# Centripetal force problem ( not enough givens?)

• saikisen
In summary, the question is asking for the period of the Moon's rotation around Earth, which can be solved using Newton's Second Law and the formula acp = v2/r. The problem assumes that you look up the masses of Earth and the Moon in a table. The expected answer is 22.8 days.

#### saikisen

Hey guys, I could not understand this question. There weren't enough givens for myself to understand how to get the period.

1. Earth and the Moon are separated at their centres by a distance of 3.4x10^8m. Determine the period of the Moon's rotation about Earth.

2. mv^2/r=Fc

3. I wasn't sure if MG applies in this question since the mass of the moon wasn't stated although Earth does have gravity that keeps the moon in its orbit. I'm assuming something like mass is supposed to be canceled in the formulas, I just don't know how or which one.

The supposed answer is supposed to be 22.8 days

You're exactly right, one of the masses will cancel.
There's a pretty straight forward way to do this problem with an application of Newton's Second Law and the formula acp = v2/r

Just do a force summation for the Moon. Look for a velocity, then consider the circumference.

Good Luck!

EDIT: Also, you mentioned that the problem did not give the mass of the Earth or Moon. Typically physics problems like this assume that you look the masses up in the tables in your book.

Also, you may need to do a center of mass calculation because the Moon does not orbit around the Earth's center but rather the systems center of mass.

This is true, however I don't believe this problem expects that. I calculated the answer using the R value that he gave and got the answer: 22.8 days

Brilliant said:
This is true, however I don't believe this problem expects that. I calculated the answer using the R value that he gave and got the answer: 22.8 days

Ok then, also if you ignore the center of mass you don't need to look up the mass of the moon (which is what the problem seems to want).

## What is a centripetal force?

A centripetal force is a force that acts on an object moving in a circular path, pulling it towards the center of the circle. It is necessary for an object to maintain its circular motion.

## What is the formula for calculating centripetal force?

The formula for calculating centripetal force is F = m * v^2 / r, where F is the centripetal force, m is the mass of the object, v is its velocity, and r is the radius of the circular path.

## What are the common givens needed to solve a centripetal force problem?

The common givens needed to solve a centripetal force problem are the mass of the object, its velocity, and the radius of the circular path. Other givens may include the frequency or period of the circular motion.

## What happens if there are not enough givens to solve a centripetal force problem?

If there are not enough givens to solve a centripetal force problem, it is not possible to find the exact value of the centripetal force. However, you can still use the given information to make estimations or calculate the range of possible values for the centripetal force.

## What are some real-life examples of centripetal force problems?

Some real-life examples of centripetal force problems include the motion of planets around the sun, the rotation of a washing machine, and the movement of a roller coaster on a loop.