A stone is dropped from the roof of a tall building. A person measures the speed of the stone to be 49 m/sec when it hits the ground. The height of the building is closest to:
Select one:
a. 24 meters.
b. 49 meters.
c. 122 meters.
d. 245 meters.

- omnomnom

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- anonymous

\[mgh = \frac{ 1 }{ 2 }mv ^{2}\]

- anonymous

you can subtract the mass from both sides which shows that everything falls at the same rate

- anonymous

you know what g is

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## More answers

- omnomnom

Gravity?

- anonymous

yes

- anonymous

so if you plug in the velovity and solve for h which is the height

- anonymous

is this for conservation of energy

- omnomnom

No this is only for the height of the building

- anonymous

The formula I gave you is the conservation of energy

- anonymous

In this case \(g\) is the gravitational acceleration we talked about in the previous problem.

- anonymous

You cannot solve this with kinematics there are too many unknowns

- anonymous

What do you mean you can't solve it with kinematics?

- anonymous

Yeah I guess you could solve for time in the acceleration then plug it in the average velocity.

- anonymous

I was talking about the \(g\) in your formula.

- omnomnom

??

- anonymous

Though technically speaking, depending on whether they are on the 'energy' part or the 'kinematic' part of the course decides which method they should use.

- anonymous

that is why I was asking

- anonymous

Last question was a 'kinematic' one so maybe it must be kinematics.

- anonymous

acceleration of gravity = velocity final minus velocity inital divided by time.

- anonymous

@omnomnom Have you talked about work or energy in your class yet?

- anonymous

Like potential energy or kinetic energy?

- omnomnom

No whats work?

- anonymous

It is something you will learn in the future, most likely.

- anonymous

yes solve for time then plug into \[\frac{ d }{ t }=\frac{ v _{f}+v _{i} }{ 2 }\]

- anonymous

conservation is so much easier to solve

- omnomnom

it confused me though
the first equstion

- omnomnom

equation*

- anonymous

Don't worry about it.

- anonymous

@omnomnom This is a two step problem. First you want to figure out how long it took to fall. Then you want to use that to figure out how far it fell.

- omnomnom

oh okay so what equation do i use.... is is\[d =Vi( t)+ a \frac{ 1 }{ 2 }(t^2) \]

- anonymous

You can't use that equation until you find time though.

- omnomnom

Oh yeah >.<

- anonymous

\[a=\frac{ v _{f}-v _{i} }{ t }\]

- anonymous

you know a and both velocities

- omnomnom

But we need the time :( Cant i just quess the answer?

- anonymous

Yes, \(a\) is the same as last time... \(9.81m/s^2\)

- anonymous

velocity initial is 0

- anonymous

You are not supposed to guess, because there is already a way to find the answer.

- omnomnom

but...... Whats the way ?

- anonymous

t=49/9.81

- omnomnom

answer is 5

- anonymous

yes 5 seconds

- anonymous

then plug in the time into your equation and find x

- omnomnom

oh hold on

- omnomnom

i got 49 m/s

- anonymous

that is the velocity

- omnomnom

so we change the equation now?

- anonymous

you want the height of the building

- omnomnom

yeah

- anonymous

x=1/2t(vfinal-vinital)

- omnomnom

122.5 meters?

- anonymous

yes

- omnomnom

thank you :D

- anonymous

do you understand the kinematic equations

- omnomnom

The first one ? NO the others Yes

- anonymous

http://www.physicsclassroom.com/

- anonymous

this is a helpfull tool

- omnomnom

oh kay thank you :D

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