I keep getting “Relative velocity” questions wrong in Kinematics. What's the trap?
adding speeds as plain numbers instead of subtracting vectors.
The usual trap: adding speeds as plain numbers instead of subtracting vectors. How to avoid it: for opposite directions the relative speed adds; for the same direction it subtracts. Go back to the definition — the velocity of A as seen from B equals vector v(A) − v(B).
Key point
Relative velocity
Common mistake
adding speeds as plain numbers instead of subtracting vectors.
Memory tip
for opposite directions the relative speed adds; for the same direction it subtracts.
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