Contents
Physics Topics can help us understand the behavior of the natural world around us.
What is Friction? What is Meant by Limiting Friction?
Introduction
It is a common experience that, when a book lying on a table is pushed, it moves some distance and then stops. But according to Newton’s first law of motion, after being pushed, the book should have continued moving with a uniform velocity in absence of any external force. The fact that the book stops, clearly shows the existence of some external force which affects the motion of the body.
This force is the force of friction, or simply friction, or dry friction. Also, if the book is pushed slightly, it often does not move at all. If the force on the book is gradually increased, it begins to move after a while. So, initially the frictional force can balance the applied force. But, later, it is not enough to resist the motion.
A large number of phenomena in our daily lives, prove the existence of frictional force. Discussions in this chapter will be restricted to friction between two dry surfaces only.
Definition: When two surfaces are in contact with each other and there exists a relative motion between them, or an attempt is made to impart a relative motion, then a force comes into play that resists this motion or this attempt. This force is called the force of friction, or simply, friction.
The respective areas of the two bodies which remain in contact with each other are called the surfaces of contact [Fig.(a)]. Friction acts parallel to the surfaces of contact in a direction opposite to the direction of relative motion that is attempted or actually occurs.
Origin of Friction
From our experience we find that, friction between two smooth surfaces is less than that between two rough surfaces. An apparently smooth surface when examined under a powerful microscope, also shows considerable roughness on the atomic scale [Fig.(c)], At the points of contact A1, A2, A3, ………. atoms of the two surfaces come very close to each other and inter atomic forces act between them. Hence, adhesion takes place between the planes. Such adhesion is the source of friction. Also, due to the relative motion between the surfaces, the shape and size of the grooves change continuously. This creates waves and atomic motion which also give rise to friction.
Static And Kinetic Friction
Coulomb, a famous scientist in the eighteenth century was the first to mention static and kinetic (or sliding) friction, separately. Let a wooden block of weight W rest on a horizontal table. Weight W of the block acts on the table vertically downwards [Fig.]; by Newton’s third law of motion the table also exerts an equal and opposite reaction force R. Force R is called normal force or normal reaction. If no other force acts on the system, weight and normal force balance each other and the block rests on the table.
Let a horizontal force F be applied on the block. If F is below a certain magnitude, the block remains at rest. At the surface of contact, and opposite to the direction of F, a frictional force f develops, which opposes the motion of the block. In the table below, the variation of the frictional force f with the change in the magnitude of the applied force F, and the corresponding state of motion of the block, are illustrated.
No. of observations | Horizontal force applied F(dyn) | Frictional force f(dyn) | Resultant horizontal force(F- f) dyn | State of motion of bank | Nature of friction |
1 | 0 | 0 | 0 | Stationary | No existence of friction |
2 | 50 | 50 | 0 | Stationary | Static friction |
3 | 80 | 80 | 0 | Stationary | Static friction |
4 | 100 | 100 | 0 | Stationary | Limiting friction |
5 | 101 | 80 | 21 | Moving with increasing speed | Kinetic friction |
6 | 80 | 80 | 0 | Moving with a uniform speed | Kinetic friction |
A study of the table above brings out the following important properties related to friction :
i) When no external force acts on the block, the block remains stationary and there is no existence of friction [observation number 1 ].
ii) Observations 2 to 4 point out an important fact—as the applied force is increased, the frictional force also increases automatically to balance it. The block, therefore, remains at rest. Frictional force by itself, increases by the exact amount required to balance the applied force, before the motion starts. Hence, before any relative motion between the surfaces begins, the frictional force is a self adjusting force. The frictional force acting under this condition is called static friction.
Definition: When two surfaces are relatively at rest but one is trying to slide over the other, then the force which comes into existence between the two surfaces and tries to oppose the motion is called static friction.
iii) As the applied horizontal force is increased, the frictional force also increases gradually to its maximum limit. When the applied force exceeds this limit, the block can no longer be at rest [observation number 4]. This maximum value of the static friction is called limiting friction or limiting value of static friction.
Definition: The maximum possible magnitude of static friction is called limiting friction.
iv) As soon as the magnitude of the applied horizontal force exceeds the limiting friction, the horizontal resultant force is no longer zero. Because of this resultant force, the block starts moving over the table with an acceleration. This motion is termed as sliding. During sliding, the frictional force that comes into play to oppose the motion, is called sliding or kinetic friction. An important property of sliding friction is that the magnitude of this frictional force falls a little as soon as motion sets in. Hence, the magnitude of sliding friction is less than that of limiting friction for a given pair of surfaces [observation number 5].
Definition: When two surfaces slide against each other, the force developed at their surfaces of contact, opposing the relative motion, is called kinetic or sliding friction.
v) To keep the block moving with uniform velocity, the resultant of the horizontal forces must be zero (observa-tion 6) [Fig.], To achieve this, the applied force is reduced so that it becomes equal to the kinetic friction.
It is a common experience that, to move a heavy stone over the ground, a large force has to be applied. But once the stone is moving, a comparatively smaller force (push) is required to maintain its motion.
Kinetic friction between two surfaces is independent of the speed of their relative motion, and therefore it is nearly a constant for a specific pair of surfaces.
In Fig., the graph represents the force of friction acting between two surfaces in contact with each other, against the applied force that acts parallel to the surface of contact.
The static friction is initially equal to the applied force. This is represented by the line OA. As the magnitude of the applied force becomes equal to OD, the block is on the verge of moving. So AD is the magnitude of limiting friction. On increasing the applied force further, the block begins to move and kinetic friction comes into play. Its value is slightly less than that of the limiting friction. In the graph, EG represents the magnitude of this kinetic friction which is approximately a constant.