## omnomnom 3 years ago 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.

1. anonymous

$mgh = \frac{ 1 }{ 2 }mv ^{2}$

2. anonymous

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

3. anonymous

you know what g is

4. omnomnom

Gravity?

5. anonymous

yes

6. anonymous

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

7. anonymous

is this for conservation of energy

8. omnomnom

No this is only for the height of the building

9. anonymous

The formula I gave you is the conservation of energy

10. anonymous

In this case $$g$$ is the gravitational acceleration we talked about in the previous problem.

11. anonymous

You cannot solve this with kinematics there are too many unknowns

12. anonymous

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

13. anonymous

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

14. anonymous

I was talking about the $$g$$ in your formula.

15. omnomnom

??

16. 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.

17. anonymous

that is why I was asking

18. anonymous

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

19. anonymous

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

20. anonymous

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

21. anonymous

Like potential energy or kinetic energy?

22. omnomnom

No whats work?

23. anonymous

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

24. anonymous

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

25. anonymous

conservation is so much easier to solve

26. omnomnom

it confused me though the first equstion

27. omnomnom

equation*

28. anonymous

29. 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.

30. omnomnom

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

31. anonymous

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

32. omnomnom

Oh yeah >.<

33. anonymous

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

34. anonymous

you know a and both velocities

35. omnomnom

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

36. anonymous

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

37. anonymous

velocity initial is 0

38. anonymous

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

39. omnomnom

but...... Whats the way ?

40. anonymous

t=49/9.81

41. omnomnom

42. anonymous

yes 5 seconds

43. anonymous

then plug in the time into your equation and find x

44. omnomnom

oh hold on

45. omnomnom

i got 49 m/s

46. anonymous

that is the velocity

47. omnomnom

so we change the equation now?

48. anonymous

you want the height of the building

49. omnomnom

yeah

50. anonymous

x=1/2t(vfinal-vinital)

51. omnomnom

122.5 meters?

52. anonymous

yes

53. omnomnom

thank you :D

54. anonymous

do you understand the kinematic equations

55. omnomnom

The first one ? NO the others Yes

56. anonymous
57. anonymous