# Difference Between Static Friction and Kinetic Friction

When you try to push a very heavy block, it doesn’t move. What do you think stops it from moving?

It is the friction force that overcomes the force that you apply.

Friction or frictional force or the force of friction is the force that opposes the relative motion or the tendency of relative motion of two surfaces in contact.

Friction is a contact force. It is of greater strength for dry and rough surfaces and lesser strength for smooth and wet surfaces.

The force of friction acts tangentially along with the contact of the two bodies. Friction exists in a pair and is due to the irregularities of the surfaces.

## Static Friction vs Kinetic Friction

The main difference between static friction and kinetic friction is that static friction acts on a body when it is at rest and kinetic friction acts on a body when it is in a state of motion. Static friction is variable while the value of kinetic friction is constant.

## What is Static Friction?

Static friction as the name suggests is the friction or force acting on stationary things.

The definition of static friction is the force acting on a stationary object preventing it from moving. It’s a self adjustable force.

Static friction force ranges from zero to its maximum limit which is the limiting friction.

Limiting friction is the frictional force applied just as the body is about to move. Its formula is as follows:

fl = μs × N where,

1. fl is the limiting friction
2. μs is the coefficient of static friction and
3. N is the normal reaction force.

Initially, when an external force is applied to a body, the value of static friction is equal to the external force applied as it increases as the external force increases.

When the external force equals the limiting frictional force, the body comes on the verge of moving. And if the external force exceeds the limiting friction, the body starts moving.

## What is Kinetic Friction?

Kinetic friction is the force of friction applied to a body when it is in a state of motion. It’s a constant force. Kinetic friction opposes the relative motion of two objects.

It always acts in the opposite direction of the external force applied.

Kinetic friction can never have a value greater than that of the limiting friction. The formula of kinetic friction is as flows:

fk = μk × N where,

1. fk is the kinetic friction
2. μk is the coefficient of kinetic friction and
3. N is the normal reaction force

Kinetic friction is sometimes also called sliding friction or dynamic friction. The coefficient of kinetic friction, that is μk is usually less than the coefficient of static friction μs.

According to Wikipedia, new models are beginning to show that kinetic friction may be greater than static friction but what we study right now is that kinetic friction is always less than the limiting friction, which is the uppermost limit of static friction.

## Main Differences Between Static Friction and Kinetic Friction

1. Static friction tends to keep a body at rest while kinetic friction tends to slow down a moving object.
2. Static friction is a variable force while kinetic friction is a constant force. Moreover, static friction is a self-adjustable force.
3. Static friction can be zero while kinetic friction can never be zero.
4. The magnitude of static friction ranges from zero to limiting friction while the magnitude of kinetic friction is constant and less than that of the limiting friction.
5. The magnitude of static friction increases with the increase in force applied up to a limit while kinetic friction is a constant but both of them are directly proportional to the normal force.

## Conclusion

Without the frictional force, we just can’t imagine how the world would have been.

We can walk because of the frictional force provided by the ground. We couldn’t even hold objects if frictional force would not have existed.

Static friction is the force that keeps a body at rest and its value varies with the variation in the external force up to a limit while kinetic friction is the force that slows down a moving object and its magnitude is constant.

Static friction acts when the two bodies in contact are at rest relative to each other while kinetic friction acts when there is a relative motion between the two objects in contact.