Understanding Frictional Force Concepts

Frictional force is a fundamental concept in physics that affects how objects move, especially when they are in contact with surfaces. Let's examine each statement provided in the question to determine which one is not true about frictional force.

We need to analyze each statement carefully based on the properties and types of friction.

Analysis of Statement A: Definition of Friction

Statement A says: "Friction is the force which opposes the relative motion of two surfaces in contact."

• This is the generally accepted definition of friction.
• When two surfaces slide or attempt to slide over each other, a force acts between them that opposes this relative motion. This opposing force is called frictional force.

Based on this understanding, Statement A is true.

Analysis of Statement B: Sliding Friction

Statement B says: "The force of friction that acts when a body is moving (sliding) on a surface is called sliding friction."

• Friction can be categorized based on the state of motion.
• When an object is already moving and sliding across a surface, the friction acting on it is known as kinetic friction or sliding friction.
• This type of friction opposes the ongoing sliding motion.

Based on this understanding, Statement B is true.

Analysis of Statement C: Effects of Friction in Machines

Statement C says: "Friction in machines wastes energy and also causes wear and tear."

• In many machines, moving parts are in contact, and friction acts between them.
• Friction converts some of the kinetic energy of the moving parts into heat energy, which is dissipated into the surroundings and is not useful work. This is considered energy wastage.
• The rubbing of surfaces against each other due to friction causes material to wear away over time, leading to wear and tear of the machine parts.

Based on this understanding, Statement C is true.

Analysis of Statement D: Comparing Rolling and Sliding Friction

Statement D says: "Rolling friction is much more than sliding friction, the use of ball bearings in a machine considerably reduces friction."

• Let's break this statement into two parts.
• Part 1: "Rolling friction is much more than sliding friction." Rolling friction occurs when an object rolls over a surface. Sliding friction occurs when an object slides over a surface. It is a well-established fact that rolling friction is generally much less than sliding friction for the same surfaces and normal force. This is why wheels are used in transportation – to replace sliding motion with rolling motion, making movement easier.
• Part 2: "the use of ball bearings in a machine considerably reduces friction." Ball bearings consist of small balls placed between moving parts. They work by replacing the sliding motion between the parts with rolling motion of the balls. Since rolling friction is less than sliding friction, ball bearings effectively reduce friction in machines. This part of the statement is true.

Since the first part of Statement D claims that rolling friction is much more than sliding friction, the entire statement is rendered false.

Here is a comparison:

Therefore, the statement that is NOT true about frictional force is Statement D.

Revision Table: Key Facts about Friction

Additional Information on Frictional Force

Frictional force depends on several factors:

• Nature of Surfaces: Rougher surfaces generally have higher friction than smoother ones.
• Normal Force: The magnitude of the friction force is usually proportional to the normal force ($\(N\)$) pressing the surfaces together. For sliding friction, \( \(F_k = \mu_k N\) \), where \( \(\mu_k\) \) is the coefficient of kinetic friction.
• Area of Contact: Surprisingly, the area of contact does NOT significantly affect the magnitude of kinetic or maximum static friction, provided the normal force is constant.

Understanding friction is crucial in many engineering applications, from designing brakes to improving the efficiency of machines. While friction is often seen as a force that hinders motion, it is also essential for many activities, such as walking (static friction between your shoes and the ground) or gripping objects.