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CBSE Class 11-science Answered

Q1)A plank is moving on a ground with a velocity v and a block is moving on the plank with a velocity u. What is the velocity of block wrt ground- v-u towards right, v-u towards left, u towards right Q2) A man can swim in still water with a speed of 2m/s. If he wants to cross a river flowing with a speed root3 m/s along the shortest possible path, then in which direction should he swim-120 deg to the water current, 150 deg to the water current, 90 deg to the water current Q3)A train 150 m length is going towards north direction at a speed of 10m/s.a birdflies at a speed of 5m/s towards south direction parallel to the railway track. The time taken by the bird to cross the train is equal to -12s, 8 s, 15s,10s
Asked by Agrima | 15 Jun, 2013, 10:37: AM
answered-by-expert Expert Answer
Please ask 1 question/ query. So, here is the answer to your first question 
 
1. In this question, it is not clear if the given velocity of the block i.e. u is with respect to the plank or ground. If it is with respect to ground, then you don't need to solve anything, but the answer would be u itself (option 3)
 
However, if the given velocity of block is with respect to plank, then 
velocity of block wrt ground = velocity of block wrt plank + velocity of plank wrt ground
velocity of block wrt ground = v+ u towards the same direction as the plank is moving. 
 
In that case, none of the options you listed would be correct. 
Answered by | 15 Jun, 2013, 05:21: PM

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