9th Physics Force and law of motion

 

 Class 9th Science -Physics

ICSE/ CBSE Notes-

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Force and laws of motion

Force

• The push or pull on a body is known as force 
•  The direction in which the body push or pull is called direction of force
• It  enables us to do any  work 
• Unit of force is Newton
• Force = mass  x acceleration  (F = ma)

Eg- Stretching a rubber band

       A football player kicking the ball

       A boy lifting the bucket 

      Open or close the door   etc

Effect of  Force

• Force can Move a stationary body and  stop the moving body
• Force can change the speed and direction of the body
• Force can change the shape and size of the body

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Resultant Force or Net force

Resultant force is  single force acting on the body which is obtained from multiple forces acting on the body

Types of  Forces

Force are mainly two types -

1.  Balanced force
2. Unbalanced force

1- Balanced force

• If the resultants of all the forces acting on a body is zero then forces are called balanced force
• Balanced forces are equal in magnitude and opposite in direction
• Balanced  force can change the shape and size of the object 

Eg- In a tug war when the force exerted by both teams on a rope , that force are equal and in opposite direction ,it is called balanced force.

When balloon is pressed between hands then balanced force applied ,balloon change there shape and size

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2- Unbalanced force

• If the resultant of applied forces are greater than zero ,the forces are called unbalanced force 
• Due to unbalanced force stationary object can move
• It increase or decrease the speed of a moving body
• It can change the shape and size of the object

NOTE- There are two types of forces according to contact

1. Contact force
2. Non contact force

1- Contact force

It is the force that is exerted by one object to another object when they are contact to each other is called contact force.

Eg- Frictional force between road and  vehicle 

2- Non contact force

It  is the force exerted by one object over the another when both two objects are not physically in contact are called non contact force

  

Eg- Gravitational force ,magnetic force etc

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Newton's Laws of motion

Newton gives the three laws of motion of bodies.

1. Newton's first law of motion
2. Newton's second law of motion
3. Newton's third law of motion

Newton's first law of motion

According to Newton's first law of motion if any body at rest will remain at rest, and a body in motion will continue in motion in a straight line unless any external force exerted to change its state.

• It means if an object is in rest then it will remain in rest
• If any object is in motion will  remain in motion 
• The state of an object change by applying any external forces on the object
• Inertia is the property by which
• Newton's  first law of motion is also called inertia law

Inertia

It is the tendency  of a body to remain at rest or if moving to continue moving in a straight line is called inertia.

• Inertia is that property of the body due to which it resist a change in its state of rest or of uniform motion..
• If the body is heavier it has more inertia.
• If the body  is lighter it has less inertia.
• Inertia of any body is directly praportional to mass of the body

Inertia ∝ Mass of the body

Eg- 1- A toy car has a small mass so it has small inertia and hence can be moved easily by pushing

  2- A big car  or real car has large mass so it has large inertia and hence difficult to move by pushing.

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Momentum

The property that a moving object has due to its mass and its motion is called 

momentum of the object.

• Momentum is Product of mass of a particle and its velocity.
• Momentum is a vector quantity.
• It has both magnitude and direction
• SI Unit of momentum is - kg m/s

                   Momentum =  mass of the  object  x   velocity of the object

                     Momentum  p  = m v

   where   m = mass of the object  in   kg

               v  =  speed or velocity of the object  in   m/s 

NOTE-

• If the body is at  rest ,its velocity is zero so it momentum is also zero
• Every moving body have some momentum.
• If velocity increases then  momentum also  increases with same ratio 

    P = m v

if  v  = 2 v then

P  = 2 m v

   if velocity double, momentum also double

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Eg-  

1. A cricket ball hit by a bat
2. A tennis ball hit by a racket
3. A man injured by fast moving bullet
4. A karate  player can break the pile of tiles by one short force .