2. Are we moving right now?

3. You BET! 460 m/s 1670 km/hr 1070 miles per hour!!

4. In a train, looking out….

5. Outside a train, looking in.

6. Why it can be important June 1, 2009 – Air France flight 447 disappeared without warning All 232 people on board were killed. Disaster was triggered because pilots were not able to accurately determine their wind speed relative to the air. Measurement instrument, called a Pitot tube, iced up and did not give accurate readings. Planes fly high, in less dense air, so narrow range of safe speeds.

7. “True Ground Speed” Measured by GPS Air Speed Measured by Pitot Tube

12. Possible Application…

13. Deeper Understanding…

15. Addition of velocities to describe relative motion Since absolute motion cannot be unambiguously measured, the only way to describe motion unambiguously is to describe the motion of one object relative to another. Symbolically, we can write v PQ for the velocity of object P relative to object Q. Velocities measured with respect to different reference points can be compared by addition. In the figure below, the ball's velocity relative to the couch equals the ball's velocity relative to the truck plus the truck's velocity relative to the couch: v BC =v BT +v TC =5cm/s+10cm/s =15cm/s The same equation can be used for any combination of three objects, just by substituting the relevant subscripts for B, T, and C. Just remember to write the equation so that the velocities being added have the same subscript twice in a row. In this example, if you read off the subscripts going from left to right, you get BC...=... BTTC. The fact that the two "inside" subscripts on the right are the same means that the equation has been set up correctly. Notice how subscripts on the left look just like the subscripts on the right, but with the two T's eliminated. Figure y: These two highly competent physicists disagree on absolute velocities, but they would agree on relative velocities. Purple Dino considers the couch to be at rest, while Green Dino thinks of the truck as being at rest. They agree, however, that the truck's velocity relative to the couch is v TC =10 cm/s, the ball's velocity relative to the truck is v BT =5 cm/s, and the ball's velocity relative to the couch is v BC = v BT + v TC = 15 cm/s.