# Homework Help: Orbital speed of Earth help!

1. Feb 28, 2008

### rsyodoom2005

Orbital speed of Earth!! help!

The Earth is 150 million km from the sun. IT completes one orbit in a period of approximately 365.25 days. Calculate the orbital speed of earth in mph?

OK guys so they give me the formula which is 2*pI*r /Period r= distance from the planet to the sun. and circumference= 2*pi*r

so i set it up 2*3.1416*150^6 /8766 hours = 8.12^9 km/hour? then just convert into mph ?

2. Feb 28, 2008

### Staff: Mentor

Redo your arithmetic. r = 150 000 000 km.

3. Feb 28, 2008

### rsyodoom2005

okay so redid it
2**3.1416*150 000 000
/ 8766 hours = 107515.4004 km/hour?

4. Feb 28, 2008

### rsyodoom2005

i dont understand why they give me the circumference formula!?

5. Feb 28, 2008

### Staff: Mentor

Good.

To find the speed, you need distance divided by time. The circumference is the distance traveled in one year.

6. Feb 28, 2008

### rsyodoom2005

ok doc now im just lost. i thought i had gotten the orbital speed by 2*3.1416*150000000km / 8766 hours (365 days) = 107515.4 km/h? do i i need to get the circumference of the earth then divide by the 107515.4004 km/h (?

7. Feb 28, 2008

### D H

Staff Emeritus
The circumference in question is not the circumference of the Earth itself. The circumference needed here is the circumference of the Earth's orbit about the Sun. That is why you need to use 150 million kilometers for the radius rather than 6378 kilometers.

8. Feb 28, 2008

### rsyodoom2005

okay so my final answer will be just 107515.4004 km/hour converted into mph?

9. Feb 28, 2008

### D H

Staff Emeritus
Correct.