MediumPhysicsHigh School (11–12)Mechanics › Relative MotionMechanics › Motion in a Straight LineNumerical
A goods train 240 m long runs at 15 m/s on one track.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A goods train long runs at on one track. An express long runs at on the next, parallel track.
- If they move in the same direction, how long does the express take to pass the goods train completely?
- If they move in opposite directions, how long do they take to pass each other?
Need a reminder? The key ideaShow
Along a line, the velocity of A relative to B is
Same direction: the difference of the speeds. Opposite directions: their sum.
For two trains to pass completely, the distance to gain is the sum of their lengths.
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. Distance. .
Step 2. Same direction. Relative speed : .
Step 3. Opposite directions. Relative speed : .
Answer: (a) . (b) .
Check. In the express goes and the goods train : a gain of .
Watch out. One length alone is the usual slip: the trains must clear each other end to end.
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