Contact Forces: To understand the concept of contact forces, Let us first understand what force is.  A force is either a push or a pull. A force is an external agent that is capable of changing the state of rest or motion of an object. Force can bring a moving body to the state of rest and a body at rest to motion. It can act between two bodies that are in close contact and also between two bodies that are far apart. To understand this, let us recall the story of apple and Newton.

Contact Force & Non-Contact Force

Newton and the Fallen Apple

We have all heard how an apple falling on his head gave Newton the idea that the earth attracts every object around it. Let us understand all the forces that are acting here. The earth attracts the apple, which was quite far from its surface towards itself, and that apple falls on newton’s head, i.e. applies force on his head before reaching the ground.

The force applied by the earth on the apple did not require the two bodies to be in close proximity, but the force applied by the apple on his head would not have been possible if the bodies were not in close contact. From this example, we can conclude that force acting between two bodies can be of two types:

1. Contact Force
2. Non-Contact Force

Contact Forces

As the name suggests, these forces act between the objects in close contact with each other. It acts at the point of direct contact between the two surfaces. Newton’s laws of motion govern contact forces. These are present everywhere, and most of the macroscopic interaction between two objects can be attributed to these forces. Picking a book, clicking on phone keys, pulling a cart, and hitting a ball with a bat, a contact is established between the two objects in all these forces.

Thus, these forces can be continuous (pulling a cart) or non-continuous (impulse is generated when a bat hits a ball at great speed). The components of contact forces can be broken down along the orthogonal axis, as seen in the case of a block kept over an inclined plane. Normal force acts perpendicular to the surface on the block while the friction force acts parallel to the surface.  In our day to day life, we deal with various types of contact forces, and these are:

1. Friction Force
2. Muscular Force
3. Tension Force
4. Normal Force
5. Drag Force
6. Air resistance
7. Upthrust or Buoyancy
8. Surface Tension

Let us discuss some of them:

Friction Force

The friction force arises when there is relative motion between two surfaces. This force is exerted by a surface as an object moves across it or makes an effort to move across it. Friction is everywhere. We will not be able to walk if there were no force of friction. Broadly the force of friction is of two types:

Static Friction: Static friction occurs between two bodies when an object is placed on a surface.

Kinetic Friction: Kinetic friction arises between two bodies when an object starts to move over a surface.

There is a general conception that friction resists the motion of an object or friction opposes the motion. Consider the following examples:

1. When we roll a ball on the floor, it eventually comes to rest. Here friction causes the ball to stop. 

2. Place a pen on a notebook and keep the notebook on a table. Slowly and carefully try to slide the book over the table. You will find that the pencil starts moving! Here friction acts between pencil and book and causes the pencil to start moving. Thus, friction initiated the motion!

Friction results from the two surfaces being pressed together closely, causing attractive intermolecular forces between molecules of different surfaces and preventing the ‘relative motion’ between them. The amount of friction between the two surfaces depends upon the nature of the surfaces.

Muscular Force

All the tasks that we perform in a day involve the muscular forces of our body. From picking up a pen to kicking a ball, from holding a cup to arranging utensils, all the tasks that we perform involve the movement of one muscle or the other and is associated with a muscular force. Thus, the muscular force can be defined as the force exerted by the muscles of our body. Human beings perform various activities like jumping, running, swimming, twisting, laughing, lifting, stretching, squeezing.

All these activities involve intensive use of our muscles. The muscular force provided by our arms helps us pick a bat and then hit a ball with it. Muscular forces are working round the clock within our bodies to ensure our survival. Digestion, excretion and respiration, and all the vital functions are possible by the muscular forces applied.

Normal Force

The contact force exerted on an object when placed on a stable surface is called the normal force. This force comes up to support the object that is being placed on a stable object. It is called the ‘normal’ force because it acts perpendicular to the two surfaces in contact. For example, if a book is resting upon a surface, the surface exerts an upward force upon the book to support the book’s weight.

There are examples in which a normal force may be exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person. The normal force equals the object’s weight if the object is not under acceleration or retardation. When an object is about to fall, the normal force will be determined by the position at which the object falls on the ground.

Air Resistance Force

As the name suggests, air resistance refers to the opposition offered by the molecule of air against the motion of an object across it. Air resistance is a type of friction force that acts upon objects as they travel through the air. Like the force of friction, air resistance often tends to oppose the motion of an object. Although generally, the value of this force is almost negligible and hence it is often neglected. But air resistance becomes noticeable for objects that travel at high speeds, divers or skiers, or objects with large surface areas; planes and race cars are designed, keeping the air resistance into account.

Tension Force

The objects placed in physical contact exert forces on one another, and these are contact forces. Based on the types of surfaces that are in contact, we get various types of contact forces. When the force a rope, string, wire or chain exerts a force on an object, such a force is called Tension force. In the pulley’s case, the ropes that pull the blocks exert a tension force on them. Thus tension force is a pull force and can not be used to exert a push. While drawing the body diagrams of objects under forces, people draw the force of tension in the wrong direction, so here, it is important to remember that tension can only pull on an object.

Non-Contact Forces

As the name suggests, the forces that act between two bodies that are not in contact with each other are called ‘non-contact’ forces. These forces act between two bodies that are not physically touching each other. The magnitude of these non-contact forces between two bodies varies with the distance, but the range of non-contact forces ideally is taken to be infinity. However, their strength increases as the objects are brought closer. We come across various kinds of non-contact forces. Examples of non-contact forces are:

Magnetic Force: The force that arises from attraction or repulsion between the poles of a magnet is called Magnetic force.

Gravitational Force: Every object in this universe attracts every other object with force called the gravitational force proportional to their masses and inversely proportional to the square of the distance between them.

Electrostatic Force: The force of attraction or repulsion arising from the static charges present in the body is called the electrostatic force. As this force is present without direct contact between the two bodies, this is a non-contact force.

Summary

Contact force acts between the bodies that are physically touching each other. It acts at the point of direct contact between the two surfaces. Newton’s laws of motion govern contact forces. We deal with various types of contact forces, and these are:

1. Muscular Force
2. Friction Force
3. Tension Force
4. Normal Force

Non-Contact forces: The forces that act between two bodies that are not in contact with each other are called ‘non-contact’ forces. These forces act between two bodies that are not physically touching each other. For example, gravitational force and electric force.

FAQs on Contact Forces

Q.1. The electricity that runs the devices around our homes and offices will be associated with which type of force?
Ans: Electricity is the flow of electric charges associated with electric force, and electric force is a non-contact force.

Q.2. A kid at school threw a ball up in the air, the ball reaches a certain height, and after that, it falls back towards the ground, which forces act on the ball throughout the whole process?
Ans: The kid applies muscular force on the ball to throw it upwards, and the force of gravity acts on the ball continuously, and so does air resistance. Here muscular force and air resistance are contact forces, while gravitation force is a non-contact force.

Q.3. Define force. 
Ans: It can be defined as a push or pull that often results in interaction between two objects. It can cause motion and stop it too. 

Q.4. Give a few examples of non-contact forces.
Ans: 1. Magnetic force
2. Electric force
3. Gravitation 

Q.5. Is buoyancy a contact or a non-contact force?
Ans: It is a contact force exerted by a fluid on an object when it is submerged in it. Thus, buoyancy is a contact force.