WBCS Preliminary(Physics):Mechanics

• Motion: In physics, motion is change of location or position of an object with respect to time. Mechanical motion is of two types, transitional ( linear ) and rotational ( spin).
• SPEED: The speed of a moving body is the rate at which it covers distance i.e. the distance it covers per unit of time.
• Speed: (distance travelled/ time required.) The S.I. Unit of speed is m/s.
• VELOCITY: The distance covered by an object in a specified direction in unit time interval is called velocity. The S.I. Unit of velocity is m/s.
• Average velocity can be calculated by dividing displacement over time.
• The instantaneous velocity shows the velocity of an object at one point.
• The difference betwwn speed and velocity is: Speed is the distance travelled by an object in a particular time. Velocity is the speed in a particular direction.
• ACCELERATION: When an object's velocity changes, it accelerates. Acceleration shows the change in velocity in a unit time. Velocity is measured in meters per second, m/s, so acceleration is measured in (m/s)/s, or m/s ², which can be both positive and negative. The symbol for acceleration is a (boldface).
• When the velocity decreases the body is said to undergo retardation or deceleration.
• Acceleration Due to Gravity: Galileo was the first to find out that all objects falling to Earth have a constant acceleration of 9.80 m/s² regardless of their mass. Acceleration due to gravity is given a symbol g, which equals to 9.80 m/s².
• FORCE: Force can be defined as a push or a pull. (Technically, force is something that can accelerate objects.) . Force is measured by N (Newton). A force that causes an object with a mass of 1 kg to accelerate at 1 m/s is equivalent to 1 Newton.
• Newton's law of universal gravitation states that every massive particle in the universe attracts every other massive particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
• In equation form, the gravitational force F = G(m₁ m₂)/ r² where r is the distance between two bodies of masses m₁and m₂ and G the universal gravitational constant. The value of G is 6.67X 10^-11 SI units.
• Centripetal Force: For a body to move in a circle there must be a force on it directed towards the centre. This is called the centripetal force and is necessary to produce continuous change of direction in a circular motion.
• The magnitude of the centripetal force on an object of mass m moving at a speed v along a path with radius of curvature r is given by the relation F = mv²/r The direction of the force is toward the center of the circle in which the object is moving. Centrifugal force is equal and opposite to centripetal force, i.e it acts outwards.
• WEIGHT: the weight of a body is the force with which the earth attracts the body towards its centre. The weight of a body should not be confused with its mass, which is a measure of the quantity of matter contained in it. Mass shows the quantity, and weight shows the size of gravity. The weight of a body is maximum at the poles and minimum at equator.
• If you know your mass, you can easily find your weight because W = mg where:
  • W is weight in Newton (N),
  • m is mass in kg, and
  • g is the acceleration of gravity in m/s².
  Weight is measured by Newton (N).
• It is now obvious that the value of g is maximum at poles and minimum at equator. At the centre of earth, g would be zero.
• It should be noted here that on the surface of the moon the value of the acceleration due to gravity is neraly one-sixth of that on earth, and therefore, an object on the moon would weigh only one-sixth its weight on earth.
• Newton's Laws of Motion:
  1. Newtons First Law of Motion:
     • Newton's first law of motion states that "An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force." . Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
     • In fact, it is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion is described as inertia.
     • Inertia: Inertia is the tendency of an object to resist changes in its state of motion. But what is meant by the phrase state of motion? The state of motion of an object is defined by its velocity - the speed with a direction. Thus, inertia could be redefined as follows:Inertia: tendency of an object to resist changes in its velocity.
     • There are many more applications of Newton's first law of motion.
     • Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.
     • The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.
     • While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object which abruptly halts the motion of the skateboard.
  2. Newton's Second Law of Motion:
     • The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
     • The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.
  3. Newton's Third Law of Motion:
     • For every action, there is an equal and opposite reaction.
     • The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.
     • The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force.
     • There's also the example of shooting a cannonball. When the cannonball is fired through the air (by the explosion), the cannon is pushed backward. The force pushing the ball out was equal to the force pushing the cannon back, but the effect on the cannon is less noticeable because it has a much larger mass. That example is similar to the kick when a gun fires a bullet forward.
• Friction : Friction is a force that resists the movement oof one surface over another. The force acts in the opposite direction to the way an object wants to slide. If a car needs to stop at a stop sign, it slows because of the friction between the brakes and the wheels.
• Measures of friction are based on the type of materials that are in contact. Concrete on concrete has a very high coefficient of friction.That coefficient is a measure of how easily one object moves in relationship to another. When you have a high coefficient of friction, you have a lot of friction between the materials.

RETURN TO:

• SCIENCE INDEX
• PRELIMINARY STUDY MATERIALS INDEX
• HOME PAGE