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NCERT Solutions for Class 9th Science


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Chapter 9. Force and Laws of Motion

Chapter Review




 Chapter 9. Force And Law Of Motion

Q-  What is force ?

A – It is a push or pull which change or has a tendency to change the position of body. It’s SI unit is kgm-2 .

Q- Write the Newton’s law of motion?

A –

  1. A body continues to maintain its position of rest or motion unless an antennae force  acts upon it
  2. The rate of change of momentum of body is directly proportional to the force applied and takes placed in the direction of force.
  3. For every action there is equal and opposition reaction.

Q:- What is balance force ?

A:- If the resultant of the several force acting a body is zero than force are said to be  balance force.

Q - What is unbalance force?

A- If the resultant of the several force acting a body is not zero than force are said to be balance force.

Q – What is momentum?

A – it is quantity of motion possessed by a body is equal to the product of the mass and velocity of the body. Its SI unit is kgms-1 Momentum = mass x velocity (p=mv)

Q – Define Newton?

A - It is the unit of force.

Q – What is inertia?

A – The tendency of undisturbed objects to stay at rest or to keep

 moving with the same velocity is called inertia.

Q – How can we reduce the effect of friction force ?

A – 

  1. Using a some their plane or surface.
  2. Using a lubricant.
  3. Polishing.

Q-  Why is the Newton’s first low of motion also called “Low of inertia”?

A – According to Newton first low of motion an object by itself is not able to change its state of rest or of uniform motion. This property of n is called inertia. That is why the Newton first low of motion is called low of Inertia.

Q- Define mass and inertia?

A –Inertia is the natural tendency of an object to result of motion. The mass of an object is a measure of its inertia.

Q- What are the different types of inertia?

A – Three types are inertia:-

  1. Inertia of rest.
  2. Inertia of motion.
  3. Inertia of direction.

Q- What is the unit of mass and inertia?

A – K. g.

Q- Which scientist proved first that object move with constant speed when no force acts on them?

A – Galileo.

Q- Why does moving marble stops after travelling some distance?

A -  A moving marble stops after travelling some distance because of the friction. Force acting opposite to the direction of motion.

Q- Who introduced the concept of momentum?

A – Newton.

Q- Which low of motion gives the mass are of force?

A – Second low of motion.

Q  - What happen when an unbalanced force is applied on the object ?

A  - when an unbalanced force is applied on the object:-

  1. Change its speed .
  2. Change its direction of motion.

Q – Why safety belts are worn during driving ?

A- A sudden application of breaks may cause injury to us by impact or collisions with the panels in front safety belt are worn to prevent from such accidents. Safety belts exert a force on our to make the forward motion slower.

Q – When does an object move?

A  - An object moves when the object  is under the continues application of an unbalanced force.

Q – Define inertia of rest and give some example?

A- the tendency of a body to remain  its position  of rest is called inertia of rest. Example;-

  1. Dust is removed from a hanging carpet by beating with a stick. As the carpet is beaten it suddenly moves forward while the dust particle tend to remain at rest due to inertia and so fall off.
  2. When we snake a tree, the fruits or dry to inertia of rest.

Q  - Example how Newton’s first law of motion is a special case of second law?

A  - According to Newton’s second law ;-

                                    F= ma

                                    F =

                                    Ft= mv – mu


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

Q – how many types of force in the third law of motion?


  1. Action
  2. Reaction

Q – What types of force dose make us possible to walk?

A – Reaction

Q- Name the weakest and strongest force from in nature?

  • Weakest = gravitation force.
  • Strongest = Nuclear force

Q – What is the SI unit of weight?

A- Newton  


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