# Homework Help: Orbital angular disp. in 1 day & centrip. force and Acceleration of planets help!

1. Feb 28, 2012

### kaoztheory

As from the title, I am trying to find for Venus, Earth, and Neptune:

a) the orbital angular displacement in one day in radians
b) the centripetal acceleration
c) the centripetal force

Relevant Equations:

Centrip accel: ac=v^2/r=4pie^2/T^2
Centrip force: Fc=ma=mv^2/r=4pie^2mr/T^2
related equations
Spin speed at equator: (2pie/T)*r=wr
ang. speed: w=change in theta/ change in time
orbital per: T=2pie*r/v

I know:
length of day in earth days--and in seconds
length of year in earth days--and in seconds
Radius of orbit around sun in meters

My attempt at find centrip accel
Earth: in process
Venus: in process
Neptune: length of year in secs=(2pie*rad of orbit around sun in meters)v
=v=(2pierad of orbit around sun in meters)/length of a year in secs
=31466412=(2pie1.49E11m)v
=297371. m/s
a=v^2/r

I used the same method for finding the other planets but didn't write them in order to check whether or not it was right.

My attempt at find centrip force:

...yeah I don't even know where to begin....

mv^2/r so......mass (of earth of a person of what???) * velocity of earth^2/radius of planet..? It seems so easy...but is it? inputting for mass of earth I get (1024 kg)((3.5E4m/s)^2)/6378000m=.001967x10^8

as for orbital angular displacement in one day...I'm lost...

So why didn't I just learn this in physics class? Because I was sick on the days my class started learning rotational mechanics--am STILL sick--and have been trying to teach myself by looking at online sources and videos. But this homework was assigned (I know because my friend sent it to me a few mins ago)....now he isn't online and I don't really have anyone else to go to help for...plus he's one of those "learn by sweat and tears" kind of people--in other words, not a very good one (or a very good one depending on how you look at it)...so even if he was online I don't think he would be much help....I just can't learn easily from that kind of "help".....However...though giving me the answers might seem like a bad way to teach me I am in fact VERY good at working backwards and understanding, so if someone was to give step by step instructions on how to solve the above questions (imputing the known variables I listed in word form), then I know I could understand the relationship better.

Anyhow...can anyone help me? I would GREATLY appreciate help as right now I am on the verge of tearing my hair out...OK not true...but I've been blowing my nose for hours and I feel extremely dizzy and I HAVE to go to school tomorrow so.....

I digress......way too much...sorry :(

2. Feb 28, 2012

### Staff: Mentor

For angular displacement in one day express your planet year in days. Then its a straightforward ratio. Suppose for some planet there are n days in its year. In one year it passes though $2 \pi$ radians as it circles the Sun once. So in one day,

$\frac{1}{n} = \frac{\theta}{2 \pi}~~~~$ Solve for θ.

When you're looking for the centripetal acceleration and force you want to use the mass of the planet in question, its orbital velocity, and its orbital radius (not it's planetary radius).