# Contact and Non Contact Forces

## Contact and Non Contact Forces - Definition and Examples

WHAT IS FORCE?

Force is a push or pull on an object when two objects interact which tends to change the state of rest or of uniform motion of an object or changes the direction or shape of an object. When the interaction stops, the two objects no longer experiences a force. Therefore it is a result of the interaction.

UNIT OF FORCE

SI unit of force is newton’. It is abbreviated by as 'N'.

1 Newton = 1 kg m / s2

One Newton is the amount of force required to give a 1 kg mass an acceleration of 1 m/s2.

IS FORCE A VECTOR OR SCALAR QUANTITY?

Force is a vector quantity. Vector quantity has both magnitude and direction. Because force has directions, it is represented using diagrams where arrows are used to show the direction of the force. These diagrams, which are used to represent forces and analyse them are called free body diagrams.

BALANCED FORCES When there are two forces of equal magnitude and they are opposite in direction, we call them balanced forces because they balance each other. In the picture given above, there is the gravitational pull that exerts force downwards and the push of the ground on the ball, or the normal force upwards.

The weight of the ball = Reaction force from the ground

Therefore, the ball is said to be in equilibrium.

UNBALANCED FORCES

Unbalanced forces are forces which cause a change in the state of rest or of uniform motion of an object. Unbalanced forces can alter the movement of an object in the given two ways:

• 1. When it acts on an object at rest, it may cause the object to move

• • 2. When it acts on an object which is already in motion it will either increase or decrease its speed.

• TYPES OF FORCES

There are two types of forces based on the interaction between two bodies:

• 1. CONTACT FORCES: These forces are involved only when two objects physically interact or touch each other

• Eg: normal reaction from the ground on the ball.

There are different types of contact forces:

• a. Tension force: The force transmitted along a string when the string is pulled by an object attached to the opposite end of the string is called tension force.

• • b. Spring force: The force exerted by a stretched or compressed spring on an object that is attached to it is called the spring force. An object which compresses or stretches the spring is always acted upon by a force which brings it back to the equilibrium or rest state. According to Hook’s law, the magnitude of the force is directly proportional to the distance the spring gets stretched / compressed.

• F = kx
Where,
F = force applied to the spring (Newton)
k = spring constant (Newton / meter)
x = distance stretched / compressed by the spring from its equilibrium (meters) • c. Normal reaction: When one body exerts a force on another body, the second body exerts a force perpendicular to the surface of the first body. This force is called a normal reaction.

• Example: when a book is kept on a table, the table exerts a force perpendicular to the surface of the book. This is the normal reaction.

• d. Friction: The force that resists the sliding or rolling of one object over the other. There are two types of friction

• A. Sliding friction: This friction aims to stop an object from sliding. It is created when two objects slide over each other.

• Eg) rubbing both the hands to create heat.

• B. Static friction: This is the force responsible for keeping an object at rest. This is the force that must be overcome by an object to start moving.

• e. Air resistance: This is the force experienced by objects that travel in the air. It opposes the motion of the object traveling in the air.

• Example: a skydiver experiences opposing force from the air as they move fast.

• f. Weight or Force of gravity: It is a force that pulls the body towards the center of the earth due to its gravity.

•                                    W = mg
Where,
m = mass of the body (kg)

g = acceleration due to gravity (which is 9.8 N/kg on Earth)

G) Applied contact forces: This is a force that is exerted on an object by another object or a person and executed through physical contact.

Example: When a person pushes a table, it is required to touch the table (directly or indirectly.

• 2. NON-CONTACT FORCES: These are forces that act between two bodies that does not require physical contact between the bodies.

• a. Gravitational force: It is the force which attracts any two objects with a mass. The force with which earth attracts an object of mass towards itself.

• Eg) the force that causes a ball that is thrown up in the air come back, the gravitational force.

• b. Electrical forces: These forces come into existence when there are charges present in the body. The direction of the force is determined by the charges present in the interacting bodies. The electric charge on a body causes the body attract or repel each other without any physical contact.

• Example: • • If one body is positively charged and another one is negatively charged, then the bodies will attract each other.

• • If both the bodies are negatively charged or positively charged, they will repel each other.

• c. Magnetic forces: It is the force that exists between two objects due to their magnetic characteristics. It is the attraction or repulsion that arises between two electrically charged objects due to its motion.

• Example: All electric currents produce a magnetic field and any charge that moves in that region experiences a force.