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Force
In physics, force is a push or a pull that changes or tends to change the state of rest or uniform motion of an object or changes the direction or shape of an object. Force causes objects to accelerate, add to the object's overall pressure, change direction, or change shape. Force is measured in Newtons. (N).
A force is always a push, pull, or a twist, and it affects objects by pushing them up, pulling them down, pushing them to a side, or by changing their motion or shape in some other way.
Newton's Second Law
According to Newton's Second Law of Motion, the formula for finding force is:
 [math]F = ma[/math]
where [math]F[/math] is the force,
[math]m[/math] is the mass of an object,
and [math]a[/math] is the acceleration of the object.
This formula says that when there is a force on an object then it will move faster and faster. If the force is weak and the object is heavy, then it will take a long time to increase the speed very much, but if the force is strong and the object is light, then it will move a lot faster very quickly.
Weight
Gravity is a force. Everything that weighs something is being pulled by gravity.
One can take the equation above and change [math]a[/math] to the standard gravity g, then a formula about the gravity on earth can be found:
 [math]W = mg[/math]
where [math]W[/math] is the weight of an object,
[math]m[/math] is the mass of an object,
and [math]g[/math] is the acceleration due to gravity at sea level. It is about [math]9.8m/s^2[/math].
This formula says that when you know the mass of an object, then you can calculate how much force there is on the object because of gravity. You must be on earth to use this formula. If you are on the moon or another planet, then you can use the formula but g will be different.
Force is a vector, so it can be stronger or weaker and it can also point in different directions. Gravity always point down into the ground (if you are not in space).
Gravitational force
Another equation that says something about gravity is:
 [math]{F}=\frac{Gm_{1}m_{2}}{d^2}[/math]
[math]F[/math] is force; [math]G[/math] is the gravitational constant, which is used to show how gravity accelarates an object; [math]m_{1}[/math] is the mass of one object; [math]m_{2}[/math] is the mass of the second object; and [math]d[/math] is the distance between the objects.
This equation is used to calculate how the earth moves around the sun and how the moon moves around the earth. It is also used to calculate how other planets, stars and objects in space move around.
The equation says that if two objects are very heavy then there is a strong force between them because of gravity. If they are very far apart then the force is weaker.

