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Section 3-8: Relative Velocity

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1 Section 3-8: Relative Velocity

2 Section 3-8: Relative Velocity
A useful example of vector addition! Example: 2 trains approaching each other (along a line) at 95 km/h each, with respect to the Earth. Observers on either train see the other coming at = 190 km/h. Observer on ground sees  95 km/h.  Velocity depends on reference frame!!

3 Velocities not along the same line
Need to use full vector addition. A common error is adding or subtracting wrong velocities A method to help avoid this is: Proper subscript labeling of velocities CONVENTION: Velocities with 2 subscripts. First = object, O, Second = reference frame, R. vOR

4 Conceptual Example 3-10: Boat Crossing A River
vBS = vBW + vWS Outer subscripts on both sides are the same! Inner subscripts are the same!

5 Can extend this to more than 2 v’s
Suppose, to the previous example, we add a fisherman walking on boat with velocity vFB = velocity of the Fisherman with respect to the Boat: vFS = vFB + vBW + vWS Outer subscripts on both sides are the same! Inner subscripts are the same! Finally: Relative velocities obey: vAB = -vBA

6 Example 3-11

7 Example 3-12

8 Example: Plane with a cross wind
vPA = 200 km/h , N vAG = 100 km/h , from NE (to SW) vPG = vPA + vAG Use the rules of analytic addition: Compute components of vPA & vAG Add these to get components of vPG. Compute the length & angle of vPG


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