1. Chapter 3-4: Relative Motion

2. What is Relative Motion?
Relative motion is the motion of an aunt, uncle, cousin, or some other family member, in respect to the current location of their nearest family member. But seriously, Relative motion is the calculation of an object with regard to some other moving object.

3. Examples of Relative Motion
One car passing another car on the highway.
A canoe being rowed across a flowing river.
A plane being flown into the wind or with a wind behind it.

4. Motion is Relative

5. Relative Motion
Long distance commercial passenger planes normally fly at a speed of km/h (via wikipedia: Cruise(aeronautics)).

7. Relative Motion Notation
Subscripts (little letters after a big letter) are used to describe what frame of reference is being used. For example, if we are looking at the motion of a car compared to the Earth: vce: velocity of the car relative to the Earth vec: velocity of the Earth relative to the car

8. Example 1
A car travelling at 80.0 km/h is passed by a car travelling at 90.0 km/h. What is the velocity of the fast car as perceived by the slower car?
vfs = vfe + ves
(the subscript that ended the first variable will start the second on the right hand side of the equation)
vfs = vfe + (-vse)
If you flip the subscript, make the term negative
vfs = 90.0km/h + (-80.0km/h) = 10.0 km/h

9. Conceptual Question
A man is standing on a train that is travelling with a velocity of 15.0 m/s. He throws a baseball in the opposite direction of the trains motion with a velocity of 15.0 m/s. What is the velocity of the baseball in respect to the Earth?

10. Example 2 – The Riverboat Problem
A boat heading north crosses a wide river with a velocity of m/s relative to the water. The river has a uniform velocity of 5.00 m/s due east. Determine the boat’s velocity with respect to an observer at shore. (11.2m/s, 26.6°)

11. Example 3 – Another Riverboat Problem
A motorboat is travelling at 4.00m/s east encounters a current at 3.00 m/s north.
What is the resultant velocity of the motorboat?
If the width of the river is 80. meters wide, then how much time does it take the boat to travel to shore?
What distance downstream does the boat reach the opposite shore?

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19. A boat travels with a resultant velocity of 10
A boat travels with a resultant velocity of 10.m/s 37 degrees north of east. If the river is 36 m wide:
What is the horizontal displacement of the boat when it reaches the other side?
How long did it take the boat to make it across the river?

20. Pick a Question!