The resultant force is the single force that represents the combined effect of all individual forces acting on an object. When multiple forces are applied, they can either add together or cancel each other out, depending on their direction and magnitude. The resultant force determines how the object will move, whether it will accelerate, decelerate, or remain at rest. To calculate the resultant force, you can use vector addition. For example, if two forces of 10 N and 5 N act in the same direction, the resultant force is 15 N. If they act in opposite directions, you subtract the smaller force from the larger one to find the resultant force.