Net force

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Net force is the sum of all the forces acting on an object. If two forces pull in opposite directions, the net force is the bigger one minus the smaller one. The net force changes the object's motion: Fnet = m a.

Forces are vectors, so they have both size and direction. We draw forces as arrows; the longer the arrow, the stronger the force. To find the total force, you add the forces using the tip-to-tail method or the parallelogram rule.

When a force acts on an extended body (not just a point), it can be applied at different places. In this case we use two ideas: net force and torque. The net force affects linear motion (speed and direction), while torque (a turning effect) affects rotation. Torque depends on how far the force is from the center of mass and on the line along which the force is applied.

The overall effect on a rigid body can be described by the net force and the net torque. If we replace all the forces by one net force plus one pure torque, we get the same motion as the original forces. Sometimes there is a line of action for the net force that makes the extra torque zero, so the body moves as if it were a single particle.

Some texts use “net force” and “resultant force” as if they always mean the same thing, but they can be different in rotating or balanced situations.

A force is a vector with a direction and a size. A simple way to picture it is a line from the point where the force is applied to another point, with the line’s length representing the force’s strength.

As a quick example, a disk with mass 0.5 kg and a force of 2 N applied 0.6 m from the center gives a torque of 1.2 Nm and, since F = ma, a linear acceleration of 4 m/s^2. The same force also contributes to angular acceleration, since the torque and the moment of inertia determine how fast the disk spins.

Summary: net force tells how the motion speeds up or slows down, while torque tells how the motion spins. The combined effect of net force and torque matches what all the original forces would do together.