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Excellent! \(v_{\text{AB}} = -v_{\text{BA}}\). As \(v_{\text{CE}} = 30\), \(v_{\text{EC}} = -30\)
If you have more than two objects, there is an equation that relates all the relative velocities: \(v_{\text{AB}} = v_{\text{AC}} + v_{\text{CB}}\).
If Car X is moving at \(40 \, \mathrm{km/h}\) wrt the Earth and Car Y is moving at \(30 \, \mathrm{km/h}\) wrt the Earth, what is the velocity of Car X wrt Car Y. That is, what is \(v_{\text{XY}}\) in \(\mathrm{km/h}\)?
  • \(40 + 30\)
  • \((-40) + (-30)\)
  • \(40 + (-30)\)
  • \((-40) + 30\)