Short notes 1 of chapter 1 (Force) of class 10

 *FORCE -

A force is an external cause which changes or tends to change the state of a body either from rest to motion or motion to rest,direction,speed, shape and size.

Qualitative definition of force is given by Newton's first law of motion.

Quantitative definition of force is given by Newton's second law of motion. It tells us about the magnitude of force acting on a body.It speaks about , how much force acts on an object.

F = m × a

SI unit ,

  F=m × a

    = 1kg × 1m s‐²

    = 1N

Cgs unit,

F = m × a

   = 1g × cm s-²

   = 1 dyne

Relationship between SI and CGS unit of force-

F = m × a

1 N = 1kg × 1 m /s²

       = 1000 g × 100 cm / s²

       = 100000 g.cm.s-²

       = 10⁵ dyne

1N = 10⁵ dyne.

Gravitational unit of force-

1. In SI system,

   F = m × a

      = 1kg × 9.8 ms-²   ( m= 1kg , a = 9.8 ms-² )

      = 9.8 N

1 kgf =9.8 N

1 kgf ≃ 10N

1 N ≈ 1/ 10 kgf

       ≈0.1 kgf

2.In CGS system,

      F= m× a    ,  (m= 1g , a = 980 cm s-² )

        = 1g × 980 cm s-²

       = 980 dyne

1 gf = 980 dyne

             ≈  1000 dyne

         1dyne   ≈ 0.001 gf

*LINEAR MOTION / TRANSLATORY MOTION-

A force is applied on a body which is free to move then it moves along a straight path in such a way that each and every particle of the substance travels the same distance .

* ROTATORY MOTION -

Forces applied in an appropriate direction to a pivoted body, then body starts rotating about that fixed point and the line passing through that point is called as axis of rotation. Particles which are near to the axis of rotation covers less distance compared to the particles which are away from the axis of rotation .

* moment of force  or Torque (て) -

Force applied to a pivoted body,then body rotates about an axis passing through pivoted point is called as axis of rotation and the point at which force is applied is known as point of application of force,the line passing through point of application of force is called as line of action of force.

SI unit -

 て = Force × perpendicular distance between axis of                              rotatio and line of action of force.

て = N × m

    = Nm 

CGs unit -

  て = force × perpendicular distance

      = dyne × cm 

      = dyne cm

Gravitational unit,

て = force × perpendicular distance

    = kgf × m

     = kgf m

て = force × perpendicular distance

     = gf × cm

      = gf cm

* In torque , 1N m can not be written as 1J.

* torque is a vector quantity.

* clockwise moment of force is taken as negative.

While anticlockwise moment of force is taken as positive.

Definition of MOMENT OF FORCE or TORQUE -Turning effect produced by a force on a rigid body about a point, pivot or fulcrum is called moment of a force or torque .

Factors affecting moment of a Force :

1. Magnitude of the force applied

2. The distance of line of action of the force from the axis of rotation.

*applications of moment of force-

1. It is easier to open the door by applying force near the outer edge away from the Hinges or axis of motion.

2. For turning steering wheel a force is applied tangentially on the rim of the wheel.

3. In a bicycle the tooth wheel is rotated by applying less force on the foot pedal which is at a large distance from the axle of rear wheel.

* forces in equilibrium:

An object in equilibrium if the sum of the forces acting on it and the sum of the moments of the forces or torque on it is equal to zero.

Kinds of equilibrium:

1. Static equilibrium-

When a body remains in a state of rest under the influence of several forces the body is in static equilibrium. Example book is lying on a table, in a beam balance when the beam is balanced in horizontal position, body lying on a table top is pulled by a force of equal magnitude But in opposite direction etc.,

2. Dynamic equilibrium-

When a body remains in the same state of motion under the influence of several forces the body is said to be in dynamic equilibrium. Example a rain drop richest the earth surface with a constant velocity, aeroplane moves at a constant height, stone tied at the end of a string when whirled in a circular path with uniform speed etc.,

Conditions of equilibrium:

1. Vector some of the forces acting on the body should be zero.

2. Algebraic sum of moments acting on the body should be zero.

DEFINITION OF equilibrium of a body :

An object is said to be in equilibrium if a number of forces acting on it produces no change in its state of rest or of uniform motion. It's motion maybe in translation of rotational equilibrium.