# A2 physics help circular orbit

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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

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

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#2

You've got radius

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

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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(Original post by

You've got radius

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

**Callicious**)You've got radius

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

do i rearrange for V?

Last edited by MARTIANAlLIENS; 3 weeks ago

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(Original post by

yup

**Callicious**)yup

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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#6

(Original post by

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

**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

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(Original post by

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.

**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.

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#8

(Original post by

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

**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

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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(Original post by

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.

**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.

the answer is A

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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#10

(Original post by

Could you also please help me with this q?

the answer is A

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.

**MARTIANAlLIENS**)Could you also please help me with this q?

the answer is A

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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(Original post by

If it's a new Q I recommend making a new post (makes it easier for future people looking for the same answer)

**Callicious**)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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#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

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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