Force and Motion



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Chapter 14 : Force and Motion



Force arrow_upward


  • Force is the process that cause any object to undergo some sort of changes.
  • Force is the push or pull you apply to make things move.
  • A force can:
    • Make an object move.
    • Change the direction of a moving object.
    • Stop a moving object. 
  • Force can also change the shape of an object.
  • Force is also defined as Mass and acceleration vector product.
  • The ratio of speed over time gives Acceleration of the motion.

  • Types of force arrow_upward



    Applied Force

  • An applied force is a force that is applied to an object by a person or another object.
  • If a person is pushing a desk across the room, then there is applied force acting upon the object.

  • Gravitational Force

  • The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself.
  • This is the weight of the object.
  • All objects upon earth experience a force of gravity.
  • It is directed "downward" towards the center of the earth.

  • Normal Force

  • The normal force is the support force exerted upon an object that is in contact with another stable object.
  • On occasions, a normal force is exerted horizontally between two objects that are in contact with each other.
  • For instance, if a person leans against a wall, the wall pushes horizontally on the person.

  • Friction Force

  • The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it.
  • The friction force often opposes the motion of an object.

  • Tension Force

  • The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends.
  • The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.

  • Balanced Forces arrow_upward


  • Two equal and opposite forces applied on an object are called balanced forces.
  • For example pushing and pulling the object with same force on both sides as shown in the figure.
  • Forces are shown by arrows in diagram. The direction of the arrow shows the direction in which the force is acting.
  • If two forces are balanced, it means that both the forces are equally applied but are acting in opposite directions.
  • Balanced forces do not change motion
  • If two balanced forces are acting on an object, that object will not change its motion. If it is still, it will stay still.
  • If it is moving, it will continue moving, in the same direction and at the same speed.

  • Unbalanced forces arrow_upward


  • Unbalanced forces do change the way something is moving.
  • Unbalanced forces can make objects start to move, speed up, slow down, or change direction.

  • Friction arrow_upward


  • The friction force acts in a direction parallel to the area of contact, and opposes the motion or the tendency to move.
  • There are two types of friction force:
    • Sliding friction
    • Static friction
  • Friction results when the two surfaces are being pressed together closely.
  • It is because of intermolecular attractive forces between molecules of different surfaces.
  • As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together.

  • Sliding Friction

  • Sliding friction results when an object slides across a surface.
  • Sliding friction results when object is in motion so it is also known as Kinetic Friction.

  • Static Friction

  • Friction forces can also exist when the two surfaces are not sliding across each other. Such friction forces are referred to as static friction.
  • Static friction results when the surfaces of two objects are at rest relative to one another and a force exists on one of the objects to set it into motion relative to the other object.

  • Motion arrow_upward


  • Motion is a change in position of an object with respect to time.

  • Kinds of Motion

  • We can classify the motion of bodies as:
    • Translatory (linear)
    • Rotatory
    • Vibratory or Oscillatory
    • Circular
    • Periodic
    • Random

    Translatory Motion

  • The motion of an object is said to be translatory if the position of the object is changing with respect to a fixed point or object.
  • Example:
    • A boy running
    • A car moving

    Rotatory Motion

  • Motion is said to be rotatory when the object rotates on its own axis
  • Example:
    • Spin Top
    • CD or DVD playing

    Oscillatory Motion

  • Oscillatory Motion is repetitive and fluctuates between two locations.
  • The to and fro motion of an object about a fixed point is called oscillatory motion or vibratory motion.
  • Example:
    • Seesaws
    • Clock pendulum

    Circular Motion

  • A motion in which the body traverses a circular path is called Circular Motion.
  • This is a kind of translatory motion where the body reaches the initial position each time it completes traversing the circle.
  • Example:
    • Hands of a clock

    Periodic Motion

  • Any motion that repeats itself at regular intervals of time is called Periodic Motion.
  • Example:
    • Heart beat
    • A water wave

    Random Motion

  • Irregular Motion of bodies changing the nature of motion frequently is called Random Motion.
  • Example:
    • Motion of football player on the ground
    • A flying butterfly

    Newton's Laws of Motion arrow_upward



    Newton's First Law of Motion

  • An object at rest will remain at rest unless acted on by an unbalanced force.
  • An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
  • This law is often called "the law of inertia".

  • Newton's Second Law of Motion

  • Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).
  • It can be expressed as a mathematical equation:
  • Force=Mass × Acceleration


    Newton's Third Law of Motion

  • For every action there is an equal and opposite re-action.


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