Calculating earth's speed using radius and speed of light, etc.

DDRchick

Ole Roemer found that the average increased delay in the disappearance of Io from one orbit around Jupiter to the next is 14 s.
(a) How far does light travel in 14 s?
1 m

(b) Each orbit of Io takes 42.5 h. Earth travels the distance calculated in part (a) in 42.5 h. Find the speed of Earth in km/s.
2 km/s

(c) Check to make sure that your answer for part (b) is reasonable. Calculate Earth's speed in orbit using the orbital radius, 1.5 108 km, and the period, one year.

d=vt
speed of light = 3x10^8

For part (a) i got 4.2e9, which was correct. d=(3x10^8)(14)
For part (b) I got 27.45 km/s which was correct. 4.2e9m = 4.2e6km
Part (c) I dont know...

Delphi51

The Earth goes around the sun in a circle. Find the distance around the circle. The time taken to go around, in seconds. Use v = d/t to find the speed.

DDRchick

Well I figure, radius is half a diameter, right?
So i multiplied the radius (1.5 x 10^8) by two.
then i found the number of seconds in a year, which is 31,536,000.
I even tried just doing (1.5x10^8)/# seconds in a year
and it wa s still wrong.
:(

**Because it won't let me edit the first post....**
orbital radius, 1.5 108 km = 1.5x10^8

LowlyPion

Quote by DDRchick

Well I figure, radius is half a diameter, right?
So i multiplied the radius (1.5 x 10^8) by two.
then i found the number of seconds in a year, which is 31,536,000.

Isn't the circumference of a circle = 2πr ?

HallsofIvy

Light doesn't travel around the circumference of a circle- travels across the diameter.

DDRchick

I used LowlyPion's equation and plugged in the radius, and then divided by the number of seconds.
It marked it correct. :D
Thanks so much!!

LowlyPion

Quote by HallsofIvy

Light doesn't travel around the circumference of a circle- travels across the diameter.

That may well be, but the question is asking for Earth's speed, and the circumference/period. Earth's speed is relevant for explaining the 14 s interval, and represents an exceedingly small arc of earth's orbit right?

monty37

hey it is the angular velocity you need to consider i.e omega,not linear velocity
then v=d/t cannot be used .

LowlyPion

Just curious would you have the θ for 14 sec divided by a year load of seconds?

And could you identify the error difference between the arc of θ, and the chord of θ?

Similar Threads for: Calculating earth's speed using radius and speed of light, etc.
Thread	Forum	Replies
Can photons be accelated or do they always travel at one speed (speed of light)?	General Physics	25
calculating angular speed out of tangential speed	Introductory Physics Homework	1
Speed of ship as fraction of speed of light	Introductory Physics Homework	2
Speed of Wire faster than Speed of Light???	General Physics	2
Calculating the Speed of the Earth's Rotation.	Precalculus Mathematics Homework	5

Delphi51 Recognitions: Homework Help	The Earth goes around the sun in a circle. Find the distance around the circle. The time taken to go around, in seconds. Use v = d/t to find the speed.

DDRchick	Well I figure, radius is half a diameter, right? So i multiplied the radius (1.5 x 10^8) by two. then i found the number of seconds in a year, which is 31,536,000. I even tried just doing (1.5x10^8)/# seconds in a year and it wa s still wrong. :( Because it won't let me edit the first post.... orbital radius, 1.5 108 km = 1.5x10^8

HallsofIvy Recognitions: Gold Member Science Advisor Staff Emeritus	Light doesn't travel around the circumference of a circle- travels across the diameter.

monty37	hey it is the angular velocity you need to consider i.e omega,not linear velocity then v=d/t cannot be used .

LowlyPion Recognitions: Homework Help	Just curious would you have the θ for 14 sec divided by a year load of seconds? And could you identify the error difference between the arc of θ, and the chord of θ?