Question

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?

Answer 1

u see i got first part

Answer 2

10 s

Answer 3

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

Answer 4

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 ...

Answer 5

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.

Answer 6

hw u got pos of boy as 5t??

Answer 7

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

Answer 8

wow i got the old eqn

Answer 9

:)

Answer 10

right, exactly

Answer 11

thx

Answer 12

so shall i make it general

Answer 13

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??

Answer 14

Yes.

Answer 15

k

Answer 16

wt if it was not const speed?

Answer 17

Then you won't have equality.

Answer 18

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

Answer 19

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

Answer 20

Yes

Answer 21

wow

Answer 22

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??

Answer 23

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

Answer 24

oh bye