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I just got my physics test back and am hoping I can be helped with two questions. My instructor gave it back at the end of class leaving no time for going over it.

1) Suppose the period of the moon in its orbit about the earth were 41.1 days and the moon were 768,000 km from earth. Suppose the earth's radius were 15,950 km. Use Kepler's third law to determine what the period of an artificial satellite orbiting near the earth's surface would be.

Kepler's third law is: T^2=Kd^3 or from what I understand D^3/T^2

Tm= 41.1 days

Dm= 768,000 km

De= 15,950 km

So first I did:

Dm/De= 768,000 km/15,950 km = 48.15047022 km

Then (Dm/De)^3= 48.15047022^3 km = 111635.3139 km

Next I did (Tm/Te)^2= 111635.3139 km

so the square root of 111632 = 334.1187123

334.1187123 x 41.1 days = 13732.27908 days

now I have to get in into hours so:

13732.27908 days x 24 hr/day = 329574.6978

Now what? Have I done something wrong or do I have more steps to take because the answer should be one of these four choices:

(a)1 hr (b) 2 hr (c) 3 hr (d) 4hr (e) 5 hr

2) Calculate the carrying capacity (number of cars passing a given point per minute) on a highway with three lanes (in one direction) using the following assumptions: The average speed is 100 km/hr, the average length of a car is 7.0 m, and the average distance between cars should be 75 m.

v= 100 km/hr

d= 75 m

I thought I shoud begin the problem by looking for the time it takes so I did t=d/v and got t= 0.75 s. Now I am stuck. I know I have to use another formula somewhere. Please let me know what I am looking for now and which formula I should use so I can try to get the answer.

The answer needs to include cars/min as the choice for the answers are:

(a) 21 cars/min (b) 31 cars/min (c) 41 cars/min (d) 51 cars/min

(e) 61 cars/min

Is there any type of clue you can give me to know when to use which formula when answering physics question. Unless I see certain words or symbols mentioned like centripetal acceleration, average acceleration, or N in questions I am at a loss. There are so many formulas and I am not clear on when which formula should be used.

Thank you in advance.

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# Homework Help: Kepler's third law and multiple formulas

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