Question

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.

Answer 1

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

Answer 2

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

Answer 3

you know what g is

Answer 4

Gravity?

Answer 5

yes

Answer 6

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

Answer 7

is this for conservation of energy

Answer 8

No this is only for the height of the building

Answer 9

The formula I gave you is the conservation of energy

Answer 10

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

Answer 11

You cannot solve this with kinematics there are too many unknowns

Answer 12

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

Answer 13

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

Answer 14

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

Answer 15

??

Answer 16

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.

Answer 17

that is why I was asking

Answer 18

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

Answer 19

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

Answer 20

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

Answer 21

Like potential energy or kinetic energy?

Answer 22

No whats work?

Answer 23

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

Answer 24

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

Answer 25

conservation is so much easier to solve

Answer 26

it confused me though
the first equstion

Answer 27

equation*

Answer 28

Don't worry about it.

Answer 29

@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.

Answer 30

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

Answer 31

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

Answer 32

Oh yeah >.<

Answer 33

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

Answer 34

you know a and both velocities

Answer 35

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

Answer 36

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

Answer 37

velocity initial is 0

Answer 38

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

Answer 39

but...... Whats the way ?

Answer 40

t=49/9.81

Answer 41

answer is 5

Answer 42

yes 5 seconds

Answer 43

then plug in the time into your equation and find x

Answer 44

oh hold on

Answer 45

i got 49 m/s

Answer 46

that is the velocity

Answer 47

so we change the equation now?

Answer 48

you want the height of the building

Answer 49

yeah

Answer 50

x=1/2t(vfinal-vinital)

Answer 51

122.5 meters?

Answer 52

yes

Answer 53

thank you :D

Answer 54

do you understand the kinematic equations

Answer 55

The first one ? NO the others Yes

Answer 56

http://www.physicsclassroom.com/

Answer 57

this is a helpfull tool

Answer 58

oh kay thank you :D