## AravindG 4 years ago A boy standing on a stationary lift (open from above) throws a ball upwards with the maximum initial speed he can, equal to 49 m/s. How much time does the ball take to return to his hands? If the lift starts moving up with a uniform speed of 5 m/s and the boy again throws the ball up with the maximum speed he can, how long does the ball take to return to his hands?

1. AravindG

u see i got first part

2. AravindG

10 s

3. AravindG

but i dont undestand y we get same answr in 2nd case

4. JamesJ

The position of the ball at time time is $y(t) = 49 t - \frac{1}{2}gt^2$ $= 49 t - 4.9t^2$ $= 0$ when $$t = 0, 10$$ seconds, right. Now in the second case, the speed of the ball is (49 + 5) m/s = 54 m/s, so it's position is $y_{ball}(t) = 54 t - 4.9t^2$ ... but ...

5. JamesJ

the position of the boy is $y_{boy}(t) = 5t$ Hence the question is: for what t is $y_{ball}(t) = y_{boy}(t)$ Solve that equation and you'll get back $$t = 0, 10$$ seconds.

6. AravindG

hw u got pos of boy as 5t??

7. JamesJ

because the lift is moving up at a constant velocity of 5 m/s

8. AravindG

wow i got the old eqn

9. AravindG

:)

10. JamesJ

right, exactly

11. AravindG

thx

12. AravindG

so shall i make it general

13. AravindG

that if someone is in a lift and the lift moves with const speed then time to throw and catch the balll is same as when lift is at rest??

14. JamesJ

Yes.

15. AravindG

k

16. AravindG

wt if it was not const speed?

17. JamesJ

Then you won't have equality.

18. JamesJ

Frames of reference moving at a constant speed are called inertial frames. The laws of Newtonian physics work the same in all inertial frames.

19. AravindG

u mean any system with a=0 is an inertial frame?

20. JamesJ

Yes

21. AravindG

wow

22. AravindG

wel james hav u seen a type of problem in which a rope is given and we can slide down that and the breaking tension is given and max a with which we can slide down??

23. JamesJ

Yes, but I'm going to go now. I'm sure gogind or jemurray or others can help you with that.

24. AravindG

oh bye