Physics General | Science & Technology for UPSC CSE PDF Download

Physics General

• A body is said to be moving with a uniform acceleration when its velocity changes at a uniform rate.
• Acceleration is defined as the rate of change of velocity with time. Uniform acceleration acting on a body means that a continuous external force is acting on the body.
• If the same weight of stones and paper are falling freely on the ground, the stones will reach the ground earlier than the paper. It is because of the friction of air that slows down the piece of paper. But, if the same experiment is performed in absence of air or, in vacuum both the bodies would reach the ground at the same time.
• When a moving train stops suddenly, a man tends to fall forward; or, when a train at rest moves suddenly a man tends to fall backward. A dirty carpet can be cleaned by beating. The above all examples are in accordance with Newton’s First Law of Motion, which states that a body continues in its state of rest or uniform motion unless compelled by some external force to act otherwise.
• The definition and derivation of Force comes from the Newton's Second Law of Motion.
• Launching of rocket vehicles and jet planes follow the Newton’s Third Law of Motion.
• One gram weight is equal to 980 dynes.
• In C.G.S. (Centi-Gram-Second) system, the unit of Force is Dyne and in S.I. or MKS (Metre-Kilo-Second) system, the Newton.
• If a body is allowed to fall from the window of a moving train, the body will fall on the ground after following a parabolic path.
• A hunter should aim his gun to hit the target a little higher than target. Because the bullet moves on a parabolic path. Similarly, projectile has the parabolic path.
• Archimedes’ Principle: When a body is immersed partly or completely in a fluid (liquid or gas), it experiences an upward buoyant force which is equal to the weight of the fluid displaced by it.
• The principle finds application in the floatation of ship, Iceberg, Submarines, Whales or a bucket immersed in water, rise of balloon in air, etc.
• If a piece of ice is in a cylinder containing water, and ice melts completely, the level of water remains unchanged (According to Archimedes’ Principle).
• Hydrometer is an instrument for measuring relative density of liquids. The deeper the hydrometer floats in the liquid, the smaller is the relative density of the liquid. Density of a substance is the mass of a unit volume of that substance.

Relative Density of a substance =

• Kinematics is the branch of mechanics, which is concerned with the phenomenon of motion without reference to mass or force.
• Dynamics is the branch of mechanics which deals with the description of motion in terms of force.
• Brownian movement is the random motion of small particles in a fluid. This irregular motion of particles arises from the thermal motion of the molecules of material in which the particles are present.
• A Scalar is a quantity that has only magnitude. Examples—mass, time, density, temperature, etc.
• A vector is a quantity that has magnitude as well as direction. Examples—linear displacement, linear velocity, force, etc.
• A bicycle brake, see-saw, crow-bar, scissors, etc. work on the principle of first class lever (fulcrum in between the work load and effort).
• Nut cracker, wheel barrow, mango cutter, bar of a boat, etc. work on the principle of lever of second class (load in between the fulcrum and effort and the load is closer to fulcrum than the effort).
• Firetong, forearm of a person holding a load, use of knife, etc. are the examples of lever of third class (effort in between fulcrum and load and the effort is closer to the fulcrum than the load).
• Common balance works on the principle of equality of moments of the forces, because the masses placed in the pans are subject to pulls of gravity. When these forces are equal, as indicated by the horizontal beam, the masses themselves must be equal.
• A body moving in circular path is due to centripetal force. Centripetal force is the necessary radial force to keep the body moving in its circular path.
• When a bucket containing water ‘loops a loop’ water does not fall down when it is at the highest point, because its weight is supported by centrifugal force. Centrifugal (flying from the centre) force is the outward force acting on a body rotating in a circle, round a central point.
• A cyclist moving on circular path bends inward to avoid excessive centrifugal force.
• Newton’s law of gravitation is universal and it states that every particle in the universe attracts every other particle with a force which is directly proportional to the product of the masses of the particle.
• A man inside the lift feels— heavy, when lift is going up ; light, when lift is coming down ; weightless, when lift is falling freely.
• Time period of a simple pendulum T = 2π √//g is an expression for simple harmonic motion.
• A body is said to move in a simple harmonic motion when it oscillates along a line about a central point.
• Length of a second’s pendulum is approximately 100 cm.
• A pendulum vibrating in an evacuated chamber will oscillate forever without change in frequency and amplitude, because the force of friction will be Zero in an evacuated chamber.

