4.6 Friction

Have you ever tried to walk on a wet, slippery floor? It's difficult because there is very little friction between your shoes and the floor. On the other hand, friction is what allows you to walk on a normal surface, drive a car, or even hold a pencil. Friction is a force that opposes motion between surfaces in contact. It is both a nuisance and a necessity.

Definition

Friction (denoted by the letter 'f') is the resistance to relative motion that occurs whenever two materials are in contact with each other, whether they are solids, liquids, or gases. It is a force that acts in a direction to oppose motion. Since it is a force, it is a vector quantity and has the unit of newton (N).

Main Idea

The main idea is that friction is a force that opposes relative motion between surfaces in contact. It arises due to the microscopic roughness of surfaces, which interlock and resist sliding. Friction can be both advantageous (e.g., walking, braking) and disadvantageous (e.g., wear and tear, energy loss). Understanding friction is essential for designing machines, vehicles, and everyday objects.

Core Concept

Microscopic Description of Friction The text explains: "Every surface is rough, even surfaces that appear to be highly polished can actually look quite rough when examined under a microscope as shown in figure 4.15. There is no such thing as a perfectly flat surface. As a result the two surfaces that are touching are not really touching across the entire area that appears to be touching."

The roughness of both surfaces interlock, which makes friction. This means that friction arises from the microscopic irregularities on the surfaces in contact.

Direction of Friction Friction always acts in a direction to oppose motion. The text states: "If you push a solid block along a floor to the right, the force of friction on the block will be to the left. When an object falls downward through the air, the force of friction, air resistance, acts upward."

Advantages and Disadvantages of Friction The text lists several advantages and disadvantages:

• Friction between the soles of our shoes (or feet) and the ground helps us walk.
• Friction between tyre and road helps to drive cars.
• Friction holds the screw and nails in wood.

• It slows down moving objects and causes heating of moving parts in machinery.
• Energy is wasted to overcome friction in machinery.
• It produces wear and tear.

• By polishing: Polishing rough surfaces makes them smooth, reducing friction.
• By using Ball Bearings: This method converts sliding friction into rolling friction, which is much smaller.
• By applying Lubricants (oil or grease): Lubricants reduce friction between moving parts.

Detailed Explanation

Friction is a fundamental force that affects almost every aspect of our daily lives. It arises from the electromagnetic interactions between atoms on the surfaces in contact.

The Microscopic View Even the smoothest surfaces have microscopic bumps and valleys. When two surfaces are placed together, these irregularities interlock. To slide one surface over the other, these interlocking points must be broken, which requires force. This is the origin of friction.

• Static friction is the force that must be overcome to start an object moving.
• Kinetic friction is the force that opposes the motion of an already moving object.

Rolling Friction The text states: "If we set a heavy spherical ball, ring or cylinder rolling, it experiences an opposing force called rolling friction. When a body rolls over a surface, the force of friction is called rolling friction. For the same weight, rolling friction is much smaller (even by 2 or 3 orders of magnitude) than static or sliding friction."

This is why the invention of the wheel was a major milestone in human history. It is much easier to move a heavy object on wheels than to slide it.

Fluid Friction The text also discusses fluid friction: "When an object moves through a fluid, the fluid exerts a retarding force that tends to reduce the speed of the object. The moving body exerts a force on the fluid to push it out of the way. By Newton's third law, the fluid pushes back on the body with an equal and opposite force. This retarding force experienced by an object moving through a fluid is called the drag force, which is the result of fluid friction."

• Size, shape, and orientation of the object.
• Type (properties) of the fluid.
• Speed of the object relative to the fluid.

Terminal Velocity The text states: "During free fall the objects does not speed up indefinitely. The speed of free falling object initially increases because of weight of the object, but as the speed increases the drag force also increase, slowing the object down. A point reaches where both the weight and drag force become equal and dynamic translational equilibrium is achieved. The object has now attained its maximum velocity termed as terminal velocity."

At terminal velocity, the acceleration is zero and the object falls at a constant speed.

Frictional Dissipation The text discusses dissipative forces: "Dissipative force decreases the mechanical energy in a system. Dissipative forces acting on an object always oppose the motion of the object. For example in case of the sky diver, when the parachute opened some energy is dissipated into the air thereby increasing its temperature."

Examples include rubbing hands together (which generates heat) and shooting stars (meteors heating up due to friction with the atmosphere).

