REASONS FOR FRICTION (i) Inter-locking of extended parts of one object into the extended parts of the other object. (i) Bonding between the molecules of the two surfaces or objects in contact. FRICTION FORCE IS OF TWO TYPES. a. Kinetic (a) Kinetic Friction Force b. Static Kinetic friction exists between two contact surfaces only when there is relative motion between the two contact surfaces. It stops acting when relative motion between two surfaces ceases. DIRECTION OF KINECTIC FRICTION ON AN OBJECT It is opposite to the relative velocity of the object with respect to the other object in contact considered Note that its direction is not opposite to the force applied it is opposite to the relative motion of the body considered which is in contact with the other surface.
STATIC FRICTION It exists between the two surfaces when there is tendency of relative motion but no relative motion along the two contact surface. For example consider a bed inside a room; when we gently push the bed with a finger, the bed does not move. This means that the bed has a tendency to move in the direction of applied force but does not move as there exists static friction force acting in the opposite direction of the applied force.
FRICTION When two bodies are kept in contact, electromagnetic forces act between the charged particles (molecules) at the surfaces of the bodies. Thus, each body exerts a contact force on the other. The magnitudes of the contact forces acting on the two bodies are equal but their directions are opposite and therefore the contact forces obey Newton's third law N normal force R contact force f friction The direction of the contact force acting on a particular body is not necessarily perpendicular to the contact surface. We can resolve this contact force into two components, one perpendicular to the contact surface and the other parallel to it (figure. The perpendicular component is called the normal contact force or normal force (generally written as N) and the parallel component is called friction (generally written as f Therefore if R is contact force then
Find the direction of kinetic friction force (a) on the block, exerted by the ground. (b) on the ground, exerted by the block w.r.t to ground 5m/s w.r.t to block .f where f, and f2 are the friction forces on the block and ground respectively. In above example correct relation between magnitude off, and f2 is (A),2 (C)f, f2 (B) f2>f, (D) not possible to decide due to insufficient data.
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