Mtr Parts
a boy standing in a stationary lift, throws a ball up with a velocity of 49 mtr/sec. find time of flight?
my teacher solved using s=ut-1/2at sqare. how can we take “a” as -9.8mtr/sec. when the ball goes up a=-g= -9.8mtr/sec. but when it returns, it is a free falling body and a=g= +9.8mtr/sec. So how one can use the eqation s=ut-1/2at sqare to solve this problem? Please help.
Second part :- if the lift moves up with a velocity of 5 mtr/sec and the boy again throws the ball up exactly as before. how long will it take for the ball to raech his hand again?
You can’t use s = ut + 1/2at^2. For one thing, you do not know s, and you would need to to solve for t.
The up bit is easy. Its just the time taken for the ball to come to rest. You can see this is just 49m/s divided by g – ie t = v/g.
Now you could just appeal to symmetry to say the same must apply on the return trip. Substitute and you do indeed get s = 1/2 gt^2 = v^2/2g or t = v/g.
Hence the total trip takes 2v/g = 10secs.
Now for the second bit.
It is an axiom of physics that inertial frames are indistinguishable. As the lift moves at constant speed it is inertial and so the result *must* be exactly the same.
You can see why this is so because from outside the lift, the boy now throws the ball up with speed 54 m/s but the lift rises to meet it on the way down. The two compensate each other.
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