velocity can be negative, SPEED can not. speed is the abs val of V.
So a ball is dropped from a roof, passing a window 1.2 m tall. It takes 1.25s to pass the said window. It takes 1 more second below the window before it hits the ground. How tall is the building?
, where is the starting velocity, is the distance, is the time, and is the acceleration.
Therefore, , or
for the starting velocity when it passes the top of the window. But this results in -5.165, which isn't possible unless down is taken to be negative, which it isn't.
I completed the problem without realizing this, only substituting a variable, z, for this number.
Another formula gave me the time taken to travel the distance above the window: , where, this time, v is the starting speed when it was dropped (0).
Then we add all the times together to find the total fall time, , which, for simplicity, we will call b.
Then the last formula we use is , which i think is the same one we used earlier. this gives us
but it's supposed to be 20.4
thanks to anyone who helps!
You make a good point--if it was dropped from a higher point (the roof) then it must be going faster, and thus take less time to fall.
For it to take more time, it must be going slower, thus justifying the negative velocity.
Well, it may be a typo in the book. if so, it wasn't thee first