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Chapter 3

Chapter 3 . 3-4 Relative Motion. Objectives . Describe situations in terms of frame of reference. Solve problems involving relative velocity. Relative Motion . What is relative motion?

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Chapter 3

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  1. Chapter 3 3-4 Relative Motion

  2. Objectives • Describe situations in terms of frame of reference. • Solve problems involving relative velocity.

  3. Relative Motion • What is relative motion? • The laws of physics which apply when you are at rest on the earth also apply when you are in any reference frame which is moving at a constant velocity with respect to the earth. For example, you can toss and catch a ball in a moving bus if the motion is in a straight line at constant speed. • The motion may have a different appearance as viewed from a different reference frame, but this can be explained by including the relative velocity of the reference frame in the description of the motion.

  4. Relative Velocity • What is relative velocity? • One must take into account relative velocities to describe the motion of an airplane in the wind or a boat in a current. Assessing velocities involves vector addition and a useful approach to such relative velocity problems is to think of one reference frame as an "intermediate" reference frame in the form:

  5. Relative velocity • Put into words, the velocity of A with respect to C is equal to the velocity of A with respect to B plus the velocity of B with respect to C. Reference frame B is the intermediate reference frame. This approach can be used with the airplane or boat examples.

  6. Relative motion and velocity • Velocity of the moving objects with respect to other moving or stationary object is called “relative velocity” and this motion is called “relative motion”.

  7. Example#1 • A boat heading north crosses a wide river with a velocity of 10km/h relative to the water. • The river has a uniform velocity of 50 km/h due east .Determine the boat’s velocity with respect to an observer on shore.

  8. Example#2 • A river flows due east at 1.50 m/s. a boat crosses the river from the south shore to the north shore by maintaining a constant velocity of 10 m/s due north relative to the water. • A. What is the velocity of the boat as viewed by an observer on shore? • B. if the river is 325 m wide , how far downstream the boat when it reaches the north shore?

  9. Example#3 • The pilot of an aircraft whishes to fly due west in a 50 km/h wind blowing toward the south. • The speed of the aircraft in the absence of wind is 205 km/h. • In what direction should the aircraft head? • What should its speed relative to the ground be?

  10. Example#4 • A swimmer can swim in still water at a speed of 9.50 m/s . He intended to swim directly across a river that has a downstream current of 3.75 m/s • A. What is the swimmer’s direction? • B. What is his velocity relative to the bank?

  11. Student guided practice • Lets work on worksheet problems

  12. Homework!!! • Do problems 1-4 from page 103

  13. closure • Today we saw about relative motion and relative speed and how we can solve for them. • Next class we are going to start chapter 4

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