cp-9 force and laws of motion for IX


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FORCE In physics, a force is any influence that causes a free body to undergo a change in speed, a change in direction, or a change in shape .


Force Generally pull and push is known as force.

Examples of force:

Examples of force

Types of Force:

Types of Force There are two types of forces Balanced Force- Which is cause of rest Unbalanced Force – Which is cause of Motion


Force Force is a vector quantity It is the cause of acceleration and deceleration in a object F=ma It’s unit is Newton

Linear momentum:

Linear momentum The product of mass and velocity of moving object is known as its linear momentum. It is a vector quantity It represented by p P = mv

Inertia :

Inertia The property of an object by virtue of which it is unable to change its rest state or uniform motion state itself, is known as Inertia.


Inertia Inertia is the natural tendency of an object to resist a change in its state of motion or of rest . The mass of an object is a measure of its inertia.

Mass and Inertia:

Mass and Inertia Heavy object has large inertia. Light object has less inertia. So Inertia of an object is directly proportional to its mass

Law of motion:

Law of motion Galileo’s observation By observing the motion of objects on an inclined plane Galileo deduced that objects move with a constant speed when no force acts on them. He observed that when a marble rolls down an inclined plane, its velocity increases

Newton’s Laws of motion:

Newton’s Laws of motion Law of Inertia Law of momentum Law of action and reaction Sir Isaac Newton made a systematic study of motion and proposed three law of motion witch are known as Newton's laws of motion.

Law of Inertia:

Law of Inertia Law of inertia on rest If an object is in rest , it will remain in the rest untill and unless an external force is not applied on it.

Example 1:

Example 1 When a horse suddenly starts running, the rider falls backward.

Example 2:

Example 2 Dust is removed from a hanging carpet by beating it with a stick.

Example 3:

Example 3 Coin falls into the tumbler when the card is given a sudden jerk.

Example 4:

Example 4 Only the carom coin at the bottom of a pile is removed when a fast moving carom coin (or striker) hits it.

Example 5 :

Example 5 Fruits falls down on sacking a tree .

Law of Inertia:

Law of Inertia Law of inertia on motion If an object is in uniform motion in a straight path , it will remain in uniform motion , untill and unless an external force is not applied on it.

Example 1:

Example 1 When horse running fast suddenly stops, the rider is thrown forward.

Example 2:

Example 2 An athlete runs for a certain distance before taking a long jump.

Example 3:

Example 3 A ball thrown upward in a moving train comes back into the thrower’s hands.

Example 4:

Example 4 A person getting out of a moving bus or train falls in the forward direction.


QUESTIONS FOR PRACTISE 1. Which of the following has more inertia : (a) a rubber ball and a stone of the same size? (b) a bicycle and a train? (c) a five- rupees coin and a one rupee coin? 2. In the following example, try to identify the number of times the velocity of the ball changes : “ A football player kicks a football to another player of his team who kicks the football towards the goal . The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team”. Also identify the agent supplying the force in each case. 3. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch. 4. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

Law of Momentum:

Law of Momentum The rate of change of linear momentum is equal to the force applied and the direction of force is always in the direction of change in momentum. F = ma



Why a cricket player pulls his hand gradually while holding a catch of a ball?:

Why a cricket player pulls his hand gradually while holding a catch of a ball? Answer- In order to increases reaction time so, he felt less force on his hands.

I law of motion from II law:

I law of motion from II law That is, when F = 0, v = u for whatever time, t is taken. This means that the object will continue moving with uniform velocity, u throughout the time, t. If u is zero then v will also be zero. That is, the object will remain at rest.

Law of action & reaction:

Law of action & reaction Every action has its equal and opposite reaction

Example 1:

Example 1 While walking ,we press the ground (action) in backward direction ,it pushes us in forward direction with equal force but in the opposite direction

Example 2:

Example 2 While swimming , a person pushes water his hands in the backward direction and water pushes him in forward direction.

Example 3:

Example 3 The recoiling of gun also due to action and reaction

Example 4:

Example 4 As the sailor jumps in forward direction, the boat moves backwards due to action and reaction law

Activity :

Activity -Take a big rubber balloon and inflate it fully. Tie its neck using a thread. Also using adhesive tape, fix a straw on the surface of this balloon. •Pass a thread through the straw and hold one end of the thread in your hand or fix it on the wall. •Ask your friend to hold the other end of the thread or fix it on a wall at some distance . This arrangement is shown in Fig. •Now remove the thread tied on the neck of balloon. Let the air escape from the mouth of the balloon. •Observe the direction in which the straw moves.

Questions for practice :

Questions for practice 1. If action is always equal to the reaction , explain how a horse can pull a cart. 2. Explain , why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity. 3. From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m s –1 . Calculate the initial recoil velocity of the rifle .

Questions for practice :

Questions for practice 4.Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of 2 m/s and 1 m/s, respectively. They collide and after the collision, the first object moves at a velocity of 1.67 m/s. Determine the velocity of the second object.

Conservation of Momentum:

Conservation of Momentum According to II law of motion F = If external applied force F in zero (F=0) Then F = =0 i.e. P = Constant In the absence of ext. applied force net linear momentum of a system is always constant.

Collision of two balls:

Collision of two balls According to the third law of motion, the force F exerted by ball A on ball B (action) and the force F exerted by the ball B on ball A (reaction) must be equal and opposite to each other.

Collision of two balls:

Collision of two balls So, the total momentum of the two balls remains unchanged or conserved provided no other external force acts.

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