What is kinetic friction?
Kinetic friction is the retarding force experienced by a body when in motion. In other words, it is the minimum force required to keep an object in motion. The requisite for kinetic friction is that the objects should be in relative motion. If the object is in a state of rest then the friction will be called static friction.
The value of this force depends on the value of the coefficient of kinetic friction. Kinetic friction is directly proportional to the normal force, like any other frictional force. You can read this post to learn more about the factors affecting friction in detail.
fk = μk*N
fk is the value of kinetic friction and μk is the value of the coefficient of kinetic friction. N is the net normal force acting on the bodies at contact. Normal force means the net perpendicular forces acting on the bodies at the point of contact.
The unit of friction will be Newton, and the coefficient of kinetic friction is a unitless constant.
Frictional forces are always resistive in action, so kinetic friction will also act in a direction opposite to the direction of force applied.
What are the laws of kinetic friction?
- It always reduces the speed of the object in motion
- It acts in the direction opposite to force applied
- It is independent of the surface area of contact, design, and velocity.
Synonyms of kinetic friction
- Dynamic friction
- Moving friction
- Friction in motion
- Sliding friction
10 Examples of kinetic friction
- A rock rolling off a slope
- A man running in a 100 m race
- A moving truck or car
- A rolling football
- Movement of a snake
- Moving on roller skates
- Mountain Skiing
- Sliding down a kid’s slide
- Striking a matchstick against a surface
For more examples of friction in general, read our post on 50 examples of friction.
What is the origin of kinetic friction?
Irregularities in the surfaces of objects give rise to an interlocking at the microscopic scale. It is impossible to find a perfectly smooth surface. The weak chemical bonding established during the movement also adds to the resistive force in action.
Friction is created due to the electrostatic interactions between the molecules of the two bodies. When a body is at rest, this interaction will be very strong. But as the object begins to move, the chemical interactions become weaker. You can understand this by looking at the friction plot.
Kinetic friction in an inclined plane
An object placed in an inclined plane will slide down the slope with a constant velocity due to the earth’s gravitational pull. Friction is directly proportional to the normal force acting upon it. In an inclined plane, the sine component of the normal force is equal to the frictional force.
fk = μk * mg sinθ where μk is the coefficient of kinetic friction.
What is the angle of friction?
When an object is sliding across an inclined plane, the angle of incline is generally referred to as the angle of friction.
Can an object move along with an incline move with uniform velocity?
An object sliding in an inclined plane will experience continuous gravitational pull downwards and will accelerate. But, when there is a force pulling the object in the backward direction then we can maintain a constant velocity. This principle is applied in braking vehicles while driving down a slope.
Numerical problems on kinetic friction
Q1: A car of weight 300 Kg is moving up a slope with an incline of 30 °. Find the value of friction.
Ans: f = 300 * sin *30° = 150 N
Q2: A car is parked stationary on a slope. Calculate the kinetic friction in this case.
Ans: Zero
What is the difference between static and kinetic friction?
| Static friction | Kinetic friction |
| Applies to a stationary object | This applies to a stationary object |
| It is a self-adjusting force | it is not self-adjusting |
| The value of static friction is greater compared to kinetic | The value of kinetic friction is less compared to static friction |
| Coefficient of static friction | Coefficient of kinetic friction |
What are the factors affecting kinetic friction?
- Irregularities in the surface
- Irregularities in the sliding object
- weight of the object
You can read more factors affecting friction here.
Why is static friction greater than kinetic friction?
Static friction is a self-adjusting force. There are a lot of chemical bonds and interlocking at the microscopic scale. All these bonds have to be broken in order for the object to move. Hence, static friction is always greater than kinetic friction.
Research on kinetic friction:
In this paper, the authors have designed a smart device to measure the kinetic friction coefficient. It is based on a bi-tilt vibration isolation system. It is a low-cost alternative and also easily accessible. The kinetic friction coefficient has to be measured while working on lubricated materials or designing a material that will be in motion over a surface. Link:https://www.hindawi.com/journals/ace/2019/4392506/
In dentistry, brackets are small pieces that are bound to the front of our teeth with a special band called orthodontic bands. The friction experienced within this bracket is very important. The authors have studied the kinetic friction involved in silica inserted ceramic brackets in an artificial saliva environment. You can read the article here.
In this paper, the authors determine kinetic friction coefficient values using the disk-block technique on different wood samples. link: http://pesjournal.net/journal/v1-n1/36.pdf
Hydroplaning is very dangerous. It happens when the road is wet and the car moves at high speed, and the hydrodynamic force uplifts the tires. Authors try to identify the factors affecting hydroplaning by using numerical simulations. They use the coefficient of kinetic friction of the tires to study the hydroplaning phenomenon in detail. Link:https://www.hindawi.com/journals/amse/2017/5843061/
What is the simple definition of kinetic friction?
It is the Resistive force acting against the motion of objects. So, if you are moving in a particular direction, friction acts in the opposite direction to stop you.
What is coulomb’s law of friction?
Coulomb’s law of friction states that, in the case of dry friction where two bodies slide over each other, the kinetic friction is independent of the velocity of sliding surfaces.
Does kinetic friction apply to objects at rest?
Kinetic friction comes into play only when the object is in motion. Even when the object is just about to move static friction is only applicable. Kinetic friction comes into the picture after the value of limiting friction is reached. You can understand this by seeing the friction plot.
Can rolling friction be considered a type of kinetic friction?
Rolling friction occurs only when an object rolls. And the rolling of an object is a type of motion. So, in a way, we can consider rolling friction as a type of kinetic friction.
How are kinetic energy and kinetic friction related?
Kinetic energy and kinetic friction should not be confused. Kinetic energy is the energy possessed by an object due to motion. On the other hand, kinetic friction is the resistive force developed due to motion.
