Force and Laws of Motion

• Unbalanced Forces:

  • If a block is pulled from both sides with different forces, the block will not be stationary
  • The forces applied are unequal and opposite to each other
  • The resultant of the forces acting on the block is now not zero as the block will shift
• Balanced Forces:

  • In tug of war games, when both teams pull the rope with equal and opposite forces, the rope remains in place
  • The forces acting on the rope are equal and opposite, resulting in a zero resultant force
  • When you squeeze a rubber ball between the palms of your hands:
  • The shape of the rubber ball changes
  • The forces applied on the ball are equal and opposite
  • The resultant of these forces does not lead to motion, instead, the object gets deformed and continues to be deformed as long as the force is applied
  • This deformation is temporary
• Galileo's observations contradicted Aristotle's belief that the natural state of bodies is rest

  • When a ball rolls down an inclined plane, its speed increases; when rolled up, its speed decreases
  • Galileo observed that a ball on a horizontal plane continues to move with constant speed when no external force or friction acts on it
  • All bodies have a natural tendency to oppose any change in their state of rest or motion
• Inertia is the property of a body that opposes any change in its state of rest or uniform motion along a straight line

  • Inertia is classified into:
  • Inertia of rest: e.g. a passenger leaning backwards in a bus when brakes are applied suddenly
  • Inertia of motion: e.g. a man leaning forward when alighting from a moving train
  • Inertia of direction: e.g. water particles flying off tangentially from a cycle tyre
  • The inertia of a body depends on its mass, with heavier objects possessing more inertia than lighter ones
• First Law of Motion:

  • The first law of motion states that "A body continues to be either in a state of rest or of uniform motion along a straight line unless an external force is applied on it."
• Momentum:

  • It is defined as the product of the mass and velocity of the object or body
  • It is a vector quantity and direction of momentum will be the same as that of velocity
  • It is represented by p. p = mv here, m= mass of the object, v is velocity
  • SI unit=kg m/s
• Newton's Second Law of Motion:

  • The rate of change of momentum is directly proportional to the applied force and takes place in the same direction as the applied force
• Impulse:

  • The mathematical representation of the second law of motion is F = mv−mu/t, Ft = mv - mu
  • When forces are acting on a body for a short interval of time then it is defined as an impulse
  • SI unit of impulse = kg m/s
• Newton's third law of motion states that “To every action, there is an equal and opposite reaction"
• Applications of Law of Conservation of Momentum:

  • The Recoil of a Gun
  • The Motion of a Rocket
  • Rocket Propulsion
• Muscular Force:

  • Muscular forces are the forces exerted by the muscles in the human body
• Gravitational Force:

  • Gravitational force is the force by which the body is attracted towards the earth, pulling the body in a downward direction
• Frictional Force:

  • The force that helps to oppose the motion of a particular object while it is in contact with another object or surface
• Air Resistance:

  • The frictional force exerted on a flying object by the air
• Concept of System:

  • The part of the universe chosen for analysis is known as a system
  • The environment is an umbrella term for everything outside the system
• Conservation of Momentum:

  • The total momentum of an isolated system is always conserved
  • Isolated system: the net external force on the system is zero
• Inertial and Non-Inertial Frames:

  • In a non-inertial frame of reference, Newton’s Laws of Motion do not hold
  • A non-inertial frame of reference is a reference that has undergone acceleration with an inertial frame
  • A frame of reference in which Newton’s Laws hold is known as the inertial frame of reference