# NCERT Class 9 Physics Formula and Important Terms Board Sample Problems Part 2

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## What is Force

When we push or pull anybody we are said to exert force on the body

Push or pull applied on a body does not exactly define the force in general. We can define force as an influence causing a body at rest or moving with constant velocity to undergo acceleration

 S.No Quantities Description 1 Balanced Forces If the resultant of all forces acting on a body is zero, then the forces are called balanced forces. 2 Unbalanced Forces If the resultant forces acting on a body is not zero, the forces are called unbalanced forces. 4 Friction Force The force that opposes the relative motion between the surfaces of two objects in contact and acts along the surfaces in contact is called the force of friction or simply friction. 5 Momentum The momentum, p of an object is defined as the product of its mass, m and velocity, V That is,Momentum p=mvMomentum has both direction and magnitude so it is a vector quantity. Its direction is the same as that of velocity, VSI unit of momentum is kilogram-meter per second (kg m )

### Newtonâ€™s Law of Motion

 Concept Description Newtonâ€™s First Law of motion An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force. Newtonâ€™s Second Law of Motion The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of forceNewtonâ€™s second law of motion can be expressed asThis can be written as after some derivationF=maThe unit of force is kgms2 or Newton, which has the symbol N. The second law of motion gives us a method to measure the force acting on an object as a product of its mass and acceleration. Newtonâ€™s third law of motion Whenever a body exerts a force on another body, the second body exerts an equal and opposite force on the first body

### Law of Conservation of Momentum

When two or more bodies acts upon each other their total momentum remains constant provided no external forces are acting

When this law is applied for a collision between two bodies, the total momentum of the colliding bodies before collision is equal to the total momentum after collision.

Total initial Momentum = Final Total Momentum

### Some Important Point to Note

 Why can dust be removed by shaking it, or beating it by a carpet? Before the carpet was shaken, the dust was at rest. When the carpet was set in motion, the dust tends to remains at rest due to inertia. As a result, dust got removed from carpet Why do passengers in the bus tend to fall back when it starts suddenly? When the bus accelerates from rest, the lower part of our body comes into motion along with the bus while the upper part of body tends to remain at rest due to inertia of motion and as a result which we fall backwards. Why is it difficult for a fireman to hose, which ejects large amount of water at a high velocity? Water Âˇs ejected with a large forward force (action). As we know by Newtonâ€™s third law of motion that every action has an equal and opposite reaction so, because of this action fireman experiences a large backward force or reaction. That is why he feels difficulty in holding the hose. Why is it advised to tie a rope on the luggage while you travel by the bus? Bus starts and stop suddenly during the journey and luggage can move because of law of inertia Why do passengers jumping out of a rapidly moving bus fall forward with his face downwards, if he does not run forward? The passengerâ€™s upper portion will remain in motion due to inertia even on falling on the ground and his lower portion will come to rest. So it will fall forward Why the cricket player moves his hand backwards on catching a fast cricket ball This is to prevent injuries to the hand. The time taken to reduce the momentum is increased in this way and less force acts on the hand What is the use of seat belt in Cars 1) If the car is stopped due to emergency braking, then the driver and passengers are no thrown forward so as to hit the steering wheel or wind screen2) The slightly stretchable seat belts worn by the passengersincrease the time taken by the passengers to fall forward and thusless stopping force acts on them