Net force [pdf] [69mgllars6f0]

The net force acting on a system such as a particle, collection of particles, or rigid body is defined as the vector sum of all of the forces acting on the system. People are interested in net force because a net force acting on a system affects its motion. More specifically, a nonzero net force causes the system's Linear Momentum to change over time in the direction of the net force, as described by Newton's Second Law: the Momentum Principle.

For a system of constant mass, this change in momentum takes the form of Acceleration in the direction of the net force. Conversely, a net force of 0 acting on a system means that, according to Newton's Second Law, the system is travelling with a constant momentum. For a system of constant mass, this means that the system is travelling at constant speed, which may be zero or nonzero.

The fact that motion is possible with a net force of 0 may be be counter-intuitive, but recall that Newton's First Law states that objects in motion stay in motion unless a nonzero net force acts on it.

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For example, consider a rock thrown in empty space. It will travel at a constant speed in a straight line forever unless it encounters something that can exert a force on it. The effects of a net force do not depend on the forces comprising it. In both cases, the net force is the same, so momentum of the system will be affected in the same way for both net forces.

Sometimes the individual force vectors are simply given in component form. In this case, recall that to add the force vectors, their corresponding components of the vectors should be added to yield the components of the resultant vector. At least as often, the individual force vectors are not given in component form but have known magnitudes and directions.

In this case, coordinate axes should be chosen and the forces should be decomposed into components along those axes. If it is possible to predict the direction of the net force using physical intuition, the axes should be chosen so that one of them is parallel to this net force. Otherwise, The axes should be chosen so that as many forces as possible are parallel to them an axis, reducing the need to decompose them.