# Calculating Earth Force

## Main Question or Discussion Point

Hi,

If at a particular time, I have the r distance from the Sun to Earth, the value of the velocity of Earth, and the angle between the velocity vector and the r vector, how can I calculate the force that Earth has regarding the tangent? Suppose I wanted to go againts the movement of the Earth. Which force should I go against and how do I calculate it?

Kind regards,

Kepler

## Answers and Replies

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In a direction tangential to its orbit , earth has an obital velocity vector prependicular to the vector 'r' . This velocity vector can be calculated easily.

Considering the system to be considering only of Sun and Earth , therefore , Sun's gravitational pull provides the necessary centripedal force :

$\frac {GMm}{r^2} = \frac {mv^2}{r}$

Calculate the velocity from here . This the velocity you have to counteract by going against the motion of earth. To bring earth to rest , you can calculate the work needed by the work-energy principle.

BJ