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Static Friction/Limiting Friction

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When two bodies that are not moving relative to each other are in contact, the friction force acting between their surfaces is known as static friction or limiting friction. This friction force will prevent one of the bodies from moving over the other, or sliding down a slopped surface, except an applied force is greater than it.

Static friction Fs, also known as limiting friction, is the maximum friction force that is produced and which must be overcome for a body to move over, or slide down the surface of another body.

Laboratory experiment to demonstrate static friction: experiments can be carried out in the lab to show the presence of static friction on the surfaces of two unmoving bodies as follows:

Consider a body A on a plane surface, and a force is gradually and increasingly applied on it using a spring balance, as shown in the figure below. At any instant, the friction force F between the surfaces adjusts itself to be equal and opposite to P, so that the body is in equilibrium.

As the force P is gradually increased, a point is reached when the body A is just about to move. The body is said to be in limiting equilibrium at this stage, and the value of P, which is noted by the spring balance represents the value of the maximum frictional force, which is also known as the static or limiting friction, Fs that is acting on the surfaces.

If P is increased further, beyond the static or limiting friction, the body begins to move with steady speed. The frictional force now acting is known as Kinetic Friction, also known as Sliding or Dynamic Friction, Fd. Dynamic friction is usually found to be less than static friction.

A good example of the action of static friction is at the contact between a car’s tire and the ground. Even when the car is in motion, the section of the tire in contact with the ground is not actually moving (it is the wheel that is moving) relative to the ground, therefore static friction is present, and enables the car’s wheel to roll on the ground without losing balance.