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How do unbalanced forces change an object’s motion?

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A diagram with labeled vector illustrations. Panel A shows vector addition with two green arrows A and B forming a parallelogram, and their sum C as a red diagonal. Panel B shows vector subtraction, with A and negative B forming resultant D. Panel C demonstrates vector components using angles and projections. Panel D uses the right-hand rule with thumbs to show the direction of the cross product E = A x B and its opposite, -E = B x A. — mechanics Figure 1: (A) The vector sum C = A + B = B + A. (B) The vector difference A + (−B) = A − B = D. (C, left) A cos θ is the component of A along B and (right) B cos θ is the component of B along A. (D, left) The right-hand rule used to find the direction of E = A × B and (right) the right-hand rule used to find the direction of −E = B × A.

In classical mechanics, unbalanced forces—also known as net forces—cause an object to undergo acceleration.1

According to Newton’s second law of motion, when a net force acts on an object with constant mass, the object accelerates in the direction of that force. This relationship is expressed by the equation:1

$F = m a$

In this formula, F represents the net force, m is the mass of the object, and a is its acceleration. Because force and acceleration are vector quantities, an unbalanced force can change an object's motion in several ways:1

Change in speed: The magnitude of the object's velocity may increase or decrease.1
Change in direction: The force can alter the path of the object.1
Combined change: The force can simultaneously change both the speed and the direction of the object's momentum.1

If an object is not accelerating, it can be concluded that there is no net force acting upon it, meaning all forces are balanced.1