Physics

• Refraction of light is the bending of an oblique ray of light when it passes from one transparent medium to another of different optical densities
• Refraction occurs because light travels with different speeds in different media
• Refraction of light is a surface phenomenon with partial reflection and partial refraction at the boundary of two media
• When light travels from a rarer medium to a denser medium, it bends towards the normal
• When light travels from a denser medium to a rarer medium, it bends away from the normal
• If a ray of light is incident normally on a surface, it passes undeviated
• Laws of Refraction:

  • Incident ray, normal at the point of incidence, and refracted ray all lie in the same plane
  • The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant, known as the refractive index
• Refractive index of a material depends on the nature of the material, color or wavelength of light used, and physical conditions like temperature
• Principle of Reversibility of Light states that the path of a ray is reversible
• Lateral Displacement is the perpendicular shift in the path of an incident ray emerging out of a glass block
• Objects placed in denser mediums when viewed from rarer mediums appear to be at a depth less than their real depth due to refraction of light
• Prism:

  • A piece of glass or transparent material with two refracting surfaces inclined at an angle
  • The line where two refracting surfaces meet is the refracting edge
  • The angle between the two refracting surfaces is the angle of the prism
  • The angle between the incident ray and emergent ray is the angle of deviation
• Total Internal Reflection is when a ray of light traveling from a denser medium to a rarer medium is reflected back into the denser medium if the angle of incidence is more than the critical angle
• Movable pulley has a mechanical advantage of 2 in an ideal case
• Effort needed to lift a load in a movable pulley is equal to half the load in an ideal situation
• Direction of tension in a string supporting a load is equal in both downward and upward directions
• Block and Tackle System:

  • Consists of two blocks of pulleys: one fixed (upper) and one movable (lower)
  • Load is always hung on the lower block
  • Number of pulleys in the movable lower block is either equal to or one less than the number of pulleys in the upper fixed block
  • Mechanical advantage in the ideal case is equal to the number of pulleys in both blocks
  • Velocity ratio is equal to the number of strands of tackle or sections of the string supporting the load
• For greater efficiency in a block and tackle system, pulleys in the lower block should be light and friction in pulleys should be minimized with lubricants
• Gears:

  • Gears are toothed wheels transferring rotational motion
  • Driver gear is closer to the power source
  • Driven gear receives motion from the driver
  • External contact between gears results in opposite rotation, internal contact results in same direction rotation
  • Gear ratio is the ratio of teeth on the driving wheel to the driven wheel
• Work:

  • Work is done when a force causes a displacement of a body
  • Work = Force x Displacement in the direction of force
  • Work done is zero if no motion is produced by a force
  • Work done at an angle to the force direction: Work = Force x Component of displacement in the direction of force x cos(angle)
  • Work done by gravity: W = mgh
  • Work is a scalar quantity with SI unit joule (J)
• Power:

  • Power is the rate of doing work
  • Power = Work done / Time taken
  • Power is a scalar quantity with SI unit watt (W)
• Energy:

  • Energy is the capacity to do work and is a scalar quantity
  • SI unit of energy is joule
  • Kinetic energy = 0.5 x mass x velocity^2
  • Potential energy = mgh
  • Law of conservation of energy states energy can neither be created nor destroyed, only changes form
• The CGS unit of force is dyne
• One dyne is the force that acts on a body of mass 1g and produces an acceleration of 1cm/s^2 in the direction of force
• The gravitational units of force are kgf and gf
• 1 kgf = 1000 gf = 9.8 N and 1 gf = 980 dyne
• Frictional force opposes the relative motion between two surfaces in contact
• The force of friction has three types: static friction, kinetic friction, and rolling friction
• Factors affecting the force of friction include the materials of bodies, the roughness of surfaces, and the weights of bodies
• Disadvantages of friction include energy dissipation in the form of heat and sound, wear and tear of machine parts, reduced machine efficiency, and seizing of moving parts due to excessive heat
• Unwanted friction can be reduced by using lubricants, polishing, using roller or ball bearings, and streamlining
• Advantages of friction include enabling walking on roads, allowing vehicles to run on roads, aiding in gripping objects, assisting in writing, facilitating the rotation of machine parts, helping in tying knots, and aiding in billiards shots
• Friction can be increased by making surfaces rough, using dry surfaces, and increasing weight
• The motion of a body moving in a straight line in the direction of force is called translational motion
• When a body is pivoted at a point and the applied force rotates it about a certain axis, the motion is called rotational motion
• Turning force is the force that rotates a body about a fixed point
• The moment of force or torque is the turning effect of a force acting on a body about an axis
• The moment of force is measured as the product of force and the perpendicular distance of the point of application of the force from the axis of rotation
• The SI unit of moment of force (torque) is newton metre (N m) and its CGS unit is dyne cm
• The gravitational units of moment of force in MKS and CGS systems are kgf m and gf cm respectively