# MOTION: FORCE AND FRICTION

Friction is the force that resists the relative movement of solid surfaces, fluid layers, and material elements sliding against each other.

CONTENT

A) FORCE

DEFINITION OF FORCE

TYPES

B) FRICTION

DEFINITION OF FRICTION

TYPES OF FRICTION

COEFFICIENT OF FRICTION

WAYS OF REDUCING FRICTION

### DEFINITION OF FORCE

In physics, force is any effect that, when not opposed, changes the movement of an object. It can cause an object with mass to change its speed(including moving from a stationary state), i.e. acceleration. It can also be intuitively described as pushing or pulling and has size and direction, making it a vector quantity. IT MEASURES IN SI UNITS OF NEWTON (N). The symbol is represented as F

F = Mass x Acceleration

Force is a physical cause that can change the state of motion or the dimensions of an object. There are two types based on their applications:

1. Contact Force
2. Non-Contact Force

Contact Force

They that act on a body either directly or through a medium are called contact forces.

Some examples are:

• Muscular
• Mechanical
• Frictional

Contact force

Non-Contact Force

Forces that act through spaces without making direct contact with the body are called non-contact forces.

Examples of non-contact forces are:

• Gravitational
• Electrostatic
• Magnetic

Non contact force

#### FRICTION

Friction is defined as the resistance provided by the surfaces in contact when they move with each other.

TYPES OF FRICTION

There are four types of friction and they are classified as follows:

• Static friction
• Sliding friction
• Rolling friction
• Fluid friction

All four types of frictions are different from each other.

Friction acting on a block being pulled on a surface

STATIC FRICTION: Static friction is defined as the friction force acting on the surface relative to each other when the surface is stationary. When a small amount of force is applied, the size of the static force is equal in the opposite direction. When the force increases, the maximum static friction is reached at some point.

Following are the examples of static friction:

• Creating heat by rubbing both the hands together
• Table lamp resting on the table

KINETIC FRICTION: Kinetic friction is defined as the resistance that is created between any two objects when they are sliding against each other. Following are some examples:

• Sliding of the block across the floor
• Two cards sliding against each other in a deck

ROLLING FRICTION: Rolling friction is defined as the force which resists the motion of a ball or wheel and is the weakest types of friction. Following are the examples of rolling friction:

• Rolling of the log on the ground
• Wheels of the moving vehicles

FLUID FRICTION: Fluid friction is defined as the friction that exists between the layers of the fluid when they are moving relative to each other. Following are the examples of fluid friction:

• The flow of ink in pens
• Swimming

### COEFFICIENT OF FRICTION

The coefficient of friction (COF), usually represented by the Greek letter μ, is a dimensionless scalar value that describes the ratio of the friction between two objects to the force that presses them together. The coefficient of friction depends on the material used; for example, ice on steel has a low coefficient of friction, while rubber on the road has a high coefficient of friction. The coefficient of friction ranges from close to zero to greater than one.

For surfaces at rest relative to each other µ = µs  where µs is the coefficient of static friction.

For surfaces in relative motionµ = µk, where µk is the coefficient of kinetic friction.

It helps us to stop and walk

It helps for the transfer of energy

It helps to start a fire

It helps to hold unto surfaces

It produces unwanted heat

It reduces efficiency of machines

It causes wear and tear

It produces unwanted noise

WAYS OF REDUCING FRICTION

1. Make surfaces smooth by grinding.
2. Lubrication
3. Streamlined body
4. Reduce pressure or weight on the object.
5. Reduce contact between surfaces by electrostatic magnetic levitation etc.
6. Use rolling friction instead of sliding friction.
7. Use fluid friction instead of the dry friction.

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