Friction and viscosity

Friction

Friction is a force that acts at the interface of two surfaces in contact, opposing their relative motion.

Types of Friction

1. Static or Limiting Friction: The maximum friction that must be overcome for a body to start moving.
2. Dynamic or Sliding Friction: The friction that must be overcome to keep a body in motion, also known as Kinetic Friction.

Static Friction

Static friction is the frictional force that acts between two surfaces when they are at rest relative to each other. It is the maximum force that must be overcome before an object can start moving over another surface. Static friction prevents an object from moving when placed on a surface.

For example, static friction keeps a parked car from sliding down a hill. The magnitude of static friction is equal and opposite to the applied force when a small force is exerted. However, as the applied force increases, a point is reached where the maximum static friction is exceeded, allowing motion to begin. Static friction varies based on the force applied but has a definite maximum value.

Examples of Static Friction

• A pencil resting on a desk.
• Rubbing palms together to create heat.
• A washing machine being pushed along the floor.
• Skis pressing against snow before sliding.

Coefficient of Static Friction

For static friction, the force can vary up to a certain maximum value, beyond which the object starts to move. Maximum static friction is reached when the inequality becomes an equality, at which point kinetic friction takes over as the object begins to move.

The maximum force of static friction is given by:

\[ F_{s_{\max}} = \mu_s N \] \[ F_s \leq \mu_s N \]

Where:

• \( F_{s_{\max}} \) = maximum force of static friction
• \( F_s \) = force of static friction
• \( \mu_s \) = coefficient of static friction
• \( N \) = normal force between the surfaces

Kinetic Friction

Kinetic friction, also known as dynamic friction, is the force that acts between moving surfaces. It opposes the motion of two or more objects sliding against each other. Kinetic friction is present in everyday situations and plays a crucial role in motion.

For example, when a car needs to stop, applying brakes generates friction that slows it down. Similarly, when walking, friction helps us come to a halt. However, if we try to stop in the middle of a puddle, reduced friction makes it harder to stop. In general, static friction is greater than kinetic friction.

Coefficient of Kinetic Friction

The force of kinetic friction is given by:

\[ F_k = \mu_k N \]

Where:

• \( F_k \) = force of kinetic friction
• \( \mu_k \) = coefficient of kinetic friction
• \( N \) = normal force

Laws of Friction

1. Frictional force opposes the relative motion between two solid surfaces.
2. Frictional force is independent of the area of contact.
3. Limiting friction is directly proportional to the normal reaction.
4. Frictional force depends on the nature of the surfaces in contact.
5. Frictional force is independent of the relative velocity of the surfaces in contact.

Advantages of Friction

1. Prevents slipping while walking.
2. Provides air resistance for parachutes.
3. Holds nails, screws, and nuts in place.
4. Keeps car tires firmly on the road.

Disadvantages of Friction

1. Causes wear and tear in machine parts.
2. Wastes kinetic energy by converting it into heat, reducing efficiency.
3. Produces unwanted noise.

Ways to Reduce Friction

1. Lubricating moving parts.
2. Using rollers or ball bearings.
3. Streamlining objects.
4. Smoothing surfaces.
5. Polishing surfaces.

Viscosity

Viscosity is the internal friction that exists between layers of molecules in a moving fluid (liquid or gas). It is a measure of a fluid's resistance to flow and is expressed in pascal-seconds (Pa.s). Viscosity is a vector quantity and can be mathematically defined as:

\[ \eta = \frac{\text{Force}}{\text{Area} \times \text{Velocity Gradient}} \]

Velocity Gradient

The velocity gradient is given by:

\[ \text{Velocity Gradient} = \frac{\text{Velocity}}{\text{Length}} \]

Forces Acting on a Falling Object in a Fluid

At terminal velocity:

\[ W = U + V \]

Rearranging:

\[ V = W - U \]

where:

• W - Weight of the object
• U - Upthrust (buoyant force)
• V - Viscous force

Examples of High and Low Viscosity Substances

Low Viscosity: Water, kerosene, petrol, ethanol

High Viscosity: Glue, syrup, grease, glycerine

Experiment: Determination of Terminal Velocity

Aim

To determine the terminal velocity of a steel ball falling through a jar of glycerine.

Apparatus

• Steel ball
• Cylindrical calibrated jar
• Glycerine

Conclusion

Terminal velocity is reached when the net force acting on the object becomes zero, meaning:

\[ W = V + U \]

At this point, the object moves at a constant speed through the fluid.

Precautions

• The steel ball should be gently released into the liquid.
• The experiment should be conducted at a constant temperature.
• Measurements should be taken carefully to avoid errors.

Terminal Velocity and Drag Force

Terminal velocity is the maximum velocity an object can attain when the viscous force balances the apparent weight of the object in the fluid.

Drag force is the force that keeps the object moving after reaching terminal velocity.

Stokes' Law

Stokes' law states that at terminal velocity, the frictional force acting on a sphere moving through a viscous fluid is given by:

\[ F = 6\pi \eta r V_t \]

where:

• F - Frictional (Drag) force
• \( \eta \) - Viscosity of the fluid
• r - Radius of the sphere
• V_t - Terminal velocity

Factors Affecting Viscosity

Several factors influence viscosity, including:

1. Surface Area of the Body: A smaller surface area facing the fluid experiences less friction, allowing the fluid to cut through more easily. For example, a horizontally moving body moves through a liquid more smoothly than a vertically moving body.
2. Temperature: An increase in temperature reduces viscosity.
3. Nature of the Fluid: Thinner fluids have lower friction. For instance, water has less viscosity compared to honey.
4. Shape of the Body: Objects with streamlined shapes experience less viscosity, making them more efficient in fluid environments.
5. Speed of the Body: viscosity increases with speed, meaning that higher speeds result in greater resistance.
6. Viscosity of the Fluid: viscosity is directly proportional to the viscosity (thickness) of the liquid. Denser fluids create more resistance.

Examples and Applications of Fluid Friction

1. A submarine moves through water, experiencing external fluid friction.
2. Lubricants are used in hinges to reduce friction.
3. A seagull soaring through the air experiences fluid friction.
4. Honey's viscosity affects its fluid friction.
5. When a wet surface exists between two thin glass plates, they stick together, preventing the bottom plate from falling when holding only the top plate.
6. Ink flows smoothly in ball pens due to controlled fluid friction.
7. Air resistance slows down falling objects.
8. Lighter dust particles move quickly on the surface of a flowing river due to a high-velocity gradient and lower dynamic fluid friction at the top layer of the water.
9. When an object is dropped in a fluid, the size of its splash depends on fluid friction.
10. Water slides use fluid friction to ensure a gentle descent.