Q1. Define the force of friction.

The force of friction is a resistive force that opposes the relative motion between two surfaces in contact. It acts parallel to the contact surface and arises due to microscopic irregularities on the surfaces. Friction can be static prevents motion from starting, sliding acts against moving objects, or rolling resists rolling motion. It depends on:

• Nature of surfaces roughness increases friction.
• Normal force more weight = more friction.
  Friction is essential for walking, driving, and holding objects but also causes wear and energy loss.

2. Why do athletes use shoes with spikes?

Athletic shoes have spikes to increase friction between the shoe and the ground. Spikes dig into the track or field, providing better grip and preventing slipping during rapid movements. This is crucial for:

• Sprinters to push off forcefully.
• Long jumpers to avoid sliding at takeoff.
  Spikes convert sliding friction into static friction, enhancing performance. Without spikes, athletes might lose traction, especially on wet or grassy surfaces.

Long Answer Questions 300-400 words

1. Explain the disadvantages of friction and methods to reduce it.

Disadvantages:

1. Energy Loss: Friction converts kinetic energy into heat e.g., engine parts overheating.
2. Wear and Tear: Moving parts in machines wear out faster e.g., brake pads, soles of shoes.
3. Reduced Efficiency: Friction opposes motion, requiring extra force/energy e.g., cars burn more fuel to overcome air resistance.

Methods to Reduce Friction:

1. Lubrication: Oils/grease fill surface gaps, forming a slippery layer e.g., engine oil in vehicles.
2. Polishing: Smooth surfaces minimize irregularities e.g., marble floors.
3. Ball Bearings: Replace sliding friction with rolling friction e.g., in ceiling fans.
4. Streamlining: Aerodynamic shapes reduce air/water drag e.g., airplanes, submarines.

Example: In bicycles, chain lubrication reduces friction, ensuring smooth pedaling and longer part life.

2. What is streamlining? How does it help ships and boats?

Answer. Streamlining is the design of smooth, tapered shapes to minimize fluid friction drag when objects move through air or water.

Application in Ships/Boats:

1. Reduced Drag: A streamlined hull cuts through water efficiently, decreasing resistance.
2. Fuel Efficiency: Less drag means engines expend less energy, saving fuel.
3. Speed: Ships achieve higher speeds with streamlined designs e.g., speedboats.
4. Stability: Tapered fronts prevent turbulent water flow, improving balance.

Scientific Principle: Streamlining aligns with Bernoulli’s principle—fluid pressure decreases with increased speed, allowing smoother motion.

Example: Submarines use torpedo-shaped bodies to navigate underwater with minimal resistance.