## YumYum247 one year ago Kelp me Please!!!! :")

1. YumYum247

can someone please check the Net force between the person (65kg) and the earth with a mass of 5.98X10^24Kg and radius of 6.38X10^6m. And how do i find the net force between them the second way...i'm not getting the right answer!!!

2. YumYum247

3. YumYum247

is it.....|dw:1442282566185:dw|

4. anonymous

Hello whats going on :D

5. YumYum247

nothing much, just another day stuck on physics as usual :D

6. YumYum247

i think i got it.... but i don't quite know how!!!! :"D

7. YumYum247

i was suppose to get the net force between the earth and the person....i got it the first way by applying the conventional formula.... F = G(m1)(m2)/d^2 and got 635N

8. YumYum247

sorry i forgot to tell you...it's question "b"

9. YumYum247

and the other way i got the net force was by multiplying the mass off the person with the gravitational force of wearth...9.80N and i got 637N

10. YumYum247

but what do you call that in terms of an actual formula...?!?!?!?

11. anonymous

Ok so the way I would do it is as follows: Since the force is the same (the force of gravity I can cheat and use the ratio of the two.... On Earth F=weight=mg=(65kg)g On Jupiter F'=jupiter weight=mj = (65kg)j Now divide the equations which give you F/F' = g/j

12. anonymous

So clearly j=g*(F'\F)

13. YumYum247

My dear what is F' and m/j, i'm not familiar to those variables.....:"(

14. YumYum247

i'm sorry if that makes things difficult....but i don't know what they mean...!!! :"(

15. anonymous

Ok sorry I am hoping around to much

16. YumYum247

did i do part b right..?!?!?!? please check ")

17. anonymous

Oh that I defined F to be the force of gravity on earth (the weight on earth, F' to be the weight on jupiter and g is the earth grav accel and j is the jupiter grav accel (just made that letter up)

18. YumYum247

OMG.....i'm sorry, i'm really dumb....please don't mind that :")

19. anonymous

Ahhh no your'e part be isnt TOTALLY correct. If you put jupter's mass in a ball with the radius the size of the galaxy it's gravitational acceleration on the surface wouldn't be close to what it is on earth's surface.... what matters is that its Mass/Radius^2 which is honestly why I chose the approach I did...

20. anonymous

Oh wait I read the question a bit backwards but my reasoning is still sound... The smaller planet here (earth) definitely DOES NOT have more mass than jupiter... google it ~10^24kg for earth and ~10^27 for jupiter

21. YumYum247

so your saying it's partly correct?!?!! ?O_oo_O

22. anonymous

No now that I reread the question it is totally incorrect sorry :/

23. YumYum247

Aaawwnnn!!!! :"(

24. anonymous

I just glanced at it and thought it was saying the weight on jupiters "surface" was greater than that on earth

25. anonymous

but anways i digress

26. YumYum247

so what have i done wrong dear?!??! Are you sure you read the right part....question A is on the left side of the paper and B is on the right side....i'm sure B is correct because i've double checked it....

27. anonymous

28. YumYum247

tow different ways and i', getting the right answer@@@

29. anonymous

Ok you will see why I do it this way in a second... it saves having to calculate a lot of numbers

30. YumYum247

31. anonymous

$j=g \frac{F_{G_{Jupiter}}}{F_{G_{Earth}}}=g\frac{\frac{GmM_{Jup}}{R_{Jup}^2}}{\frac{GmM_{Earth}}{R_{Earth}^2}} = g \frac{\frac{M_{Jup}}{R_{Jup}^2}}{\frac{M_{Earth}}{R_{Earth}^2}}= g \frac{M_{Jup}R_{Earth}^2}{{R_{Jup}^2M_{Earth}}}$

32. anonymous

So now we have removed all those pesky extra factors... and have expressed it in terms of what we know

33. anonymous

Awwww darn I just realized they dont give you the earth mass so you have to go through all the lame calculations..... (thinking) hold on

34. YumYum247

but it looks scary tho :(

35. anonymous

Actually I can fix it hold on:

36. YumYum247

please don't fix it, let it be..i have a habit following those lame and long calculations....

37. anonymous

Oh wait... were you allowed to just look up the values of the radius and mass of the earth?