• As shown in the figure above, water squirts horizontally out of two small holes in the side of cylinder and the two streams strike the ground at the same point. If the hole is at a height h above the ground and the level of water stands at height H above the ground, then the height of P would be H-h.
• Two Ping-pong balls are suspended as shown in the figure below.

A stream of air is directed in the space between them. The balls will come closer.

• The area of the shaded portion in the given velocity—time graph for an object gives its distance moved during acceleration.

As from the graph, the velocity is changing for a certain period of time after which the velocity is constant. The rate of change of velocity is acceleration.

Velocity *Time = distance (displacement).
Hence, the area of the shaded portion gives the distance travelled during acceleration.

• The diagram shows an experiment done in a laboratory on the earth. With the weight as shown the system is in equilibrium when the angle between the strings is 90o. If the arrangement were set up on the surface of the moon where the gravitational attraction is about one-sixth of earth value, then the lead would remain exactly as on earth.
• Two weights W₁ and W₂ are suspended from the ends of a light string passing over a smooth fixed pulley. If the pulley is pulled up at an acceleration g, then the tension in the string will be

• A vessel containing water is given a constant acceleration ‘a’ towards the right along a straight horizontal path. The following figure will represent the surface of the liquid

• Vessels A and B have the same base area but different shapes. Both are filled with water upto the same height. The thrust on the base will be equal in both. 

Because, Thrust = Pressure/Area or, Thrust = height *density *g/Area; and since the factors are equal for both the vessels, the thrust is same. 

• Hydraulic machines like Hydraulic press, Hydraulic brake and Hydraulic jack are devices that are based on Pascal’s Law.

Pascal’s law states that when pressure is applied to an enclosed liquid or gas, it is transmitted equally in all directions.

• Elasticity : The property of a body or material of resuming its original form and dimensions when the forces acting upon it are removed. If the forces are sufficiently large for the deformation to cause a break in the molecular structure  of the body or, material, it loses its elasticity and the elastic limit is said to have reached. 
• Hook’s law applies only within the elastic limit.
• For an elastic body, the ratio of the stress to strain produced is known as Young’s modulus of elasticity.
• Small liquid drops is due to the fact that liquid tends to have minimum surface area due to surface tension. Surface Tension :  An open surface of a liquid is under a state of tension, causing a tendency for the portions of the surface to separate from each other. The tension is an effect of the forces of attraction existing between the molecules of a liquid. A surface tension exists in any external surface of a liquid.
• Surface tension of water is greater than that of mercury.
• Surface tension of liquid is reduced by dissolving substances in the liquid or, by increase in temperature.
• Soap makes the water easier to wash clothes because it decreases its surface tension.
• Viscosity is the resistance in a liquid to flow. It may be called fluid friction. Stronger the inter molecular forces, higher is the viscosity. When the temperature is raised the viscosity of liquid decreases. This is because increase in temperature increases the average kinetic energy of molecules which overcomes the attractive forces between them.
• According to Bernoulie’s Principle when the speed of the fluid (liquid or gas) in a pipe line increases, the pressure decreases and conversely when the speed of the fluid decreases, the pressure increases.
• Applications of Bernoulie’s Principle

(i) Filter Pump or Aspirator, (ii) Atomizer or Sprayer, (iii) The Bunsen Burner, (iv) Lifting a disc (v) Magus effect (vi) Streamlining, (vii) Lift on an aircraft wing, etc.

• Two cars passing each other at high speed, are in danger of side swiping because decrease in air pressure in the space between them (Bernoulie’s Principle).