Force, Friction and Pressure

Overview & Fundamental Concept

Complete AISSEE Class 9 General Science guide for Force, Friction and Pressure (Topic 5). Features pointwise mechanics rules, friction hierarchy tables, pressure calculation models, Magdeburg hemisphere history, and 10 exam-standard MCQs.

• Force Definition: A push or a pull exerted on an object resulting from its interaction with another object. Force has both magnitude and direction, making it a vector quantity.
• SI Unit of Force: The SI unit of force is the Newton (N). 1 Newton is the force required to accelerate a 1 kg mass by $1\text{ m/s}^2$. (In CGS system, 1 Newton = $10^5\text{ dynes}$).
• Effects of Force: A force can:
  • Make a stationary object move.
  • Stop a moving object.
  • Change the speed of a moving object (speed up or slow down).
  • Change the direction of motion of an object.
  • Change the shape or size of an object (e.g. stretching a spring, rolling dough).
• Resultant (Net) Force Rules:
  • If two forces act on an object in the same direction, the net force is their sum: $F_{net} = F_1 + F_2$.
  • If two forces act in opposite directions, the net force is their difference: $F_{net} = |F_1 - F_2|$, directed toward the larger force.
  • If two equal forces act in opposite directions, net force is Zero (Balanced forces → no change in state of motion).
• Contact Forces: Forces that can act only when there is physical contact between two interacting objects.
  • 1. Muscular Force: Force exerted by the muscles of human or animal bodies (walking, lifting, pulling carts).
  • 2. Frictional Force: Force that opposes relative motion between two surfaces in contact.
  • 3. Mechanical / Normal Force: Force exerted by machines or supporting surfaces.
• Non-Contact Forces: Forces that can act between two objects even when they are separated by empty space (action-at-a-distance).
  • 1. Gravitational Force: Universal attractive force exerted by any mass on any other mass (e.g. Earth attracting fallen apples). Always attractive.
  • 2. Electrostatic Force: Force exerted by a charged body on another charged or uncharged body (e.g. plastic comb rubbed with dry hair attracting bits of paper). Can be attractive or repulsive.
  • 3. Magnetic Force: Force exerted by magnets on magnetic materials or other magnets (Like poles repel; unlike poles attract).

• Frictional Force: The opposing force that comes into play whenever two surfaces in contact slide or roll, or tend to slide or roll, over each other. It always acts in a direction opposite to the direction of motion.
• Microscopic Cause of Friction: Caused by the interlocking of microscopic irregularities (ridges and grooves) present on the two contacting surfaces. Even highly polished smooth surfaces have microscopic rough peaks. When pressed together, these irregularities interlock.
• Factors Affecting Friction:
  • 1. Nature of the Surfaces: Rough surfaces have deeper irregularities, producing much higher friction than smooth polished surfaces.
  • 2. Normal Force (Pressing Weight): Friction increases as the two surfaces are pressed harder together ($F_f \propto N$). A heavy wooden crate experiences far more friction than an empty crate.
  • Crucial Exam Fact: Friction is INDEPENDENT of the apparent area of contact for ordinary solid surfaces.
• Types of Friction:
  • 1. Static Friction ($f_s$): The opposing force that balances the applied force while the body remains at rest. It is a self-adjusting force that increases exactly equal to the applied force until a maximum threshold is reached.
  • 2. Limiting Friction ($f_{\text{lim}}$): The maximum value of static friction just before the body begins to slide over the surface.
  • 3. Sliding Friction ($f_k$): The opposing frictional force that comes into play when an object is continuously sliding over another surface. Sliding friction is always slightly less than limiting static friction because the irregularities do not get enough time to interlock firmly.
  • 4. Rolling Friction ($f_r$): The opposing force when an object rolls over the surface of another body (e.g. wheels, rollers, ball bearings). It is drastically smaller than sliding friction.
• The Great Friction Inequality Law:
  $$\text{Rolling Friction} < \text{Sliding Friction} < \text{Static (Limiting) Friction}$$

• Friction as a 'Necessary Evil':
  • Why Necessary? We cannot walk without friction (feet would slip backward), vehicles cannot brake or steer on roads, nails/screws cannot hold into walls, and pens cannot write on paper.
  • Why Evil? Causes energy loss as useless heat, causes wear and tear in machine gears, sole of shoes, and vehicle tyres.
• Methods of Increasing Friction:
  • Grooving tyres of vehicles (treads provide better road grip).
  • Spikes in the soles of athletes' and cricketers' shoes.
  • Kabaddi players rubbing dry soil on hands for better grip.
  • Gymnasts applying chalk/magnesium carbonate powder on hands.
  • Brake pads in bicycles pressing against wheel rims.
• Methods of Decreasing Friction:
  • Lubricants: Substances like oil, grease, or graphite applied between moving machine parts to form a thin film that prevents direct metal-to-metal interlocking.
  • Ball Bearings: Fitted between wheels and axles of bicycles, fans, and dynamos to convert sliding friction into rolling friction.
  • Talcum Powder: Sprinkled on carrom boards to smoothen the surface.
• Fluid Friction (Drag): The resistive frictional force exerted by fluids (liquids and gases) on objects moving through them.
  • Factors Affecting Drag: (1) Speed of the object relative to fluid, (2) Shape of the object, and (3) Viscosity of the fluid.
  • Streamlined Shape: A special tapered shape (pointed at front and back, rounded in middle) modeled after fish and birds that minimizes fluid drag. Used in aeroplanes, submarines, rockets, and high-speed sports cars.

• Pressure Definition: The perpendicular force (thrust) acting on a unit surface area of an object.
  $$\text{Pressure } (P) = \frac{\text{Force / Thrust } (F)}{\text{Area } (A)}$$
• SI Unit of Pressure: The Pascal (Pa), where $1\text{ Pa} = 1\text{ Newton per square metre } (\text{N/m}^2)$.
• Inverse Area Rule: For a constant force, Pressure is inversely proportional to Area ($P \propto 1/A$).
  • Small Area → Massive Pressure: Sharp sewing needles, pointed nails, and sharpened kitchen knives penetrate easily.
  • Large Area → Low Pressure: Wide straps on school bags reduce pressure on shoulders; heavy army tanks ride on continuous wide tracks (caterpillar tracks) to avoid sinking into soft mud; camel feet are broad to walk on desert sand.
• Liquid Pressure Laws:
  • 1. Depth Dependence: Liquid pressure increases directly with depth ($P = h\rho g$). Engineering consequence: Dam walls are constructed much thicker at the bottom than at the top to withstand the tremendous hydrostatic pressure at depth.
  • 2. Uniform Direction: Liquids exert equal pressure at the same depth in all directions.
  • 3. Wall Pressure: Liquids exert lateral pressure on the side walls of their container.
• Atmospheric Pressure: The pressure exerted by the weight of the blanket of air (atmosphere) surrounding the Earth.
  • Magnitude: At sea level, atmospheric pressure is approximately $100,000\text{ Pa}$ (or $10^5\text{ N/m}^2$ / $1\text{ bar}$), equivalent to a weight of 1 kg pressing on every square centimetre!
  • Why aren't we crushed? Because the blood pressure and internal fluids inside our bodies exert an equal outward pressure that perfectly balances the atmospheric pressure.
  • Otto von Guericke (1654): German scientist who invented the vacuum pump. Performed the famous Magdeburg Hemispheres Experiment using two hollow metallic hemispheres of 51 cm diameter pumped free of air; 16 horses (8 on each side) could not pull them apart against atmospheric pressure!