Need help ASAP (1 Viewer)

[wolf7]

Need help

this is a question i dont understand

suppose a child has a mass of 30kg. what is the weigt of the child on earth what would be the new weight of the child if
a)the earth were suddenly to triple in radius, its mass remaining the same?
b)the earth were suddenldy to triple in mass, its radius remaining the same

Also can some explain radius plz

 

[speed2]

Need help

this is a question i dont understand

suppose a child has a mass of 30kg. what is the weigt of the child on earth what would be the new weight of the child if
a)the earth were suddenly to triple in radius, its mass remaining the same?
b)the earth were suddenldy to triple in mass, its radius remaining the same

Also can some explain radius plz

the initial weight of the kid is 30x9.8=294N

using newtons formula F=GMm/r²
initially this equalls 294N

(a) if 'r' is tripled, sub that in to the formula and u'll get 1/9 of the original, that is 32.67N.

(b) same concept but sub in M as 3M, so u'll get 882N

 

[wolf7]

sweet, i understand that heaps better, so for question a, you can say that the weight is decrease

 

[speed2]

u mean for question (b) u can say it increases, yeah?

 

[wolf7]

oh i get, a decrease b increase

 

[wolf7]

hey im confused, did u use the gravitational constant, 6.67 x 10 of the power of 11 or did u use theg gravitation of the earth

 

[Jago]

i dont know what he uses but the gravity of a planet can be found by:

g = Gm/r²

where G = 6.67 x 10^-11
m = mass of earth
r = radius

 

[speed2]

i used G=6.67x10^-11

 

[wolf7]

so u used that constant number of 6.67 in that problem

 

[speed2]

yeah thats what the 'G' stands for, but u dont really need to sub it in, i just did it algebraically.

 

[dead]

i dont get (b), so u sub in 3M in the equation and do u just solve to find m?
can someone please show me working? i always get stcuk with these questions
oops dw i just found out how to do it!!! ty guys....im just slow

 

[speed2]

initially the kid's weight is 274N (from W=mg), i.e. 294=GMm/r² where M=mass of earth, m=kids mass.

for (b) the mass of the planet triples so the new weght, W = G3Mm/²
we know from before that GMm/r² = 294, hence the new weight = 3x294=882