You are in a car, and another car passes you. It seems to move slowly. A train that comes the other way seems to go past very fast. Relative motion is how fast one thing moves as seen from another. We learn to work it out. Then we use it to answer the question of every chase: when do they meet?
3 lessons, about 1 h 20 min3 simulations7 problems6 practice questionsPython in every lesson
Created by StemPy LabRelative velocity is how fast one object moves as seen from another. Along a line, the velocity of A relative to B is the velocity of A minus the velocity of B, with the signs kept. For two objects that go the same way, we subtract the speeds, and for objects that go opposite ways, we add them. The relative velocity is zero when both have the same velocity. To change the point of view, we add or subtract the velocity of the new observer.
Write where each object is at any time, and find the time when the two positions are the same.
When the velocities are not along one line, they add and subtract as vectors. Rain that falls straight down seems to slant towards a moving walker. A boat crossing a river moves along the sum of its own velocity through the water and the river's velocity. Heading straight across gives the shortest time; heading upstream lets it land right opposite.
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