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Define dynamics?

Difficulty: Easy


          The branch of mechanics that deals with the study of the motion of an object

and the cause of its motion is called dynamics.

Define Force.

Difficulty: Easy


          A force moves or tends to move stops or tends to stop the motion of a body. The force can also change the direction of motion of a body

                                                 F = ma

SI unit of force is newton

Note: A force can also change the shape or size of a body in which it acts.

Define inertia. Experiment to understand inertia?


What is the law of inertia?

Difficulty: Medium


The inertia of a body is its property due to which it any change in its state of rest or motion.

Galileo related the inertia of a body with its mass, the greater the mass of a body greater its inertia.

Inertia ∝   mass of the body



Take a glass and cover it with a piece of cardboard. Place a coin on the cardboard. Now kick the card horizontally, with a jerk of your finger.

The coin does not move with the cardboard due to inertia. The coin falls into the glass as the card flicks away.



Cut a strip of paper. Place it on the table. Stack a few coins at one end. Pull out the paper strip under the coins with a jerk.       

Coins stacked over remain undisturbed on pulling the paper strip quickly

Coins stacked over remain undisturbed on pulling the paper strip quickly due to inertia.


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Put a one-rupee coin over a piece of card paper placed on an empty glass. Push the card with a sudden stroke of a finger. The card will move ahead while the coin falls into the glass. Why it does do?

Difficulty: Easy

Due to inertia card will continue its motion in the forward direction while the coin will remain at rest and will fall into the glass.

Define Momentum.

Difficulty: Medium


          The momentum of a body is the quantity of motion it possesses due to its mass and velocity.

          The momentum P of a body is given by the product of its mass m and velocity v. Thus

P = mv

          Momentum is a vector quantity.

          The momentum of a system depends on its mass and velocity.

A bullet has very small inertia due to its small mass. But why does its impact so strong when it is fired from the gun?

Difficulty: Easy

According to the law of conservation of momentum, the mass of the bullet is much smaller than the gun therefore the recoil is much greater than the velocity of the gun. Therefore, the impact of the bullet is very strong.

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Why the impact of the loaded truck on a body coming it's way is very large even if the truck is moving slowly.

Difficulty: Easy

The greater the mass of the truck, the greater will be its momentum. Therefore, the loaded truck has a large impact.

Explain Newton’s first law of motion by a practical example of daily life?

Difficulty: Medium

Newton’s laws of motion:

          A body continues its state of rest or uniform motion in a straight line provided no net force acts on it.


Case I (Part I):

According to Newton's first law of motion, a body at rest remains at rest provided no net force acts on it. This part of the law is true as we observe that objects do not move by themselves unless someone moves them. For example, a book lying on a table remains at rest as long as no net force acts on it.

Case II (Part II):

 Similarly, a moving object does not stop moving by itself. A ball rolled on the rough ground stops earlier than that rolled on smooth ground. It is because rough surfaces offer greater friction. If there would be no force to oppose the motion of a body then the moving body would never stop.

Why newton’s first law of motion is also known as the law of inertia?

Difficulty: Easy

Since Newton's first law of motion deals with the inertial property of matter, therefore, Newton's first law of motion is also known as the law of inertia.

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Why do the passengers standing on a bus fall forward when the driver applies the brakes suddenly?

Difficulty: Easy

The passengers standing on a bus fall forward when its driver applies the brakes suddenly. It is because the upper parts of their bodies tend to continue their motion, while lower parts of their bodies are in contact with the bus stop with it. Hence, they fall forward. 

When a bus takes a sharp turn, passengers fall in an outward direction. Why?

Difficulty: Easy

When a bus takes a sharp turn, passengers fall in the outward direction. It is due to inertia that they want to continue their motion in a straight line and thus fall outwards.

Define net force?

Difficulty: Easy

Net force:

            The net force is the resultant of all the forces acting on a body.

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State and prove Newton’s second law of motion.


Show that F = ma.

Difficulty: Medium

Newton’s second law of motion:

           When a net force acts on a body, it produces acceleration in the body in the direction of the net force. The magnitude of this acceleration is directly proportional to the net force acting on the body and inversely proportional to its mass.


 Derivation of formula F = ma:

          If a force produces an acceleration ‘a’ in a body of mass ‘m’, then we can state mathematically that

a \: \alpha \: F...... (1)

and    $a \: \alpha\frac{1}{m}$

By combining (i) and (ii)

Or   $a \: \alpha\frac{F}{m}$

Or   $F \: \alpha \: ma$

Putting k as proportionality constant, we get

F = kma...... (iii)


 In SI units, the value of k comes out to be 1. Thus Eq. (iii)

                            F = ma

Define SI units of force?

Difficulty: Easy

SI units of force:

           SI unit of force is newton (N).


Newton (1 N):

           One newton (1 N) is the force that produces an acceleration of $1 \: ms ^{-2}$ in a body of mass of 1 kg.

        Thus, a force of one newton can be expressed as

$1 \: N \: = 1 \:kg \: \times 1 \:ms^{-2}$

Or  $1 \: N \: = 1 \:kgms^{-2}$


Differentiate between mass and weight.

Difficulty: Medium

Mass and weight:




1.The quantity of matter contained in a body is called its mass.

1.Weight is a force with which earth attracts a body towards its centre.

2.The mass of a body remains constant everywhere, whether it is measured at a point far away from the centre of the earth, or the surface at the centre of the earth.

2.The weight of a body is not a constant in quantity but its value is different at different places.

3.Mass is a scalar quantity.

3.Weight is a vector quantity and is always directed towards the centre of the earth.

4.The SI unit of mass is kilogram.

4.The SI unit of weight is newton.

5.Mass is measured by ordinary balance.

5.weight is measured by spring balance.

6.Mass of the body cannot be zero.

6. The weight of the body can be zero i,e at the centre of the earth and in space where g=0 so w=mg=m×0=0

7. It is the measure of inertia in a body i.e: m=

7. Weight is given by w=mg

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