Static Friction vs Kinetic Friction: Difference and Comparison

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 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 stronger for dry and rough surfaces and lesser for smooth and wet surfaces.

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

Key Takeaways

1. Static friction is the force that resists motion when an object is at rest, while kinetic friction is the force that resists motion when an object is in motion.
2. Static friction is greater than kinetic friction, making it more difficult to overcome and start an object moving.
3. The coefficient of static friction is greater than kinetic friction, indicating that it takes more force to overcome static friction than maintain motion.

Static Friction vs Kinetic Friction

Static friction is the force that keeps an object at rest and prevents it from moving, acting until the applied force exceeds the force of static friction. Kinetic friction, also known as sliding friction or moving friction, acts on the object when it is in motion.

Comparison Table

Parameters of Comparison	Static Friction	Kinetic Friction
Definition	The force of friction that acts between two surfaces with the tendency to move is called static friction.	Static friction is a variable force with a magnitude ranging from zero to its maximum value. The maximum value of static friction is called limiting friction.
Magnitude	Static friction is a variable force with a magnitude ranging from zero to its maximum value. The maximum value of static friction is called the limiting friction.	Kinetic friction is a constant force with a magnitude less than that of the limiting friction. Its magnitude cannot be zero.
Formula	Since it is a variable force, its formula is not fixed though the formula of limiting friction is as follows: Limiting friction = Coefficient of static friction × Normal force.	The formula of kinetic friction is as follows: Kinetic friction = coefficient of kinetic friction × Normal force
Comparison of coefficients of friction	The coefficient of static friction is always more than or equal to the coefficient of kinetic friction.	The coefficient of kinetic friction is less than or equal to that of static friction.
Examples	1. A man pushing a heavy block that does not move. 2. A boy leaning on a stationary trolley.	1. A ball rolling on a table. 2. A person skating.

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:

f_l = μ_s × N where, 

1. f_l is the limiting friction
2. μ_s 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.

The body comes on the verge of moving when the external force equals the limiting frictional force. 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 in 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 limiting friction. The formula of kinetic friction is as follows:

f_k = μ_k × N where,

1. f_k is the kinetic friction
2. μ_k 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, which is μk, is less than that of static friction μ_s.

According to Wikipedia, new models are beginning to show that kinetic friction may be greater than static friction. However, we now study 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 keeps a body at rest, while kinetic friction slows 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 limiting friction.
5. The magnitude of static friction increases with the force applied up to a limit, while kinetic friction is a constant. Still, both are directly proportional to the normal force.

References

1. https://www.sciencedirect.com/science/article/pii/002072259090014A
2. https://www.sciencedirect.com/science/article/pii/S0301679X01000500
3. https://journals.aps.org/prb/abstract/10.1103/PhysRevB.59.14313
4. https://www.sciencedirect.com/science/article/pii/S0043164803002345