Short notes-1 of chapter 3(FORCE)- class-6

 * Force-

- A force is that cause which changes the state of a body,either the state of rest or the state of motion or the direction of motion or changes the size or shape of the body.

- Force can not be seen but it can be felt.

- Force is always applied in a particular direction and has magnitude .It is a VECTOR quantity.

- Measured by a Spring balance.

- S.I unit is newton(N).

- C.G.S.unit is dyne.

- Gravitational unit

    In Mks system is kgf(kilogramforce)

    In cgs system is gf(gramforce )

- Relationship between different units of force

   1 N = 10⁵ dyne.

Or 1 dyne =10-⁵ N

   1 kgf= 9.8N

  1N = 0.102 kgf

        = 1.02 × 10-¹ kgf

 1 gf = 980 dyne

         = 9.8 × 10² dyne

 1 dyne = 0.00102 gf

               = 1.02 × 10-³ gf

1 kgf = 1000 gf

          = 10³ gf

1 gf = 1/1000 kgf

        = 10-³ kgf

* Effects of a force

-  Moves a body

- Stops a body

- Decreases speed

- Increases speed

- Changes direction

- Changes shape & size

* Kinds of force 

- Contact force :

   The force which acts on a bodies by making an actual contact.

Eg 1.Muscular force

      2.Frictional force (Friction)

      3.Tension force (force of tension)

      4. Reaction force ( force of reaction )

- Non contact force (Forces from a distance) :

   The force which act on bodies with no contact with them.

Eg 1. Gravitational force

      2. Electrostatic force

      3.Magnetic force

      4.Nuclear force

* Friction (force of friction):

-Friction is the force that opposes the relative motion between the two surfaces in contact with each other.

Cause of friction 

The interlocking of the irregular projections on the two surfaces in contact.

- Contact force

- Derived from latin word "fricare" which means " to rub "

- S.I unit & all other units are same as force. 

*Effects of Friction:

 - Friction opposes the motion.

 - Friction always acts in a direction opposite to the direction of motion.

- Friction produces heat.

- Friction causes wear & tear.

* Factors affecting the forces of friction

- The smoothness of the surface.

- The nature of medium in which it moves.

   Between solid & solid = maximum

   Between solid & liquid  = intermediate

    Between solid & gas = minimum

- The weight of the moving body on the surface.

  But  W = m×g  ,  m= mass of a body

                                g = acceleration due to gravity.