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# Kinetic Friction

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 Sub Topics A car at rest starts with the accelerator needs a good enough force to exceed the limiting friction (due to static friction) that acts on it. But once it starts moving a force of friction acts that resists the motion. This is what kinetic friction is! Let's see about it.

## Kinetic Friction Definition

When a body moves over the other then the force of friction acts between two surfaces known as kinetic friction. The maximum friction force that acts before body starts to move is called limiting friction. If the applied force acting on the body is greater than its limiting friction, then the body begins to slide. There comes the concept of kinetic or dynamic friction.

Kinetic or dynamic friction acts when there is a relative motion between the two surfaces in contact. They are of two types

Lets go through the nature of kinetic friction

• It is always opposite to the direction of velocity
• It depends on the nature of surfaces in contact
• It does not depend upon the shape or area of the surfaces in contact
• The force of kinetic friction = $\mu_k$ N where, N is normal reaction
• Coefficient of static friction $\mu_s$ > Coefficient of kinetic friction $\mu_k$.

## Coefficient of Kinetic Friction

The coefficient of kinetic friction decides how much is the kinetic friction acts on a body. It is the ratio of the kinetic friction $F_k$ that acts to the normal force $F_n$.

Kinetic Friction Formula is given by
$F_k$ = $\mu_k$ $F_n$
where,
$F_k$ is the kinetic friction force,
$\mu_k$ is the coefficient of kinetic friction and
$F_n$ is the normal force.

## Example of kinetic friction

Let's see some examples of kinetic friction:
• Car in motion on rough surface should exceed kinetic friction
• The game of billiards is an good illustration where the player applies force on the ball by striking it with the cue stick. The cue when it starts to move it should overcome the kinetic friction acting.
• Fly wheels or wind mill when moves has to overcome the kinetic friction.

## Find the coefficient of kinetic friction

Lets go through some solved problems on kinetic friction:

### Solved Examples

Question 1: A car of 800 kg mass is moving on a straight road with a constant velocity. How much force is needed to keep it so?
Solution:

Given :
Mass of a car m = 800 kg, coefficient of rolling friction = 0.04 (Assume)
To calculate the force use formula
F = $\mu_k$ N = $\mu_k$ mg
= 0.04 $\times$ 800 kg $\times$ 9.8 m/s2
= 313 N.

Question 2: A mass of 80 kg is being pulled over ice and it needs 100 N force to do so. Calculate the coefficient of kinetic friction.
Solution:

Mass of body = 80 kg
Normal reaction = Body weight
N = W = mg
= 80 kg $\times$ 9.8 $m/s^{2}$
= 784 N

Force of friction F = 100 N
Coefficient of friction $\mu$ = $\frac{F}{N}$
= $\frac{100}{784}$
= 0.12