Class 9 Science Chapter 9 Force and Laws of Motion complete explanation

 

Force  : Push of pull on an object is called force.

It is defined as the push of pull on an object with mass that causes it to change its velocity. It is a vector quantity. Example : To open or close the door .

 Force and Laws of Motion  complete explanation

Effect of Force 

·       Force can move a stationary object

·       Force can stop a moving object.

·       Force can change the direction of moving object.

·       Force can change the speed of moving object.

Type of Force

Balanced force : if the resultant of applied force is equal to zero, than it is called balanced force .

balanced force 

Unbalanced force : if the resultant of applied forces is greater than zero, than it is called unbalanced force.

F = ma

 (F : force    M : mass  a : acceleration )

 SI unit of force is newton (N)

Newton’s laws of motion

Newton studies the idea of Galileo and gave the three laws of motion

·       First law of motion – Inertia

·       Second law of motion – Momentum

·       Third law of motion 

      First law of motion – Any object at rest remains in the state of rest and any object in motion will continue in motion along a straight line with uniform speed until it is compelled y an external force to change its state.

 Inertia : It  is the tendency of object due to which it resists any change in its state of rest or uniform motion .

·        Inertia of  any object measured by its mass.

·        Inertia is directly proportional to the mass.

Example : When we stop cycling: This is an example of inertia of motion that each of us must have experienced at some point. When we ride a bicycle, it accelerates from rest. But, when we stop pedaling it continues for some time till it stops due to friction

Momentum: Product of mass and velocity of an object is called momentum.

It is also called the power of a moving object. It is a vector quantity.

It is denoted by “P”

P = m * v ( mv )

·        Momentum increase with increase of either mass or velocity of an object.

·        Vehicle running with high speed have greater momentum as compared to vehicle running with slower speed.

Small object moving with great velocity have high momentum.

Second law of motion : It state that the rate of change of momentum is directly proportional to the force applied in the direction of force .

 Force α rate of change of momentum

  Mass = m

Initial velocity  = u

Final velocity = v

Initial momentum = mu

Final momentum = mv

 

F  α   mv-mu

         ____________

                     T

 

F  α   m( v-u)

          ____________

                     T

F  α    ma

F  = K ma      ( K = 1)

F = ma 

SI unit  = kg m/s² or Newton

Third law of motion : It state that if a body exerts a force  on second body , the second body exert a force that is equal in magnitude and opposite in direction .

·       Every action has an equal opposite reaction

·       Force exerted by fist body is action and force exerted by second body is reaction.

Example : walking , jumping from boat.

Frictional Force : The force which always opposes the motion of one body over another body is called frictional force .