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First law of motion - class-XII

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Physical independence of force is a consequence of 

  1. First law of motion

  2. Second law of motion

  3. Third law of motion

  4. All of these laws


Correct Option: A
Explanation:

Physical independence of force is a consequence of first law of motion.

Mark the correct option

  1. The state of motion of an object is described by its speed only

  2. Many a time application of force does not result in a change in the state of motion of an object

  3. By sitting on a bicycle seat force is applied to the spring fixed to the seat of a bicycle

  4. All of the above


Correct Option: D
Explanation:
$(i)$ option $A$ is correct, as if to know about the unifomity of a motion and the body is moving or not speed is used as the parameter
$(ii)$Option B is correct, as the acceleration of a body depends on the net force a applied not on single $10r$ individual forces.
$(iii)$ Option $C$ is correct, while sifting on the bicycle, the spring force allached to the seat balances weight.
Option $D$ 

Consider the following statements
I) When a body is travelling with uniform speed, no force is acting on it.
II) When a body is travelling with constant velocity, no force is acting on it.
Which of the above statements is/are true?

  1. only I

  2. only II

  3. both I and II

  4. neither I nor II


Correct Option: A

Newton's law of motion cannot be applicable to the particle moving at a speed comparable to the speed of

  1. $Light$

  2. $Sound$

  3. $Rocket$

  4. $Bullet train$


Correct Option: A
Explanation:

Newton's law of motion are applicable for matter which occupy some space and have some mass. It is not possible for any matter to travel with speed of light. Secondly the mass charges as the speed approaches speed of light. Newton's law is applicable only when mass is constant so light is correct option.

Inertia of a body directly depends on its :

  1. Velocity

  2. Mass

  3. Area

  4. None of the above


Correct Option: B
Explanation:

Inertia of body directly depends on its mass.

The correct option is B. 

Choose the correct statement(s)

  1. Motion necessarily takes place along the direction of force

  2. If no force acts, the body is at rest

  3. A body in motion, need not be acted upon by a force

  4. A change in speed is always there, whenever a force is applied on the moving body


Correct Option: C
Explanation:

Motion always takes place along velocity of the body.So it does not necessarily takes place along the direction of force.
When a body moves with uniform velocity , there is no force acting on the body. So option B is incorrect and C is correct.
We know that magnetic force does not do work. It can change the direction of motion but never change the speed. So option D is incorrect.

Hair of shaving brush cling together when it is removed from water due to

  1. Force of attraction between hair

  2. Surface tension

  3. Viscosity of water

  4. Charactersitic property of hair


Correct Option: B
Explanation:

When the brush is taken out of water, the hairs cling together due to surface tension. This happens because the free surface of water tries to acquire a minimum surface area.

Hence, option B is correct.

Which of the following Newton's law(s) is/are not applicable in non-inertial reference frame. 

  1. Newton's first law

  2. Newton's second law

  3. Newton's third law

  4. All of these


Correct Option: A
Explanation:

Newton's law(s) is/are not applicable in non-inertial reference frame in newton's first law.

so the option is A.

Is force needed to keep a moving body in motion? Which one of the Newton's laws follow this ?

  1. No, first law.

  2. Yes, third law.

  3. No, second law

  4. Yes, first law.


Correct Option: A
Explanation:

No, force is not needed to keep a moving body in motion. This is according to Newton's first law of motion.

A ball moving on a table top eventually stops. What is the reason ?

  1. due to gravity.

  2. due to friction between the ball and table

  3. due to other external force.

  4. none of these.


Correct Option: B
Explanation:

Due to the force of friction between the ball and table top, it  opposes the motion of the ball. And, gradually its speed keeps on decreasing and the ball finally stops.

Which Newton's law is related to the Galileo's law of inertia ?

  1. Second.

  2. First

  3. First and second

  4. Third


Correct Option: B
Explanation:

 Galileo's Law of Inertia stated: "A body moving on a level surface will continue in the same direction at constant speed unless disturbed."

Object will remain at rest or in uniform motion in a straight line unless acted upon by an external force - which law is this ?

  1. Newton's third law

  2. Newton's first law

  3. Newton's second law

  4. None of these


Correct Option: B
Explanation:

Statement of Newton's first law of motion,

It states that a body will remain at rest or in state of motion unless an external force is applied to the object.
To have any changes in state of object , force is necessary.

What is the force on a body of mass $m$ when it is moving with a constant velocity ?

  1. $mg$

  2. $0$

  3. $\dfrac{m}{g}$

  4. $g$


Correct Option: B
Explanation:

When a body moves with constant velocity, its acceleration is zero ($a=0$).

