Kinetics

# Force

Force is the term which is used to describe such interactions which are as simple as pull or push and as complicated as that known as weak force. If one object is interacting upon other object then each of the objects has force upon each other.

In nature, we often see many natural objects interacting with each other. Those interactions happen in various forms and in various ways. The following physical phenomena are some examples of such interactions:

• The earth revolving around the sun.
• An apple falling near the surface of the earth.
• Sticking of paper pieces to recently used comb.
• Holding together of atoms in a molecule.
• Holding of protons and neutrons in a stable nucleus.
• Changing of neutron into proton and vice versa.

Depending upon the qualitative and quantitative natures of interactions between material objects, forces in nature are broadly divided into four categories;

• Gravitational Force
• Electromagnetic Force
• Nuclear Force
• Weak Force

Certain laws such as Newton’s laws of motion, Newton’s law of gravitation etc. which govern the observed physical phenomena provide the systematic study of force. According to Newton’s first law of motion, due to inertia, a body has no tendency of its own to change its state of rest or uniform motion. A force is always necessary to cause any change in the original state (either rest or uniform motion) of a body. Hence, Newton’s first law of motion defines force in the following ways;

A force is that which upon acting on a body changes or tends to change the state of rest or of uniform motion of the body.

## Forces are classified into two broad categories;

### Contact Forces

Contact forces are those forces which occur when two interacting objects are physically contacting with each other. It requires contact between the objects to occur. Frictional force, air resistance, normal force etc. are some examples of contact forces.

### Non-Contact Forces or Action at a Distance Forces

Forces can be exerted by the interacting objects even when the objects are not physically contacted. Such forces which do not need physical contact between the two interacting objects to exert force are called non-contact forces or action at a distance forces. Gravitational force, magnetic force, electric force etc. are some examples of non-contact forces.

## Forces Acting At A Point

A force can have an effect on an object. It can cause the object to move, or to stop moving or turn or twist or break or a combination of two or more of these effects. Lifting, carrying and turning are the most common effects. Experiences tell us that the effect of a force depends upon questions such as:

• How large is the force?
• In which direction the force is exerted?
• Where the force is exerted?

Therefore, a complete description of force needs;

• Its magnitude in terms of the standard unit of force.
• Its direction.
• Its point of application.

If all the forces act at a point (or particle) or all lines of action of the forces passes through a single point, then such forces are said to be concurrent forces.

A force is represented by a line-segment drawn from a point $O$ to a point $A$.

• The point $O$ represents the point of application of force.
• The direction from $O$ to $A$ represents the direction of force.
• $OA$ represents the magnitude of the force which is proportional to the length of $OA$.

Since a force has both magnitude and direction, it is a vector quantity and can be represent by $\overrightarrow{OA}$. It originates from the point $O$, so it is a position vector. A force can also be denoted by a single capital letter such as $P$, $Q$ etc. with an arrow head above it i.e. $\overrightarrow{P}, \text{ } \overrightarrow{Q}$ and its magnitude by $P$ and $Q$.

If more forces act at a single point i.e. if there is a system of concurrent forces then they may lie;

• Along a line i.e. collinear
• On the same plane i.e. coplanar
• On different planes i.e. non-coplanar

In nature, two or more forces are found to act simultaneously at a same point, but we only experience the effect of a single force. If the effect produced by a single force acting at a point is the same as that produced by a number of forces acting at that point, then the single force is the resultant and the other forces are its components. The process of finding the resultant force is called the composition of forces, and the process of finding the components is called the resolution of forces.