## omnomnom Group Title 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. one year ago one year ago

1. Decart Group Title

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

2. Decart Group Title

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

3. Decart Group Title

you know what g is

4. omnomnom Group Title

Gravity?

5. Decart Group Title

yes

6. Decart Group Title

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

7. Decart Group Title

is this for conservation of energy

8. omnomnom Group Title

No this is only for the height of the building

9. Decart Group Title

The formula I gave you is the conservation of energy

10. wio Group Title

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

11. Decart Group Title

You cannot solve this with kinematics there are too many unknowns

12. wio Group Title

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

13. Decart Group Title

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

14. wio Group Title

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

15. omnomnom Group Title

??

16. wio Group Title

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. Decart Group Title

that is why I was asking

18. wio Group Title

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

19. Decart Group Title

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

20. wio Group Title

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

21. wio Group Title

Like potential energy or kinetic energy?

22. omnomnom Group Title

No whats work?

23. wio Group Title

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

24. Decart Group Title

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

25. Decart Group Title

conservation is so much easier to solve

26. omnomnom Group Title

it confused me though the first equstion

27. omnomnom Group Title

equation*

28. wio Group Title

29. wio Group Title

@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 Group Title

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

31. wio Group Title

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

32. omnomnom Group Title

Oh yeah >.<

33. Decart Group Title

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

34. Decart Group Title

you know a and both velocities

35. omnomnom Group Title

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

36. wio Group Title

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

37. Decart Group Title

velocity initial is 0

38. wio Group Title

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

39. omnomnom Group Title

but...... Whats the way ?

40. Decart Group Title

t=49/9.81

41. omnomnom Group Title

42. Decart Group Title

yes 5 seconds

43. Decart Group Title

then plug in the time into your equation and find x

44. omnomnom Group Title

oh hold on

45. omnomnom Group Title

i got 49 m/s

46. Decart Group Title

that is the velocity

47. omnomnom Group Title

so we change the equation now?

48. Decart Group Title

you want the height of the building

49. omnomnom Group Title

yeah

50. Decart Group Title

x=1/2t(vfinal-vinital)

51. omnomnom Group Title

122.5 meters?

52. Decart Group Title

yes

53. omnomnom Group Title

thank you :D

54. Decart Group Title

do you understand the kinematic equations

55. omnomnom Group Title

The first one ? NO the others Yes

56. Decart Group Title
57. Decart Group Title