If they are unbalanced, the acceleration is not zero and the object changes its state of motion. If two forces are balanced, they cancel out and the acceleration is 0. The pure rightward component of one force is exactly compensated for by the pure left component of the other force. So what happens if you add two forces that are equal in magnitude, one in the purely right direction and the other in the purely left direction? It is easy to see that the two forces would entirely cancel each other out, and the result is if the body has no force acting on it at all (i.e. Vector quantities, like numbers, can be added and subtracted. It is not enough to just say “a 60 N force ” you must say something like “a 60 N force in the rightward direction. Remember, forces are vector quantities, meaning that they have both a magnitude (“size”) and a direction. We know that forces cause changes in motion, so why doesn’t the book move if it has two forces acting on it? The reason why is that the force from gravity and the normal force are equal in magnitude and opposite in direction. In our previous example, the book lying on the table has two distinct forces acting upon it, the downward force of gravity and the normal force of the table. Let’s examine that phrase “net force.” Strictly, speaking, Newton’s first law should not just say “force” but “net force.” The net force acting on an object is the sum total of all the individual forces acting on the object. The downward force acting on it is greater than the upward force, so the book accelerates downward. The reason the book moves is that the net force acting on it is non-zero. In this situation, the forces are considered unbalanced. So, the force of gravity takes over and causes the book to accelerate towards the ground. Once we remove the table, there is no longer any upward force acting on the book. Now, imagine what happens if we suddenly remove the table. The forces acting on the book are equal and opposite, so the book does not move. In this configuration, the book is at equilibrium and the two equal and opposite forces are called balanced. Because these forces are exactly equal and in opposite directions, they perfectly cancel out and the book does not change its state of motion. At the same time, the table, according to Newton’s third law, exerts an upward force on the book of equal magnitude and opposite direction as gravity (sometimes called the normal force). In this configuration, the force of gravity pushes down on the book. Here is a simple example: Imagine a book lying at rest on top of a table. Forces are measured in Newtons (N), named after Isaac Newton. Unbalanced forces are the things that make object deviate from their current state of motion. NL1: “An object in a constant state of motion remains in that constant state of motion unless acted upon by an unbalanced force.” Recall Newton’s first law of motion, which states: Unbalanced forces are the things that make objects speed up or slow down. In the context of physics, an unbalanced force is a force that causes a change in an object’s state of motion.
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