Intermediate Level

In physics, friction is a force that arises from the electromagnetic interactions between atoms on the surfaces of objects. It always acts to oppose relative motion. The magnitude of friction depends on the nature of the surfaces and the normal force pressing them together.

• Static friction: Opposes the start of motion.
• Kinetic (sliding) friction: Opposes the motion of sliding objects.
• Rolling friction: Opposes the motion of rolling objects. It is much smaller than sliding friction, which is why wheels are so useful.
• Fluid friction (drag): Opposes motion through fluids (liquids or gases). It depends on the speed, shape, and size of the object.

The concept of terminal velocity is important for falling objects. As an object falls, air resistance (a type of fluid friction) increases with speed. Eventually, the upward drag force equals the downward weight, and the object falls at a constant speed called terminal velocity. The acceleration becomes zero, and the object is in dynamic equilibrium.

Important Points

• - Friction: A force that opposes relative motion between surfaces in contact.
• - Microscopic roughness causes friction.
• - Direction: Always opposes motion.
• - Types: Static, sliding, rolling, fluid (drag).
• - Rolling friction is much smaller than sliding friction.
• - Fluid friction (drag) depends on size, shape, and speed of the object.
• - Terminal velocity: The constant speed reached when drag equals weight.
• - Friction can be reduced by polishing, ball bearings, or lubricants.

Remember

• - Friction: Opposes motion.
• - Microscopic roughness is the cause.
• - Types: Static, sliding, rolling, fluid.
• - Rolling < Sliding friction.
• - Terminal velocity: Drag = Weight, acceleration = 0.
• - Reduce friction: Polish, ball bearings, lubricants.

Common Mistakes

• - Thinking friction always opposes motion: Friction can also be the force that causes motion. For example, friction between a car's tires and the road propels the car forward.
• - Forgetting that friction produces heat: Friction converts mechanical energy into thermal energy (heat). This is why rubbing your hands makes them warm.
• - Confusing rolling friction with sliding friction: Rolling friction is much smaller, which is why wheels are useful.

Quick Revision

Friction is a force that opposes relative motion between surfaces in contact. It arises from the microscopic roughness of surfaces. Friction can be advantageous (e.g., walking, braking) or disadvantageous (e.g., wear and tear, energy loss). It can be reduced by polishing, using ball bearings, or applying lubricants. Fluid friction (drag) is the force that opposes motion through fluids and is responsible for terminal velocity.

60-Second Revision

• Cause: Microscopic roughness.
• Types: Static, sliding, rolling, fluid (drag).
• Rolling friction is much smaller than sliding.
• Terminal velocity: Drag = weight, acceleration = 0.
• Reduce friction: Polish, ball bearings, lubricants.

Frequently Asked Questions (FAQs)

Q1. What is friction? Answer: Friction is the resistance to relative motion that occurs whenever two materials are in contact with each other. It is a force that acts in a direction to oppose motion.

Q2. What causes friction? Answer: Friction is caused by the microscopic roughness of surfaces. Even surfaces that appear smooth have tiny irregularities that interlock with the irregularities of the other surface, resisting motion.

• Static friction: Opposes the start of motion.
• Sliding (kinetic) friction: Opposes the motion of sliding objects.
• Rolling friction: Opposes the motion of rolling objects.
• Fluid friction (drag): Opposes motion through fluids.

Q4. What is rolling friction? Answer: Rolling friction is the force that opposes the motion of a rolling object (like a ball or wheel). It is much smaller than sliding friction, which is why wheels make it easier to move heavy objects.

Q5. What is fluid friction? Answer: Fluid friction, or drag, is the retarding force experienced by an object moving through a fluid (liquid or gas). It depends on the size, shape, and speed of the object, as well as the properties of the fluid.

Q6. What is terminal velocity? Answer: Terminal velocity is the constant maximum velocity reached by an object falling through a fluid when the drag force equals the weight of the object. At this point, the net force is zero, and the acceleration is zero.

• Polishing surfaces to make them smoother.
• Using ball bearings to convert sliding friction into rolling friction.
• Applying lubricants (oil or grease) between surfaces.

• Q8. What are the advantages of friction?
• Answer: Friction is essential for walking, driving, and holding objects. It allows us to grip surfaces, brake vehicles, and fasten screws and nails.

