  8. Example #2 • An ant travels 20.0 cm [W] on a plate as a disgusted picnicker pushes the plate a distance of 60.0 cm [W 30.0° S]. What is the displacement of the ant with respect to the earth?

  9. Relative Velocity Vectors in One Dimension • The formula for velocity vectors is the same as the formula for the displacement except you use “v” as the main variable instead of “d”. • Example: ove=ovm + mve

  10. Example #1 • A train moves at 30 km/h [W]. A passenger on the train moves toward the front of the train at a velocity of 5 km/h. What is the passenger’s velocity relative to a person standing on the ground?

  11. Example #2 • A goose is flying 20 km/h [N]. A hunter is driving south at 40 km/h. Determine the velocity of the goose with respect to the hunter.

  12. Relative Velocity Vectors in Two Dimensions • These problems are just like the one dimension problems except you need to draw triangles and then use Pythagorean Theorem and Trigonometry to solve the problems.

  13. Example #1 • A swimmer jumps into a river and swims for the opposite shore. Her velocity in still water is 5.0 km/h [S]. The current in the river is 4.0 km/h [W]. Find the swimmer’s velocity relative to the shore.

  14. Example #2 • A swimmer is able to swim 5 km/h in still water. She is heading [W 30° S] in a river with a current flowing north at 3 km/h. • What is the velocity of the swimmer relative to the shore? • How long does it take her to cross the river, if the river is 1500 m wide? • How far down stream does she land?

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