EasyPhysicsHigh School (11–12)Mechanics › Relative MotionMechanics › Motion in a Straight LineConceptual
When is the relative velocity of two moving objects exactly zero?
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
- When is the relative velocity of two moving objects exactly zero?
- Two runners each run at , one east and one west. What is their relative speed?
Need a reminder? The key ideaShow
Along a line, , with signs. The same way: subtract the speeds. Opposite ways: add them. Time to meet gap closing speed. To change km/h to m/s, multiply by .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) Only when the two have the same velocity: the same speed and the same direction. Each then seems to stand still to the other.
Step 2. (b) .
Answer: when the velocities are equal; .
Check. Equal speeds are not enough: the same speeds in opposite directions give the largest relative speed.
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