So, force required, $F=m \times a=m \times 0 = 0$

An aeroplane of mass is moving uniformly at a constant height under the action of two forces (i) upward force (lift) and (ii) downward force (weight). What is the net force on the aeroplane.

  1. Zero.

  2. mg

  3. only downward force

  4. only upward force


Correct Option: A
Explanation:

If the net force is not zero the plane would be moving upward or downward. Since its height is constant the net force is 0

The acceleration produced in a body by a force of given magnitude depends on:

  1. size of the body

  2. shape of the body

  3. mass of the body

  4. none of these


Correct Option: C
Explanation:

Newton's second law of motion, $F=ma$, where $m$ is mass of an object which acquires acceleration $a$ when force $F$ is applied on it.
For a force of constant magnitude, $a\propto{\dfrac{1}{m}}$ i.e. acceleration is inversely proportional to mass.

State whether given statement is True or False.
An object moving on a frictionless horizontal surface should move with constant velocity

  1. True

  2. False


Correct Option: A
Explanation:
True,
Newtons law of motion states that a body moving with constant velocity will continue moving with same velocity if any external force is absent. In the given case above case surface is frictionless and any other external force is absemt so body will move with same constant velocity.

An object moving at constant velocity must:

  1. have a net force on it

  2. eventually stop due to gravity

  3. not have any force of gravity on it

  4. have zero net force on it


Correct Option: D
Explanation:

The object moving with constant velocity must have zero acceleration i.e. $a _{net} = 0$. From Newton's IInd law the net external force must be zero in case of zero acceleration.

Net external force acting on the object  $F _{net} = ma _{net} = 0$
Hence answer D is correct.

Every body continues to be in its state of rest or of non-uniform motion in a straight line unless compelled by some external force to act otherwise. This is stated by:

  1. Newton's 1st law

  2. Newton's 2nd law

  3. Newton's 3rd law

  4. None of the above


Correct Option: D
Explanation:

The premise of question is external force is zero and conclusion is  state of motion of body will be at rest or non uniform motion. However, the non uniform motion is only applicable in case of external force being applied to body, hence the statement is false.

So, given statement is stated by none of Newton's laws.

If no net force acts on an object, then the velocity of the object:

  1. remains same

  2. becomes zero

  3. increases

  4. decreases


Correct Option: A
Explanation:

Newton's first law indicates that if no net force acts on an object, then the velocity of the object remains unchanged.

If there are many forces acting on an object, it must have a non-zero net (unbalanced) force acting on it.

  1. True

  2. False


Correct Option: B
Explanation:

In case of many forces acting on the body, it can be in equilibrium,which implies zero external force. For example in case of ladder supported by a wall there are frictional and gravitational forces working at wall and floor, which balances each other so that ladder does not move. Hence the given statement is false.

A ball is moving over a horizontal smooth surface with a constant velocity. Sum of external forces is:

  1. zero

  2. positive

  3. negative

  4. unity


Correct Option: A
Explanation:

The ball is moving with a constant velocity, this implies the sum of external forces is zero

State whether given statement is True or False.
Is it possible to move in a curved path in the absence of a force?

  1. True

  2. False


Correct Option: B
Explanation:

Any curve path can be assumed as summation if small circular arcs  of different radii. For a path to move at circular arc, there must be some force to provide it centripetal force to remain it in circular path. It is not possible to have motion along curved path without any force.

Consider a ball at rest in the middle of a toy wagon. When the wagon is pulled forward, the ball:

  1. Rolls forward

  2. Stays stationary

  3. Rolls sidewards

  4. Rolls backwards


Correct Option: D
Explanation:

The relative motion between ball and toy wagon is zero before being pulled. As the wagon is pulled, the inertia of ball causes ball to maintain its state of rest, so the ball rolls backwards when wagon is moved in forward direction.

State whether given statement is True or False.
You fall in the forward direction when a moving bus applies brakes to stop, and fall backward when it accelerates from rest.

  1. True

  2. False


Correct Option: A
Explanation:

In a moving bus, passengers are in motion along with bus. When brakesare applied to stop a moving bus, bus comes in the position of rest. But because of tendency to be in the motion a person falls in forward direction.Similarly, when a bus is accelerated from rest, the tendency to be in rest, a person in the bus falls backwards.

Change in the state of rest or of uniform motion is brought about by:

  1. Acceleration

  2. Force

  3. Energy

  4. None


Correct Option: B
Explanation:

Whenever an external force acts on a body there is a change in motion of the body. 

