Relative velocity
- Two cities X and Y are connected by a regular bus service with a bus leaving in either direction every T min. A girl is driving scooty with a speed of 60 km/h in the direction X to Y notices that a bus goes past her every 30 minutes in the direction… Answer: 4. 15 min, 120 km/h Key idea: buses in one direction are spaced vT apart. The girl meets them at the rate set by the relative speed.... 1
- The speed of a swimmer in still water is 20 m/s. The speed of river water is 10 m/s and is flowing due east. If he is standing on the south bank and wishes to cross the river along the shortest path, the angle at which he should make his strokes w.r.… Answer: 1. 30^° west To cross along the shortest path (straight north), the swimmer's velocity component along the river flow must cancel th... 1
- Preeti reached the metro station and found that the escalator was not working. She walked up the stationary escalator in time t_1. On other days, if she remains stationary on the moving escalator, then the escalator takes her up in time t_2. The time… Answer: 2. t₁t₂/t₂+t₁ Key idea: add speeds. Let the stair length be L . Her walking speed is L/t₁ , escalator speed is L/t₂ . Combined speed... 1