orbital periods (1 Viewer)

[fashionista]

AAAARRRGGHH!!!
theres this question we have to do n its pissin me off sooo much...every time i use kepler's law (On the Alif example) i get the wrong orbital period can sum one plz plz plz help me??
heres tha question

The table contains information about two planets prbiting a distant star:
-----------------Planet Alif---------------------Planet Ba
Mass (kg) 1.21*10^25 --------------- 1.5*10^24


Orbital 4*10^11 --------------------- 8*10^11
Radius(m)


Radius of
Planet (m) 8*10^6 ------------------------ 4*10^6


Length of 9.5*10^4 -------------------- 4.7*10^7
day (s)


Orbital
Period (s) 8.75*10^7 ------------------------ ?

Find the orbital period for Ba. This can be calculated using the data from the table

is it:
A) 3.1*10^7 s
B) 5.51*10^7 s
C) 1.39*10^8 s
D) 2.47*10^8 s
every time i do it i get the orbital period as like 5.59sumthing sumthing sumthing 10^sumthing (this is calculating Alif's orbital period from the answer already given in the table so i know if i've done it right(GAH!! cant evn get that one!!!!)..that's y i havnt attempted calculating Ba)

 

[Xayma]

Ok the answer is D. Alot of irrelevant information is given if you only need to calculate Ba's orbital period.

r³/T²=k

(4x10¹¹)³/((8.75x10⁷)²)=8.34x10¹⁸

Given that k is a constant for all satellites orbiting an object.

T²=r³/k
T=sqrt(r³/k)
T=sqrt({[8x10¹¹]^3}/[8.34x10¹⁸]
T=2.47x10⁸

 

[fashionista]

THANK U THANK U THANK U!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[fashionista]

can i jus ask where u got the 8.34*10^18 from??...because u sed that k =8.4*10^18

 

[Xayma]

Typo sorry. I forgot the 3. Or I added one, cant remember.

 

[fashionista]

tats k...i think u forgot it in the first one thanx for ur help!!!