38. anonymous

because if so... all you need to do is take that equation plug in your knowns and evaluate it.... then of course you have found the acceleration on the surface of jupiter (I called it j)... then multiply by the mass to find the weight on the "surface" of jupiter

39. YumYum247

they were given to me in the question.

40. anonymous

Nope I only see Jupiter's mass and radius as givens

41. anonymous

But u were definitely using earth's mass and radius in one of the formulas

42. YumYum247

that's what i did honey.... the second way to get the net force between the person and the earth is by simply multiplying the gravitational force of gravity of earth(9.8m/sec^2) times the mass of the person...65kg...and i get the same answer as in the initial way i got the answer!!!

43. YumYum247

please ignore the last line on the paper...my second way there is wrong...........i made a mistake there....

44. anonymous

Ummm yes part a) has you find the earth weight which is just mg but you did an F_grav calculation with earth's mass and radius which weren't givens... look if your'e teacher is ok with that I am I just wasn't sure because sometimes you have to stick to the knowns in the problem.

45. anonymous

Its alright

46. YumYum247

please ignore the last line on the paper...my second way there is wrong...........i made a mistake there....

47. anonymous

So your answer looks correct using the numbers you used int he problem 24.585 which you should express as 24.6 since the largest number of sig figs used in the givens is 3

48. anonymous

I think thats how it works... it might be the minimum number of sig figs, but I think I have it right

49. anonymous

sorry 24.585 N and 24.6 N shame on me for forgetting units

50. YumYum247

btw were you referring to art B, because the mass of the earth and the radius were given in the previouse page o i used those to solve for the net force....for part A i on'y used what was given to me...

51. anonymous

OOOOO ok then yes then my way totally works... and yes I calculated the answer to part b) using my method... Note how it looks nasty, but I got rid of the extraneous factors before evaluating it

52. YumYum247

but the question is still there.... do you see the 635N on the paper...????? how do i get the same net force using another way.....

53. anonymous

Given the level of accuracy required... it probably doesn't make any difference, but in the real world a scientific calculation should be first expressed as simply as possible with as few variables as possible and then evaluated. Since each term bring with it uncertainties which stack up (though some known values can have absurdly high accuracy like charge/mass ratio of electron)

54. YumYum247

^^.agree

55. anonymous

oh... ummm I thought it was calculate earth's weight and then calculate jupiter's weight two different ways

56. YumYum247

no just the earth's.....

57. anonymous

So the first way is definitely using F_gravity in its full form... then just use mg as the second one

58. YumYum247

there is another way to find the net force between two objects.....

59. anonymous

heck all g represents is this: $g= \frac{GM_{Earth}}{R_{Earth}^2}$

60. YumYum247

i think it's 9.80N X mass of the person (65kg)

61. YumYum247

yah...that's it....i get 637N which is pretty close to 635N....

62. anonymous

So if you evaluate it long hand by using all those terms... and then evaluate it using just g technically they are the same but I dont know.... No that is incorrect the units of g are acceleration not force ( m/s/s)

63. YumYum247

i know but i get the right number tho :"(

64. anonymous

Correct... but your teacher would also point that out. Units are important

65. YumYum247

F = mXa a = F/m i know :"(

66. anonymous

I know its ok... just a simple error, but I can't let it go

67. anonymous

Anyways I have to go and get some food so I will see you next time

68. YumYum247

so is it right or wrong?!?!?!

69. anonymous

You calculated it two different ways and came to pretty much the same answer what do you think? XD

70. anonymous

Im not trying to be rude, but since your answers agreed that would suggest they are probably correct.

71. YumYum247

ok Thank you and love you infinite....GOD bless you :")

72. YumYum247

Please take care of yourself we need more people like you here on this planet :") Live long.

73. YumYum247

btw it was on the spot guess kinda thing.....no joke.!! :D

74. anonymous

:D your welcome... remember as you progress in your studies you will find it harder and harder for people to be able to help you and then maybe, eventually you will be out there on your own solving problems that don't have known answer. This is a really good time to start practicing your judgement and reasoning skills on problems that are easy, and when you do make an error to learn from it by finding out where you made the error and always being cautious of it down the road.

75. YumYum247

Again tHank you shoooooooooooooooooooo much!!! :")

76. anonymous

:D cya later

77. YumYum247

Abolutely agree with you....:") Anyways Cya amigo :)