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Physics Tools Physics for the Life Sciences Lecture #1C Racquetball Striking a Wall Mt. Etna Stewart Hall
A Nice Way to Use Vectors •Relative velocity: sometimes a velocity is ‘made up’ of several parts: you walk through a train while it moves over the track •Your total velocity is a sum of several components vperson-ground=vperson-train+vtrain-ground!vperson-trainvtrain-groundvtrain-ground!vperson-trainvperson-groundyxzvPG=vPT+vTGOR Note: !vPG=−!vGP,!vPT=−!vTP, and !vTG=−!vGT
In Two Dimensions It’s the Same •Imagine a dog swimming across a river •The dog swims at 1.2 [m/s] relative to the water •The water flows left at 1.2 [m/s] •If the dog “heads straight across” (swims relative to the waterwith his nose pointing to the opposite shore), it travels at 45º relative to the shore. vdog-watervdog-shorevwater-shorevdog-watervwater-shorevdog-shore=vdog-water+vwater-shoreor vDS=vDW+vWSyxz
•Imagine a dog swimming across a river oThe dog can swim at 2 [m/s] relative to the water oThe water flows West at 1 [m/s] relative to the shore •What direction must the dog swim to end up directly across the river as shown?