How can an unbalanced force can cause an object’s motion to change?
An unbalanced force is one which is not balanced or counteracted by another force of equal magnitude and opposite direction. In absence of a counteracting force, this force causes motion (from rest) or a change in motion (of an already moving body) of an object. For example, a ball at rest, perhaps sitting in the grass, has two forces acting on it: Earth's gravity pulling it down and the normal force pushing it up. These two forces counteract each other. Due to these balanced forces, the ball stays at rest. When we hit the ball with a golf club, a force has been imparted to the ball and friction will help to stop the ball (by opposing the motion). How far and in which direction the ball will go depends on which force is stronger. Thus, the ball will experience an acceleration. An unbalanced force causes a change in the state of motion of an object by accelerating it. Note that deceleration (by say, friction) is also acceleration, with a negative value.
In general, an unbalanced force can cause a change in the state of motion, speed and/or direction of an object.
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