Force Notes Clas 9th

 

Force And Laws Of Motion

 Force:-

            It is pull  or push acting on an object which can change  shape, size of an object  is called  force.

 Effects of force

1.      Force can change the shape of an object .

2.      Force can change the direction of an object.

3.      Force can change the size of an object.

4.      Force can change the  speed of an object.

 Types of force

·        Balanced force

·        Unbalanced force

 

 Balanced force

 A force is said to be balanced if the resultant of all the forces acting on an object is zero .

Unbalanced force

A force is said to be unbalanced if Resultant of two forces acting on an object is not equal to zero.

 

 

Newton ‘s laws of motion

 Newton first law of motion

 According to Newton first law of motion, a body is at rest remain at rest a body is in motion remain in motion and A body moving in a direction will move in same direction until some external unbalanced force is not applied to change its state.

 For example, a book is lying on a table remain on the table until some external unbalanced force is applied to change its state.

 

 Inertia

 It is defined as  the inability a body to change  change my itself its state of rest or motion or direction.

 Types of inertia

·        Inertia of rest

·        Inertia of motion

·        Inertia of direction

 

 Inertia of rest:-

Iit is the inability of a body to  to change by itself its state of rest. It means a body at rest remain at rest .

 For example,, suppose we are sitting which is at rest suddenly bus starts due to inertia of rest.

Inertia of  motion

 It is the inability of a body to  to change by itself its state of  motion. It means a body at  motion remain  in motion.

 For example,, sitting in a moving bus and suddenly brakes are applied  then we will fall forward due to inertia of motion.

 Inertia of direction

 It is the inability of a body to change by itself it's state of direction. It means a body in a direction remains in the same direction.

 For example, suppose you are sitting in a fast moving open jeep driver took a U-turn and  we may fall

  From the Jeep due to inertia of direction.

 

 Linear momentum

            It is defined as the product of mass and velocity of a body.

 Consider M is mass body  and V velocity of a body  then,

 Momentum=  mass* velocity

                                           p= m*v 

Unit=Kg m/s.

 Note:-

• 1.      It is a vector quantity.
• 2.      If a body is  at rest  then its  linear momentum is equal to zero.

Applications of Momentum:

1.      A small bullet can penetrate into the body of a man and kill him when fired from a gun because it has a large momentum due to its great velocity.

2.      A person gets severely injured when hit by a fast moving vehicle which is due to the momentum of vehicle due to large mass and high velocity.

 

 Newton’s second law of motion

            According to Newton’s second law of motion, the rate of change of linear momentum of the body is directly proportional to an external force applied.

 

 

Newton's Second Law of Motion (Derivation)

 Consider a body moving with an initial velocity 'u'. Application of a force 'F' continuous for time t' changes the velocity of the body to 'v'. If mass of this body is 'm', then

  

 Application of  Newton’s second law of motion:-

1. In cricket , while catching a ball player moves his hands backward. The reasons to do this, the time taken to caught the ball increases which  decreases force applied to  stop it. Which will help to catch the ball easily  without hurting.

2. Games like wrestling, there is a soft cushion bed. When a player falls on it will take time to move towards rough surface and hence, the force applied decrease as a time will increase.

 Show that Newton’s second law of motion is a special case of newton first law of motion:-

            When F force is applied on a body whose mass is m, due to this applied force Let a be the acceleration produced in the body.

By Newton Second law of Motion,

F = m . a 

F = m(v-u) / t    _____ ( 1 )

By Newton Ist law,

F= 0 ___________ ( 2 )

When we put this (2) in (1)

then, 0 = m(v-u) / t

0 = mv-mu / t

0 = mv-mu

mv = mu

v = u

final velocity = initial velocity

Which is one of the case of Newton's First law of motion.

 Newton’s third law motion

            According to Newton’s third law of motion, for every action there is an equal and opposite reaction.  Both the forces are  acting on two different object and they never cancel with each other. 

 For example, 

1. To start walking, you pushed ground in backward direction  with your feet. By Newton’s third law, the ground also pushes your feet,  in the forward direction. Forward directing  force  acting on your feet that accelerate  you're in for what direction.

2.Suppose you are on boat near the shore, you jump out of the boat.  You push  boat. As you jump, the boat backward with your feet .

By Newton’s third law,The boat pushes you in the forward direction. Thus, you must watch the show and both away from the shore.

When the bulletin fire from a gun, gun recoil. This is because the gun exerts a force on the bullet in the forward direction bullet exert an equal force  in the backward direction.So. Gun moves backward, or recoils.

 Law conservation of linear momentum

According to law of conservation of linear momentum, the total linear momentum of system is constant  if no external force is applied on the system.

Suppose, two objects A and B each of mass m₁ and mass m₂ are moving initially with velocities u₁ and u₂, strike each other after time t and start moving with velocities v₁ and v₂ respectively.

 

 

We know that, Momentum = Mass x Velocity

Therefore,

Initial momentum of object A = m₁u₁

Initial momentum of object B = m₂u₂

Final momentum of object A = m₁v₁

Final momentum of object B = m₂v₂

Now, Rate of change of momentum = Change in momentum/ time taken

Therefore,

 Recoiling of the gun

            When a person fire bullets a gun, there is a backward movement occurred  in the gun. That backward moment is recoiling of gun. The velocity with which it move backward, is called  recoiling velocity.

  Let m1  is the mass of bullet and m2 is mass of the gun.

u1  initial velocity of a  bullet

 u2  initial velocity of a  gun

v1 final velocity of a  bullet

v2 final velocity of a  gun.

 Formula

V₂=-m₁v₁/m₂

 Special case

m₁u₁ +m₂u₂=m₁v₁+m₂v₂

                    When object stick with the object after striking,  both will start moving  with same velocity V ,then formula become

m₁u₁ +m₂u₂=(m₁+m₂) v