omnomnom
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.
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Decart
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\[mgh = \frac{ 1 }{ 2 }mv ^{2}\]
Decart
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you can subtract the mass from both sides which shows that everything falls at the same rate
Decart
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you know what g is
omnomnom
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Gravity?
Decart
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yes
Decart
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so if you plug in the velovity and solve for h which is the height
Decart
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is this for conservation of energy
omnomnom
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No this is only for the height of the building
Decart
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The formula I gave you is the conservation of energy
wio
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In this case \(g\) is the gravitational acceleration we talked about in the previous problem.
Decart
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You cannot solve this with kinematics there are too many unknowns
wio
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What do you mean you can't solve it with kinematics?
Decart
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Yeah I guess you could solve for time in the acceleration then plug it in the average velocity.
wio
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I was talking about the \(g\) in your formula.
omnomnom
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??
wio
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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.
Decart
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that is why I was asking
wio
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Last question was a 'kinematic' one so maybe it must be kinematics.
Decart
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acceleration of gravity = velocity final minus velocity inital divided by time.
wio
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@omnomnom Have you talked about work or energy in your class yet?
wio
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Like potential energy or kinetic energy?
omnomnom
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No whats work?
wio
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It is something you will learn in the future, most likely.
Decart
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yes solve for time then plug into \[\frac{ d }{ t }=\frac{ v _{f}+v _{i} }{ 2 }\]
Decart
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conservation is so much easier to solve
omnomnom
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it confused me though
the first equstion
omnomnom
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equation*
wio
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Don't worry about it.
wio
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@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
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oh okay so what equation do i use.... is is\[d =Vi( t)+ a \frac{ 1 }{ 2 }(t^2) \]
wio
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You can't use that equation until you find time though.
omnomnom
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Oh yeah >.<
Decart
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\[a=\frac{ v _{f}-v _{i} }{ t }\]
Decart
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you know a and both velocities
omnomnom
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But we need the time :( Cant i just quess the answer?
wio
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Yes, \(a\) is the same as last time... \(9.81m/s^2\)
Decart
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velocity initial is 0
wio
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You are not supposed to guess, because there is already a way to find the answer.
omnomnom
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but...... Whats the way ?
Decart
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t=49/9.81
omnomnom
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answer is 5
Decart
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yes 5 seconds
Decart
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then plug in the time into your equation and find x
omnomnom
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oh hold on
omnomnom
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i got 49 m/s
Decart
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that is the velocity
omnomnom
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so we change the equation now?
Decart
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you want the height of the building
omnomnom
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yeah
Decart
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x=1/2t(vfinal-vinital)
omnomnom
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122.5 meters?
Decart
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yes
omnomnom
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thank you :D
Decart
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do you understand the kinematic equations
omnomnom
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The first one ? NO the others Yes
Decart
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this is a helpfull tool
omnomnom
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oh kay thank you :D