# A2 physics help circular orbit

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#1
The Earth moves around the Sun in a circular orbit with a radius of
1.5 x10^8 km.
What is the Earth’s approximate speed?

I'm not sure what equations to use?
do i do mv^2=GMmr^2/r^2

A 1.5 × 10^3ms–1
B 5.0 × 10^3ms–1
C 1.0 × 10^4ms–1
D 3.0 × 10^4ms–1
Last edited by MARTIANAlLIENS; 3 weeks ago
1
3 weeks ago
#2
You know solar mass
I assume you know the value of newton's constant
In circular motion the gravitational force of sun is the centripetal of earth
Your circular equation is a bit dodged though
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#3
(Original post by Callicious)
You know solar mass
I assume you know the value of newton's constant
In circular motion the gravitational force of sun is the centripetal of earth
Your circular equation is a bit dodged though
mv^2/r=GMm/r^2

do i rearrange for V?
Last edited by MARTIANAlLIENS; 3 weeks ago
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3 weeks ago
#4
(Original post by MARTIANAlLIENS)
mv^2/r=GMm/r^2

do i rearrange for V?
yup
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#5
(Original post by Callicious)
yup
ok i got v=root GM/r

the i subbed in values
6.67x10^-11 x 5.97x10^24 divided by the radius in m 1.5x10^11
and rooted and i got 51.52
but the answer should have been 3.0 × 10^4ms–1
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3 weeks ago
#6
(Original post by MARTIANAlLIENS)
ok i got v=root GM/r

the i subbed in values
6.67x10^-11 x 5.97x10^24 divided by the radius in m 1.5x10^11
and rooted and i got 51.52
but the answer should have been 3.0 × 10^4ms–1
I got ~29.5 kms^-1; something you should memorise actually it shows up fairly frequently iirc. Anyway, your M is wrong. What's that 5.97? The mass of the sun is 1.989x10^30 kg.
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#7
(Original post by Callicious)
I got ~29.5 kms^-1; something you should memorise actually it shows up fairly frequently iirc. Anyway, your M is wrong. What's that 5.97? The mass of the sun is 1.989x10^30 kg.
ah i was using the earth's mass, how do you know when to use the sun's mass or earth's, i assumed it was the earth since the question said earth's speed
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3 weeks ago
#8
(Original post by MARTIANAlLIENS)
ah i was using the earth's mass, how do you know when to use the sun's mass or earth's, i assumed it was the earth since the question said earth's speed
You just need to know the equations and the physics behind them. If you know that, you'll know which numbers to push through them.

I.e. in this case you did the force on the earth from the sun

is equivalent to the centripetal force on the earth necessary for circular motion

cancelled out the and put in your values for which are ~6.67e-11,1.989e30,1.5e11/astronomical-unit.
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#9
(Original post by Callicious)
You just need to know the equations and the physics behind them. If you know that, you'll know which numbers to push through them.

I.e. in this case you did the force on the earth from the sun

is equivalent to the centripetal force on the earth necessary for circular motion

cancelled out the and put in your values for which are ~6.67e-11,1.989e30,1.5e11/astronomical-unit.
how do you go about these type of question

Two satellites, P and Q, of the same mass, are in circular orbits around the Earth. The radius of
the orbit of Q is three times that of P. Which one of the following statements is correct?
A The kinetic energy of P is greater than that of Q.
B The weight of P is three times that of Q.
C The time period of P is greater than that of Q.
D The speed of P is three times that of Q.
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3 weeks ago
#10
(Original post by MARTIANAlLIENS)
how do you go about these type of question

Two satellites, P and Q, of the same mass, are in circular orbits around the Earth. The radius of
the orbit of Q is three times that of P. Which one of the following statements is correct?
A The kinetic energy of P is greater than that of Q.
B The weight of P is three times that of Q.
C The time period of P is greater than that of Q.
D The speed of P is three times that of Q.
If it's a new Q I recommend making a new post (makes it easier for future people looking for the same answer)
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#11
(Original post by Callicious)
If it's a new Q I recommend making a new post (makes it easier for future people looking for the same answer)
oh ok
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3 weeks ago
#12
I'd prefer to calculate the orbital circumference using 2 pi r (in meters)

Divided by the number of seconds in a year

You don't need to bring in extraneous information like the mass of the sun
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