FRICTION

          When an object is in contact with a surface, the surface exerts a contact force on the object. The component of the contact force that's parallel to the surface is called the friction force on the object. Friction, like the normal force, arise from electrical interactions between atoms that make up the object and those that make up the surface. 
      we'll look at two main categories of friction: (1) static friction and (2) kinetic (sliding) friction. If you attempt to push a heavy crate across a floor, at first you meet with resistance, but then you push hard enough to get the create moving. The force that acted on the crate to cancel out your initial pushes was static friction, and the force that acts on the create as it sides across the floor is kinetic friction. static friction occurs when there is no relative motion between the object and the surface ( no sliding); kinetic friction occurs when there is relative motion )when there's sliding).
     The strength of the friction force  depends, in general, on two tho things: the nature of the surface and the strength of the normal force. The nature of the surfaces is represent by the coefficient of friction, denoted by μ (mu). The greater this number is, the stronger the friction force will be. For example, the coefficient or friction between rubber-soled shoes and a wooden floor is 0.7, but between rubber-soled shoes and ice, it's only 0.1. Also, since kinetic friction is generally weaker than static friction (it's easier to keep an object sliding once it's sliding than it is to start the object sliding in the first place), there are to coefficient of friction: one for static friction(μs) and one for kinetic friction(μk). For a given pair of surfaces, its virtually always true that  μk < μs . The magnitude of these two types of friction forces are given by the following equations:




           Fstatic friction, max = μsFN

                  Fkinetic friction = μkFN

Notice that the equation for the magnitude of the static friction force is for the maximum value. This is because static friction can vary, counteracting weaker forces that are less than the minimum force required to move an object. For example, suppose an object feels a normal force of F=100N, and the coefficient of static friction between it and the surface it's on is 0.5 then, the maximum force that static friction  can exert is (0.5)(100N) = 50N. However, if you push on the object with a force of, say 20N, then the static friction force will be 20N( in the opposite direction), not 50N: The object won't move. The net force on a stationary object must be zero. static friction can take on all values, up to a certain maximum, and you must overcome the maximum static friction force to get the object ti slide. The direction of Fkinetic friction  = Ff (kinetic) is opposite to that of motion (sliding), and the direction of Fstatic friction = Ff (static) is opposite to that of the intended motion.

THE NORMAL FORCE

       When an object is in contact with a surface, the surface exerts a content force on the object. the component of the contact force that's perpendicular  to the surface is called the normal force on the object. (In physics, the word normal means perpendicular.) The normal force is what prevents objects from falling through tabletops or you from falling through the floor. The normal force is denoted by F or simply by N. ( If you use the letter notation, be careful not to confuse it with N, the abbreviation for the newton.)

NEWTON'S LAWS

THE FIRST LAW


Newton's First  Law says that an object will continue in its state of motion unless compelled to change by a force impressed upon it. If the object is at rest, then it will stay at rest, and if it is moving, then it will continue to move at a constant speed in a straight line.
            Basically, no force means no change in velocity. This property of objects, their natural resistance to changes in their state of motion, is called Inertia. In fact, the First Law is often referred to as the Law of Inertia.


THE SECOND LAW


Newton's second Law predicts what will happen when a force does act on an object: The object's velocity will change; the object will accelerate. More precisely, it says that its acceleration (a), will be directly proportional to the magnitude of the total-or net-force(F) and inversely proportional to the object's mass(m):


                                      F = ma
  This is the most important equation in mechanics!
  
THE THIRD LAW


This is the law that's commonly remembered as, to every action, there is an equal, but opposite, reaction. More precisely, if object 1 exerts a force on object2, then object 2 exerts a force back on object 1, equal in strength but in the opposite direction. These two forces, F(1-on-2) and  F(2-on-1), are called an action/reaction pair.

Physics syllabus

Physics grade 12,13 Sinhala
Physics grade 12,13 English