It is advised to tie any luggage kept on the roof of a bus with a rope because of :

  1. Newtons first law

  2. Newtons second law

  3. Newtons third law

  4. None of these


Correct Option: A
Explanation:

Luggage kept on the roof of a bus has the tendency to maintain its state of rest when bus is in rest and to maintain the state of motion when bus is in motion according to Newtons First Law of Motion. When bus will come in motion from its state of rest, in order to maintain the position of rest, luggage kept over its roof may fall down(backward). Similarly, when a moving bus will come in the state of rest or there is any sudden change in velocity because of applying of brake, luggage may fall down(forward) because of its tendency to remain in the state of motion.
This is the cause that it is advised to tie any luggage kept on the roof a bus with a rope so that luggage can be prevented from falling down.

A horse tied to a cart is standing stationary on road. Which of the following statement is CORRECT?

  1. Horse is applying force on the cart

  2. Cart is applying force on the horse

  3. Force applied by the horse is balanced by the force applied by the cart

  4. No forward force is being applied by the horse or the cart


Correct Option: D
Explanation:

The horse tied to cart is standing stationary on road denotes that the net force acting on the horse as well as the cart individually is zero. If it is standing stationary, it means that horse is applying force on the road perpendicularly and road is also applying same magnitude of force in the opposite direction, which makes the net force on horse to be zero and hence no forward force on the horse is there.

A coin flicked across a table will stop, because:

  1. it is heavy

  2. no force is acting on it

  3. earth attracts the coin

  4. table exerts a frictional force


Correct Option: D
Explanation:

The table exerts a frictional force on the coin in the reverse direction and thus the coin stops across the table.

Which law of Newton defines an ‘inertial frame of reference’?

  1. First law of motion

  2. Second law of motion

  3. Third law of motion

  4. Law of gravitation


Correct Option: A
Explanation:

First Law of Newton states that when viewed in an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity unless acted upon by an external force.

A particle is moving with a constant speed along a straight line path. A force is not required to:

  1. Increase its speed

  2. Decrease its momentum

  3. Change the direction

  4. Keep it moving with uniform velocity


Correct Option: D
Explanation:

Newton's first law of motion states that if an object experiences no net force, then its velocity is constant. So, force is not required to keep object moving with uniform velocity.

Newton's first law of motion is also called Galileo's law of:

  1. Conservation of momentum

  2. Inertia

  3. Friction

  4. Conservation of mass


Correct Option: B
Explanation:

Newton's first law of motion states that the object tends to remain in the initial state untill no external force is applied to it which is also called Galileo's law of inertia.

When a body is stationary :

  1. there is no force acting on it

  2. the combination of forces acting on it balance each other

  3. the force acting on it are not in contact with it

  4. the body is in vacuum


Correct Option: B
Explanation:

For the body to be stationary, net external force acting on it must be equal to zero which is possible only when the combination of forces acting on it balance each other.

Which law is involved in the act of cleaning a carpet by beating it with a stick ? 

  1. First law of motion.

  2. Second law of motion

  3. Law of conservation of energy

  4. Law of conservation of mass


Correct Option: A
Explanation:

The dust particles get separated out from the carpet by beating it with a stick due to their inertia of rest which is explained by Newton's first law of motion.

Idea of inertia came from:

  1. Newton's third law of motion

  2. Newton's second law of motion

  3. Newton's first law of motion

  4. Law of gravitation


Correct Option: C
Explanation:

Newton's first law of motion states that the body tends to remain in the initial state if no external force is applied to it which gave the idea of inertia.

Newton's law of motion were stated by the scientist named as Charles-Augustin 

  1. True

  2. False


Correct Option: B
Explanation:

Newton's laws of motion published in $1687$ by the scientist $Issac$ $Newton$ in his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy).

Why is it advised to tie any luggage kept on the roof of a bus with a rope?

  1. While the bus is moving, luggage tends to remain in inertia of motion state

  2. When the bus stops, the luggage tends to resist the change and due to inertia of motion it move forward and may fall off

  3. Both a and b are true

  4. None of them is True


Correct Option: C
Explanation:

While the bus is moving, luggage tends to remain in inertia of motion state. When the bus stops, the luggage tends to resist the change and due to inertia of motion it moves forward and may fall off. That's why it is advised to tie any luggage kept on the roof of a bus with a rope.

The law that gives a qualitative definition of force is :

  1. Newton's second law of motion

  2. Law of inertia

  3. Newton's third law of motion

  4. Law of gravitation


Correct Option: B
Explanation:

Law of inertia states that the body tends to remain in the same state (regarding its velocity and direction) untill no external force is acting on it.

Hence option B is correct.

Which of the following equations or principle is most suitable to solve given question.  

A block of mass $m$ remains at rest on a table when a piece of paper is pulled out from under it.

