Calculate Height of Freely Falling Object - 30m in 1.5s

[kash25]

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 have been working on this for a while and seem to be going wrong somewhere.
I am unclear as to how free fall is meant here. I know that it need not refer to an object that is simply falling and can refer to any object that is experiencing gravity alone. How can I tell whether this object was thrown upward before falling? Does it make a difference?

Thanks.

 

[chislam]

Find the velocity of the object when it is 30m above the ground. Then use this to find out the height above the 30m where the velocity was 0. Just use formulas where you have the most knowns and fewest unknowns.

 

[kash25]

i was doing that but i got different velocities depending on which equation i used...:
velocity(Average)=(vsub30m + vsubground)/2
20 = vsub30m/2
vsub30m=40m/s
if i do the same thing with the equation deltaX = vsub30m*t + .5at^2,
i get something like 12.6 for vsub30m..
where am i going wrong?

 

[alphysicist]

Hi kash25,

i was doing that but i got different velocities depending on which equation i used...:
velocity(Average)=(vsub30m + vsubground)/2
20 = vsub30m/2

This line is not right. You have assumed that vsubground=0; however, vsubground is the impact velocity (the velocity right before it touches the ground) and so is not zero.