The impending motion refers to the motion of any body which is about to overcome the force of static friction. In other words, impending motion is the exact point when the body is about to move. Friction is of two kinds: static and kinetic. Static friction acts on a body when it is at rest. The force of friction cancels the net force that is acting on the body. Friction is not a centred force, which means that friction has no defined direction. It is a vector quantity that can merely act in opposition to the relative motion of two bodies. The friction force applied by one body acts in the opposite direction in which the other body is trying to move. In this sense, friction is a purely opposing force.
Apart from defining it for only solid bodies, friction can be defined on the basis of contacts between several different phases.
Dry friction occurs between two solid bodies. This friction can either be static or kinetic. Dry friction essentially occurs because of the surface features of the two bodies. This means that friction is solely caused by how the particles on the surface of the bodies interact with each other. These surface features are also called asperities, which is an old English word for imperfection.
Fluid friction is applicable for a set of fluids with viscosities that are moving relative to each other. This can be observed in the interaction between honey and cream. Honey and cream are both very viscous fluids, and when we try to mix them together, they are not readily mixable. This happens because of the fluid friction present between both of them. Fluid friction can also cause an accumulation of charges in gas pipelines which can cause a very destructive fire in the facility housing it.
Lubricated friction refers to a situation where a layer of fluid separates two solid surfaces. This fluid acts as a lubricating medium and reduces the force of friction acting between the two bodies since fluids-solid interaction is characterized by the particles of fluid sliding in between the asperities of the solid surface. This happens because of the flexible shape of the fluid. Thus, fluid’s action on the surface of the solid helps in reducing the friction it experiences with respect to relative motion with other solid bodies. That is why lubricants are widely used in heavy industries to decrease the force of friction acting between two machine parts and to make their operation smoother.
Skin friction occurs when a solid object is moving through a fluid, like a submarine through the ocean. The surface of the solid experiences force of friction due to the drag force acting on the solid body. This drag force is the result of the viscosity of the liquid, which can be said to be the relative inertia of the particles of the fluid. The solid body needs some force to overcome this internal inertia, and that force is manifested in the form of the drag force.
Internal friction is the force of friction acting between the particles of the same solid object. This happens when the object changes its shape. This causes the particles of the object to move relative to each other, and due to this relative motion, a type of frictional force comes into effect that is called internal friction.
The motion of any object is said to be impending when the object is just about to overcome the various forces of friction described above. This occurs at the upper limit of limiting friction. Limiting friction is the maximum value that the force of friction can take when considered relative to another body. When the net force acting on the body exceeds this limiting friction, the object starts moving. To write this in mathematical language:
F = μsN
F is the force of friction acting on the body
N is the normal force acting on the point of contact
μs is the coefficient of static friction
We can see from the above equation that the net force on the object must exceed the maximum value that friction take, that is μs. When the net force is greater than that, the object will move. The impending motion refers to the condition when the net force just cancels the force of friction. In mathematical language:
Fnet = Fmax = μsN
This condition is called the condition of impending motion. The object is just about to launch in motion but has not done so because all the forces are currently in equilibrium. When this condition of equilibrium is breached, the object will move.