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`        Q .1- A projectile projected at some angle with vel. 50m/s crosses a 20m wall after 4s from the time of projection. The angle of projection of particle will be ? ( pls give full sol.)                                                                                   2. a swimmer can swim in still water at 1.5m/s , the river is flowing with 2m/s from left to right. The minimum time taken by swimmer to cross the river and drift will be? ( pls full sol. )`
5 years ago Abhishek Doyal
25 Points
```							Q2 please tell me the width of riverAbhishek doyalIIT BombayAskiitian tutor
```
5 years ago Abhishek Doyal
25 Points
```							Abhishek doyalIIT bombayAskiitian tutor
```
5 years ago
```							qn 1:	We can solve almost every projectile question using just the basic kinematics formulas.Let us take the sign convention for this problem as g (acceleration due to gravity) as -10m/s^2 in the upward direction (+y axis) and +10m/s^2 in the downward direction (-y axis).Let initial vel be UU=50m/s (given)Let time be t t=4 sec (given)let x be the angle of projection which we have to find.Here the 20 m wall is the vertical distance covered by the projectile.The range of the projectile would be the horizontal distance covered by it but we dont need to worry about it here.We can split ‘U’ into its rectangular componentsthe vertical displacement 20 m of the projectile is due to only its vertical component of U (Uy) which is Usinx.=50sinxy(vertical displacement)=Uyt+1/2(-g)(t)^2                                     =Uyt-1/2(g)(t)^2     20=50sinx-1/2(10)(4)^2=     20=50sinx-(5)(16)=      20=50sinx-80=      50sinx=100=       sinx=1/2=        x=30 degreesHope this helps!!!!
```
5 years ago
```							Small correction. The procedure and the final answer is the same but i made a calculation mistake in betweeni missed the t there20=50sinx * 4- 5*16=200sinx-80100=200sinxsinx=1/2hence x=30 degrees(in my previous calculations we are getting sinx=100/50 =2 which is obviously wrong)sorry about that
```
5 years ago Arun Kumar
IIT Delhi
256 Points
```							HelloThanks & RegardsArun KumarBtech, IIT DelhiAskiitians Faculty
```
5 years ago
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