So, what exactly is an unbalanced force? Generally speaking, an unbalanced force is not just one force, but a combination of forces that influence an object. Take for example a book sitting on a table. There are two forces acting on the book: the force of gravity on the book and the force of the table on the book. If the book is motionless, those two forces are equal, but are in opposite directions, so their sum would be zero. The book is said to be in equilibrium. But, if someone were to push the book, then there would be an unbalanced force and thus the book would accelerate. The book would no longer be in equilibrium.
Consider each of the images below, depicting the different stages in the scenario where the soccer ball starts at rest, is kicked, flies toward the goal, and then hits the net. Decide whether the forces in each situation are balanced or unbalanced. Click on the image to check your understanding.
Question
Does the presence of a specific force mean that the object's motion will change?
Not necessarily! If there is another force also acting on the object that balance that one force out, then the object's motion will not change. However, if that force is the only force acting on the object, then, yes, it will change its motion.
Question
What is one tool that you can use to help you determine if the force is unbalanced?
The free-body diagram is an excellent tool to help you determine whether the forces are unbalanced.
