## Force of Friction

If the switch off the engine of car it will stop after moving a certain distance. It means that some retarding force is acting on the car which stops it. The force opposing the motion of the car is called “force of friction”.

• The frictional force is tangential to the surface in contact and always in a direction opposite to the direction of motion of the object.
• Frictional force is a force opposing the relative motion between two surfaces which are in contact with each other.

Advantages: It is because of friction that we can walk, drive, open doors, turn taps, and write. Screws and nails do not slip off because of the friction between the wood and the metal surface of the screw A knot remains in place because of the friction of the rope with which you tie it. Bed sheets remain on the bed because of friction. You can sit on a chair without sliding off because of friction.

Disadvantages: Friction causes the moving parts in machinery and automobiles to heat up. This results in wastage of energy and fuel. Friction also causes wear and tear of the moving parts. Depending on our requirements, we may need to reduce or increase the friction between surfaces.

• Wear and Tear: In machine parts like gears, brakes when they come in contact with each other continuously, they wear out gradually, which should be replaced time to time.
• Friction reduces efficiency of the machine.
• Friction in machine produces heat and undesirable noise which damages the machine. To avoid excessive heating, water is circulated in machines generally.

### Applications of frictional forces

• We would not be able to walk if there had been no friction between the soles of our shoes and the ground.
• If there had been no friction, the wheels of a car will slip instead of rotating and stop moving. For that we have to increase the friction by making the tyres corrugated to get better grip of tyres on the road. Also, the friction is increased.
• When brakes are applied, the vehicle stops due to the force of friction between the brake lining and the drum.
• In the absence of friction, we cannot write on a blackboard with a chalk stick because the chalk stick will slide off the board without leaving any mark on the board.

## Force Of Friction With Example Problems

Example 1:    Three blocks are tied together with strings as shown. They are lying on a horizontal frictionless table. These are pulled to the right with T3 = 12N force. If m1 = 1 kg, m2 = 2kg, and m3 = 3 kg, calculate the tension T1.

Solution:

m3a = T3 – T2
⇒ 3a = 12 – T2           …(1)

m2a = T2 – T1
⇒ 2a = T2 – T1         …(2)
from (1) & (2)
5a = 12 – T1              …(3)
m1a = T1 ⇒ a = T1         …(4)
from (3) & (4)
5T1 = 12 – T1 ⇒T1 = 2N

## What Is The Normal Force

The external agent which tends to set a body in motion or which changes the speed and direction of motion of a body or which can change the shape of a body is called force.
SI unit of force is Newton.

Types of Forces:

(a) Non Contact:
These are the forces in which contact between two objects is not necessary. Gravitational force between two bodies and electrostatic force between two charges are two examples of field forces.
(b) Contact Forces:
Two bodies in contact exert equal and opposite forces on each other. If the contact is frictionless the contact force is perpendicular to the common surface and known as normal reaction.

If, however, the objects are in rough contact and move (or have a tendency to move) relative to each other without losing contact then frictional force arise which oppose such motion. Again each object exerts a frictional force on the other and the two forces are equal and opposite. This force is perpendicular to normal reaction. Thus, the contact force (F) between two objects is made up of two forces.
(i) Normal reaction (N) (ii) Force of friction (f)
and since these two forces are mutually perpendicular.
$$\text{F}=\sqrt{{{\text{N}}^{\text{2}}}\text{+ }{{\text{f}}^{\text{2}}}}$$