At it's simplest a Force, symbol F can be seen as a push or a pull in a given direction. From this we can see that Force must be a Vector Quantity as it has a direction and also has a magnitude.
The Net Force, Fnet is the resultant sum of all the vector forces acting on a body and if these are balanced then Fnet is zero and the body is subject to Newton's First Law of Motion, ie it has inertia and remains in equilibrium:
- if it is at rest it will continue to remain at rest (be stationary) and if it is moving it will continue to move at a constant speed
- all the force vectors must add up to zero
If the forces become unbalanced the Fnet no longer add to zero and the the object starts to accelerate or decelerate, according to Newton's Second Law of Motion (Force = Mass x Acceleration) and the sum of all the vector forces must produce a resultant Force Vector in the direction of the acceleration.
Newton's third law can help us with understanding some of the other forces in play and how to represent them on a Force Diagram: "every action has an equal and opposite reaction." So for example if a box is sitting on the ground stationary there is a force, due to gravity, acting in a downward direction. From Newton's 3rd Law there must be an equal and opposite reaction: in this case a force acting upwards that is equal to the gravitational force: the two forces balance out with the resultant Fnett = 0, so the box remains stationary and will only move if another force acts on the box making the Fnett grater than zero. Now the forces are no longer in balance or equilibrium!
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