Physics question

[dellcom]

ok this is starting to drive me nuts.

 

the question is:

 

"A freely falling object requires 1.50 s to travel the last 30.0 m before it hits the ground. From what height above the ground did it fall?"

 

i am assuming by 'free falling' it has a constant acceleration of 9.8 m/s^2

 

would i just try to find the velocity at 30 m ? then find the point where the acceleration is = 0?

 

this is probably simpler then i am making it out to be but any help would be appreciated.

[Jay-qu]

You are right to assume this is a constant acceleration question.

 

In these problems there are 5 variable - acceleration, starting velocity, final velocity, displacement and time. Normally it works out that you are given three of these and you have to find the forth.

 

Your infomation can be paraphrased as "it takes an object 1.5 seconds to fall 30m under acceleration of 9.8m/s/s"

 

This info can be used to find either the velocity at a hieght of 30m (:hihi:) or when its about to hit the ground (useless..)

 

From there you now have another set of infomation. An object is dropped from rest, accelerates at 9.8m/s/s and has final velocity of v (from above). These 3 bits of infomation can be then used to find a forth bit (perhaps the distance it fell ;) )

 

I like to help people solve the problems rather than just giving the answer, so let us know how you go with that and if you need more help then feel free to ask :)

 

J

[dellcom]

ok so here is what i came up with..

 

The Initial Velocity when the object is at 30 m = (Xf - Xi - .5(a)t^2) / t = (0 - 30m -.5(9.8m/s/s)(1.5s)^2)/(1.5s)

Vxi = -12.7 m/s (over the interval of 30m - 0m)

then Vxf = -12.7 m/s + (-9.8 m/s/s) * 1.5s

Vxf = -27.35 m/s

now i can use that Vxi as the Vxf velocity between the start height and 30m to find the original position.

 

Xi = - (Vxf^2 - Vxi^2)/2a + Xf

 

Xi = (-27.35^2 m/s - 0^2 m/s)/2(9.8m/s/s) + 30m

Xi = 38.2 m

 

So the final answer should be the object fell from a height of 38.2 meters?

[Jay-qu]

So the final answer should be the object fell from a height of 38.2 meters?

 

Exactly what I got :hihi:

 

Except that you may want to quote your final answer to 2 significant figures since that is the allowed accuracy from your initial information.

 

J

[dellcom]

Exactly what I got :hihi:

 

Except that you may want to quote your final answer to 2 significant figures since that is the allowed accuracy from your initial information.

 

J

 

horray lol thx for you help

 

hmm the question gives 3 significant figures, are you referring to me using 9.8m/s/s for gravity for the 2 significant figures?

[sanctus]

Actually you should use 9.81 to get the significant figures right