Let's observe nature. An apple hangs on a branch and rests there. We have already learnt that the force causes a body to move. So let’s think that no force acts on the apple.
However, it is practically never the case in nature that no force acts on a body. The Earth attracts all bodies to itself with a force we call gravity or weight. If we want a body to remain at rest despite its weight, at least one more force must act on it, which will be opposite to the weight. These two forces must cancel out each other. Their resultant will be equal to zero and only then will the body be at rest.
The same applies to all bodies that are already moving uniformly and in a straight line. A force can change the speed and direction of a moving body. If the body moves uniformly, then there is no such force to change its speed, or the resultant of the forces is zero.
The described phenomena are dealt with by Newton's first law, which we learn about below.
Newton's first law describes bodies that are at rest or moving uniformly in a straight line. In both cases, the resultant of all the forces acting on the body is equal to zero. This Newton's law is also called the law of inertia because the body persists in its motion or rest. Its speed does not change.
Newton's first law states that:
If the resultant of the forces acting on a body is equal to zero, then the body is at rest or moving uniformly in a straight line.
However, cases, where the resultant force on a body is not equal to zero, are described by Newton's second law. We will get to know it in more detail later, here we will only mention what happens if the resultant of the forces is not equal to zero. In this case, the effect of the resultant is similar to the effect of a single force. The resultant of the forces causes:
a stationary body to begin to move, or
to increase or decrease the speed of an already moving body.
If the resultant of the forces acting on the body is zero, then we say that the forces are in equilibrium. We also say that the body is in balance. In accordance with Newton's first law, we can then conclude that the body will move uniformly or remain at rest.
The reverse is also true. If the body is moving uniformly or at rest throughout the observation period, then we can conclude that the resultant of the forces acting on the body is zero.
How we check the equilibrium of forces, let's take a closer look at two examples:
two forces act on a body, and
several forces act on a body.
Two forces are in equilibrium only when they have equal magnitude and act in opposite directions. Their resultant is zero. We learned about such cases in the material Addition of Forces for KS4.
If several forces act on a body at different angles, the balance can be checked by using the:
parallelogram rule or
polygon of forces.
The forces are in equilibrium when we can obtain two oppositely directed large forces using the parallelogram rule.
In the polygon of forces, we move the forces in parallel to their directions and load them one on top of the other. If the beginning of the first force and the end of the last force are at the same point, it means that the resultant is zero. The forces are therefore in equilibrium.
