Friction :
We will learn that Whenever an object moves or tends to move on a rough surface, the component of contact force parallel to surface resists the relative motion of the object with respect to surface in contact. This property of the surface is friction and the resistive force is the frictional force. Experiments show that the frictional force arises from the nature of the two surfaces: because of their roughness, contact is made only at a few locations where peaks of the material touch.
The microscopic area of contact for these spots is substantially less than the apparent macroscopic area of contact between the surfaces-perhaps thousands of times less. At these contact points, the molecules of different bodies are close enough together to exert strong attractive intermolecular forces on one another, leading to what are known as "cold welds.
" At these locations, the frictional force arises in part the opposing surface and in part from chemical bonding ("spot because one peak physically blocks the motion of a peak from welds") of opposing peaks as they come into contact. Although the details of friction are quite complex at the atomic level, this force ultimately involves an electrical interaction between atoms or molecules. Hence, we can say that frictional forces oppose the tendency of contacting surfaces to slip one relative to the other.
Friction Force Introduction
Friction Force Introduction :
Actually, whenever the surface of one body slides or has a tendency to slide on the surface of another then each body exerts a frictional force on the other, parallel to the surfaces. The frictional force on each body is in a direction opposite to its motion relative to the other body. Frictional forces automatically oppose the relative motion and never aid it. Even when there is no relative motion, frictional forces may exist between surfaces (but there must be a tendency of relative motion).
Consider a block at rest on a horizontal table as shown in figure. We find that the block will not move even though we apply a small force. We say that our applied force is balanced by an opposite frictional force exerted on the block by the table, acting along the surface of contact. As we increase the applied force we find that there is some definite value of the applied force at which the block just begins to move.
Once motion has started, this same force (without increasing any further) produces accelerated motion. By reducing the force once motion has started, we find that it is possible to keep the block in uniform motion without acceleration; this force may be small, but it is never zero. Now, analyze the diagrams given in figure once again. In Figure (a), no force is applied on the block due to which it does not have tendency to slide over the surface of the table and hence, the surface does not apply any opposing (i.e., frictional) force on the block.
In Figure (b), we have applied a small force on the block and its magnitude is being continuously increased. It is clear from the second, third and fourth figures, that as we are increasing external force, frictional force acting upon the block also increases and prevents the block from sliding over the surface of the table. But at a certain instant friction reaches its maximum value, referred to as limiting friction, and now if we increase the external force, even slightly, the block will start sliding in the direction of the external force.
We can also say that now the external force has sufficient magnitude to break molecular bonds between the surfaces In Figure (d), frictional force has already reached its maximum value (i.e., its limiting value) and is just balancing the horizontal force
F
applied on the block. If
F
is increased further by even a negligible amount, the block starts sliding. Once the body has started sliding it is observed that the magnitude of friction (or we can say the magnitude of kinetic friction) is constant and is smaller than limiting friction (i.e., maximum value of static friction). We can write
The frictional forces acting between surfaces at rest with respect to each other are called forces of static friction. The maximum force of static friction will be the same as the smallest force necessary to start motion. Once motion has started, the frictional forces acting between the surfaces usually decrease so that a smaller force is necessary to maintain uniform motion. The forces acting between surfaces in relative motion are called forces of kinetic friction.
Opposite Frictional Force
Opposite Frictional Force :
Note that in figure the block, of course exerts an equal and opposite frictional force on the table (as shown in figure), tending to drag it in the direction of the horizontal force we exert. This frictional force is due to the bonding of the molecules of the block and the table at the places where the surfaces are in very close contact. If we focus on the table only, we see that friction tends to move the table in the same direction in which force
F
tends to move the block. This observation leads us to two very important conclusions.
(a)
friction does not always oppose motion (because it is friction only which is trying to move the table)
(b)
friction always opposes relative motion between surfaces in contact (because it is trying to move the table in the same direction in which the block has a tendency to move).
Reasons For Friction
(a)
Frictional forces arise due to molecular interactions due to which a bonding between the molecules of the two surfaces or objects in contact comes into being due to which it becomes difficult to move one surface on the other.
(b)
Inter locking of extended parts of one object into the extended parts of the other object
Regarding The Frictional Force (F) Following Points Are Worth noting
-
It is the tangential component of net contact force (
F
) acting between two bodies in contact.
-
It starts acting when there is tendency of relative motion (different velocities) between two bodies in contact or actual relative motion takes place. So, friction has a tendency to stop relative motion between two bodies in contact.
-
If there is only tendency of relative motion then static friction acts and if actual relative motion takes place, then kinetic friction acts.
-
Friction force is electromagnetic in nature.
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The surfaces in contact, however smooth they may appear, actually have imperfections called asperities.
-
When one surface rests on the other the actual area of contact is very less than the surface area of the face of contact.
-
The pressure due to the reaction force between the surfaces is very high as the true contact area is very small.
-
Hence, these contact points deform a little and cold welds are formed at these points.
So, in order to start the relative sliding between these surfaces, enough force has to be applied to break these
welds. But, once the welds break and the surfaces start sliding over each other, the further formation of these welds is relatively slow and weak and hence a smaller force is enough to keep the block moving with uniform velocity.
This is the reason why limiting value of static friction is greater than the kinetic friction.
