[wolfgang]

If there were another two planets looking exactly like our earth,and both of them having the same distance to our sun like our earth,and both rotates around the sun and around their axis at the same speed as our earth did.

Also on both of them there are the same creatures and the same mankind.

The only difference is that ,one of them(LE planet)is two times larger than our earth,and the other planet(SE)is half the size our earth.

If Three babies were born at the same time,each on one of the three planets.

15 years later,the baby on earth celebrates his 15th birthday.

How old would be the other two babies at this time?

Note:

One year=365 days

1 day= complete rotation of a given planet around its axis,One time.

[wolfgang]

I agree. However, the OP states that the planets rotate about the axis at the same speed. Speed of rotation is usually meant to mean angular velocity.

edit - mayhap Wolfy will clarify if he meant linear speed.

I meant, ( angular speed ),

[wolfgang]

Rigandy was near to right thinking.

[Peekay]

I meant, ( angular speed ),

As far as I remember, angular velocity and angular speed are essentially the same thing. Angular velocity, in the strict sense, is a vector where the direction of the vector is the axis of rotation. But for our purposes, they should be treated the same.

[Guest]

ok so i feel really stupid for over thinking this and trying to figure this out mathematically.

the size of the planets is completely irrelevant. right at the beginning you are informed that the two earths are completely identically in every way and function to our earth except in size. meaning that even though it may take more or less rotations to make up 1 day it's still just 1 day.

so long answer short...all the babies be 15!

[Akriti]

its getting a little wacky!!!!

Edited April 21, 2011 by Akriti

[Akriti]

i still feel all are

15 yrs old

:)

[Guest]

Actually, orbit speed is independent of the mass of the satellite, as the increase in gravity is balanced by the decrease in acceleration due to greater inertia. I figured that out doing a lab in astronomy. It's pretty cool, though.

it's that decrease in speed that would getcha, actually. If they are in the same plane of ecliptic and in the same orbit, they would have to be moving at the same speed around the sun. If they weren't, they wouldn't stay in orbit for long as they would crash into each other and pull themselves out of orbit.

Edited April 26, 2011 by Singerbear

[benjer3]

it's that decrease in speed that would getcha, actually. If they are in the same plane of ecliptic and in the same orbit, they would have to be moving at the same speed around the sun. If they weren't, they wouldn't stay in orbit for long as they would crash into each other and pull themselves out of orbit.

Well the acceleration I'm talking about is perpendicular to the motion of the satellite, so it doesn't affect its speed.

[wolfgang]

which planet will have a longer day?why?

[Anon26]

WE KNOW:

planet rotate with same speed,

They have different sizes

SO:

lets say 'a' distance is covered in 't' time,

the larger planet will have lets say circumference of larger planet = 4a , earth = 2a and smaller = a

so one revolution on earth takes 2t time

for 15 years, it will take (365 x 15)x 2t = 10950t

larger planet would be 21900t

smaller one would be 5475t

If 10950t = 15

then t= 15/10950 = 0.001369

so on smaller planet it would be 5475 x 0.001369 = 7.5 years

On bigger it would be 21900 x 0.001369 = 30 years

[wolfgang]

The smaller planet will have a shorter day.and by a simple math.we can find that the baby on the small planet will be about 21 years ,while the one one the big planet will be about 11 years old.

[k-man]

The smaller planet will have a shorter day.and by a simple math.we can find that the baby on the small planet will be about 21 years ,while the one one the big planet will be about 11 years old.

Wolfgang, can you explain how you came up with this answer? By your definition, the day is the time the planet rotates around its own axis. Given that the angular speed is the same, the length of the day on all three planets must be the same - it takes the same amount of time to rotate 360 degrees. Also, by your definition, a year is 365 days, so where does this difference in ages come from?

[wolfgang]

Wolfgang, can you explain how you came up with this answer? By your definition, the day is the time the planet rotates around its own axis. Given that the angular speed is the same, the length of the day on all three planets must be the same - it takes the same amount of time to rotate 360 degrees. Also, by your definition, a year is 365 days, so where does this difference in ages come from?

If you are riding a bicycle with two wheels of different sizes.

It will go at a constant speed,but each wheel will have a different number of rotations around its axis.

[Guest]

Wolfgang, can you explain how you came up with this answer? By your definition, the day is the time the planet rotates around its own axis. Given that the angular speed is the same, the length of the day on all three planets must be the same - it takes the same amount of time to rotate 360 degrees. Also, by your definition, a year is 365 days, so where does this difference in ages come from?

If you are riding a bicycle with two wheels of different sizes.

It will go at a constant speed,but each wheel will have a different number of rotations around its axis.

That's because the surface of each wheel (where it meets the road) are a different distance from their respective axes. However, you said that they rotate around the axis at the same speed, which means you measure the speed from the axis, not the surface. This means they will take the same amount of time to complete one rotation- one day as you defined it.

Edited May 5, 2011 by maurice