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This lesson aligns with Next Generation Science Standards (NGSS) PS2.B

Consider a book lying on a table, it is currently at rest. Your classmate asks to see your book. You exert force on the book to slide it toward her. Ideally, there is no force should be acting in the direction of motion. According to newton’s first law of motion, the book should keep sliding toward her. Eventually, it slows down and stops before it reaches her. It means that a force must be acting in a direction opposite to the motion of the object. This force is known as friction. In this article, we will learn about friction, factors affecting friction, the causes of friction, and the applications of friction. 

Friction is defined as a force that is exerted by two surfaces when they interact and rub against each other. All moving objects experience friction. The force of friction causes moving objects to slow down and eventually stop moving. Friction always works in the direction opposite to the direction of motion of the objects. 

For instance, when we throw a ball on the playground, it starts moving with some velocity. The ball does not continue its motion, instead, it stops after moving some distance. This is due to the frictional force that works when an equal force is exerted in the opposite direction of the moving object. As you can see in the figure mentioned below, the frictional force is moving in the opposite direction to the pushing force. It will slow down and ultimately stop the ball from continuing to roll.

Why Does Friction Occur?
Friction occurs due to the irregularities on the surfaces of two objects that are in contact with each other. Thus; when one object moves over the surface of the other, these irregularities are entangled, and cause to produce friction. The more the roughness, the more irregularities appear that result in significant friction to occur.

The frictional force always occurs because no surface is perfectly smooth. Even surfaces that may look smooth to the unaided eye may have an irregular, bumpy look when viewed under a microscope. For instance, you can see in the figure below. The aluminum foil is so smooth and looks shiny. But when viewed through a microscope, the smooth surface of metal appears to be very rough and bumpy.

Factors Affecting Friction
The frictional force depends on external factors. There are two factors on which friction depends;

The Nature of Surfaces
The frictional force depends on the smoothness or roughness of the two surfaces that strike each other. When the surfaces are smooth, there is less friction between the two as there are not many irregularities. While the rough surfaces produce more friction.

The Force acting on these Surfaces
Friction is also affected by the force that is acting on the surfaces. If surfaces push hard against each other, it increases friction. For example, if you rub your hands together quickly, you will feel them get warmer as there is more friction than if you rub your hands together lightly.

Types of Friction 
Four types of friction are mentioned below;
  • Static friction
  • Sliding friction
  • Rolling friction
  • Fluid friction  

Applications of Friction
  • Friction is a useful force e.g. when we walk on the floor, it is the frictional force that holds our feet to the floor and prevents us from slipping. Similarly, it also prevents the vehicle from skidding on the road.
  • Moving a piston in a cylinder is also an application of friction.
  • Writing is a great example of friction. When we write, the tip of the pen is in contact with the surface of the paper. The ballpoint produces rolling friction. 
  • Other applications of friction include lighting matchsticks, flying airplanes, skating, etc.

  • Friction is a force that resists the sliding of an object over the surface of other.
  • The direction of frictional force is always the opposite of the direction of motion.
  • When the irregularities of one surface come into contact with the irregularities of another’s surface, friction occurs.
  • The frictional force depends on the materials from which the two surfaces are made and the force acting on these surfaces.
  • Friction has been divided into four general types that include static friction, sliding friction, rolling friction, and fluid friction.

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