Multiple Choice Questions (MCQs)

• Q1. The force that opposes relative motion between surfaces in contact is called:
• A. Gravity
• B. Friction
• C. Tension
• D. Normal force

• Correct Answer: B
• Explanation: The text defines friction as the resistance to relative motion between surfaces in contact.

• Q2. Friction always acts in a direction:
• A. Parallel to motion
• B. Opposite to motion
• C. Perpendicular to motion
• D. Random

• Correct Answer: B
• Explanation: The text states that friction always acts in a direction to oppose motion.

• Q3. The cause of friction is:
• A. Smooth surfaces
• B. Microscopic roughness of surfaces
• C. Weight of the object
• D. Color of the object

• Correct Answer: B
• Explanation: The text states that every surface is rough at the microscopic level, and this roughness causes friction.

• Q4. Which type of friction is the smallest for the same weight?
• A. Static friction
• B. Sliding friction
• C. Rolling friction
• D. Fluid friction

• Correct Answer: C
• Explanation: The text states that rolling friction is much smaller (even by 2 or 3 orders of magnitude) than static or sliding friction.

• Q5. The force that opposes motion through a fluid is called:
• A. Normal force
• B. Tension
• C. Drag force
• D. Weight

• Correct Answer: C
• Explanation: The text defines the retarding force experienced by an object moving through a fluid as the drag force.

• Q6. The constant maximum velocity reached by a falling object when drag equals weight is called:
• A. Average velocity
• B. Instantaneous velocity
• C. Terminal velocity
• D. Initial velocity

• Correct Answer: C
• Explanation: The text defines terminal velocity as the maximum constant velocity attained by an object falling through a fluid.

• Q7. At terminal velocity, the acceleration of the object is:
• A. Maximum
• B. Zero
• C. Equal to g
• D. Infinite

• Correct Answer: B
• Explanation: At terminal velocity, the net force is zero, so the acceleration is zero.

• Q8. Which of the following is a method to reduce friction?
• A. Increasing the roughness of surfaces
• B. Using lubricants
• C. Increasing the weight of the object
• D. Removing ball bearings

• Correct Answer: B
• Explanation: The text states that applying lubricants (oil or grease) is a method to reduce friction.

• Q9. Friction between the tires of a car and the road is:
• A. Always harmful
• B. Necessary for the car to move
• C. Only present when the car is stationary
• D. Zero when the car is moving

• Correct Answer: B
• Explanation: The text states that friction between tire and road helps to drive cars. It is necessary for motion.

• Q10. Which of the following is a disadvantage of friction?
• A. It helps us walk
• B. It produces wear and tear
• C. It holds screws in wood
• D. It allows cars to drive

• Correct Answer: B
• Explanation: The text lists "produce wear and tear" as a disadvantage of friction.

Frequently Asked Questions (FAQs)

Q1. What is the difference between static and kinetic friction? Answer: Static friction is the force that must be overcome to start an object moving. Kinetic (sliding) friction is the force that opposes the motion of an already moving object. Static friction is generally greater than kinetic friction.

Q2. Why is rolling friction smaller than sliding friction? Answer: Rolling friction is smaller because the object rolls over the surface rather than sliding. The contact area is smaller, and the interlocking of microscopic irregularities is less, reducing the resistance.

Q3. How does friction affect energy? Answer: Friction converts mechanical energy into thermal energy (heat). This is why rubbing your hands together makes them warm. In machines, friction causes energy loss and wear and tear.

Q4. What is the difference between sliding and fluid friction? Answer: Sliding friction occurs between solid surfaces in contact. Fluid friction (drag) occurs when an object moves through a fluid (liquid or gas). Fluid friction depends on the speed, shape, and size of the object.

Q5. Why is terminal velocity reached? Answer: Terminal velocity is reached when the upward drag force equals the downward weight of the object. At this point, the net force is zero, and the object falls at a constant speed.

Q6. How do ball bearings reduce friction? Answer: Ball bearings convert sliding friction into rolling friction. Since rolling friction is much smaller than sliding friction, the overall friction is reduced.

Q7. What is frictional dissipation? Answer: Frictional dissipation is the process by which mechanical energy is converted into thermal energy (heat) due to friction. This reduces the mechanical energy of the system.

Q8. Is friction always undesirable? Answer: No. Friction is essential for many everyday activities, such as walking, driving, and holding objects. Without friction, we couldn't move or grip anything. However, in machines, friction can cause energy loss and wear, so it is often reduced.

You: