LANDSAILING - THE APPLICATION OF THE SCIENCE OF PHYSICS - PART II   Let us now study more important theories of Physics applicable to Landsailing.  Ø      Newton’s First Law Of Motion: It states that every body continues in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise. o       The yacht is in its state of rest till the sail is set at the appropriate angle and the wind – the external force – compels it to move. Ø      Newton’s Second Law Of Motion: It states that the rate of change of momentum of a body is directly proportional to applied force and takes place in the direction of the force. o       The applied force is wind – the body, i.e., the yacht moves in the direction of the wind.  Ø      Newton’s Third Law Of Motion: It states that to every action, there is an equal and opposite reaction. o       The power that pushes it also drags it.   Ø      Center Of Gravity: The center of gravity of a body is the point fixed relative to the body through which the weight of the body acts in whatever position the body may be placed.   If the weight of the yacht is 20 kg, then the wind force may require only half the effort to move it compared to a 40 kg yacht.  The important physical principle which governs yacht sailing is: Ø      Moment of a couple: The moment of a couple is equal to the product of one of the forces and the perpendicular distance between the lines of action of the forces. The moment of a couple is “torque”, (i.e., the force of power that makes something turn around a central point). It is a “vector” (i.e., a quantity that has a direction as well as a size usually represented by an arrow). Its direction is perpendicular to plane of rotation given by the right hand rule (Right hand rule states that if the fingers of the right hand curl round the axis of rotation in the direction of rotation, then the thumb represents the direction of angular displacement vector. The length of the arrow gives the magnitude. (Couple: A pair of equal, unlike, parallel and non-collinear forces acting on a rigid body is called a couple). The body is yacht and the force is wind. Ø      Displacement Vector: If an object changes its position from a point A to B, then the vector will tail A and Tip B and AB is the displacement vector. It may be noted that in its actual motion the object may not have traveled along the straight line A to B at all.                      Also, Angular Velocity, Work Energy Theorem, Principle of Conversion of Energy, Potential Energy (PE), Kinetic Energy, Laws of Friction, Momentum and Conservation of Linear Momentum, etc., are some of the other principles of Physics, which are closely associated with Wind Sailing both on water and on land.