  1. Newton's first law(law of inertia)

  2. Newton's second law ($F _{net}$ = ma)

  3. Newton's third law (For every action there is an equal and opposite reaction)

  4. Newton's law of universal gravitation

  5. Conservation of linear momentum


Correct Option: A
Explanation:

Newton's First Law of Motion - sometimes referred to as the law of inertia states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Hence a piece of mass remains at rest on a table when a piece of paper is suddenly pulled out from under it.

If no force acts on a body, then what will happen to it?

  1. It does not change its shape.

  2. It continues to move but with less speed.

  3. It changes its direction.

  4. It can either remain at rest or continue to move in a straight line.


Correct Option: D
Explanation:
The force is define as $F=ma$
hence acceleration is $a=\dfrac{F}{m}$
if  balanced force acts on a body F is  zero hence body must not be accelerated. hence by eq of motion $u=v=at,a=0\implies u=v$ if v=0,u=0
It can either remain at rest or continue to move in a straight line.

Physical independence of force is a con-sequence of 'X'. Identify 'X'.

  1. Newton's third law of motion

  2. Newton's second law of motion

  3. Newton's first law of motion

  4. All of the above


Correct Option: C
Explanation:

Physical independence of force is a con-sequence of 'X'.

X is Newton's first law of motion Newton's first law of motion - sometimes referred to as the law of inertia. An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force

State whether true or false.
A ball thrown upwards in a train moving with uniform velocity returns to the thrower. 
  1. True

  2. False


Correct Option: A
Explanation:

Since train is moving with uniform velocity so when the thrower throws the ball upward ball has already acquire the same velocity as that of moving train and have moved the same distance as that of thrower. 

When a bus suddenly takes a turn, the passengers are thrown outwards because of 

  1. Inertia of motion

  2. Acceleration of motion

  3. Speed of motion

  4. Both (b) and (c)


Correct Option: A
Explanation:

According to law of inertia of motion a body will continue moving with constant velocity until unless any force is acting on it. When a bus is moving straight the passengers inside are also moving straight but when suddenly bus turn left passengers still continue to move straight  because no force is acting on passengers so they will move with same magnitude and direction i.e. outwards when bus turns .

Which Newton's law of motion defines force?

  1. First

  2. Second

  3. Third

  4. None


Correct Option: A
Explanation:

Newton's first law of motion gives definition of force.

Newtons law are not valid in

  1. Both inertial as well as non- inertial frame of reference.

  2. A frame moving with constant velocity w.r.t. an inertial frame.

  3. All reference frames which are at rest w.r.t. an inertial frame.

  4. The reference frame attached to the earth.


Correct Option: A
Explanation:

Newton's Law is only valid in inertial frame of reference. If there is a non - inertial frame of reference then it won't be valid. 

Hence option $\textbf A$ is the correct answer.

A force vector applied on a mass is represented by $\vec {F} = 6\hat {i} - 8\hat {j} + 10\hat {k}$ and accelerates with $1\ m/s^{2}$. What is the mass of the body?

  1. $14\ kg$

  2. $10\sqrt {2}kg$

  3. $2\sqrt {10}kg$

  4. $20\ kg$


Correct Option: A
Explanation:
To find mass of the body we will use magnitude of force formula $,$ 
According to question$:-$
$\overrightarrow { F } =6-8\widehat { j } +10\widehat { k } $
Magnitude of Force $=\sqrt {\left( {{6^2} + {8^2} + {{10}^2}} \right)} $
                                  $= 14.14 N$
$m = F / a$
     $= 14.14 N / 1 m/s^2$
     $= 14.14 kg$
Hence,
option $(A)$ is correct answer.

A force is not required to keep a body in a 

  1. Uniform circular motion

  2. Simple harmonic motion

  3. Motion with constant acceleration

  4. Motion with constant velocity


Correct Option: D
Explanation:

If force on any body is zero than its acceleration should be zero that means its velocity would remain constant.

Which of the following is different from others?

  1. watt-sec

  2. coulomb-volt

  3. newton-metre

  4. coulomb-frarad


Correct Option: D

A rain drop of mass $0.1g$ is falling with uniform speed of $10{ cm/s }$. What is the net force the drop?

  1. ${ 10 }^{ -2 }N$

  2. ${ 10 }^{ -3 }N$

  3. $2\times { 10 }^{ -3 }N$

  4. $zero$


Correct Option: D
Explanation:

Uniform speed means that a body is traveling at a constant speed along a straight line. So it has no acceleration. Thus net weight $=ma=0$.

A car driver going at a speed v suddenly find a wide wall at a distance. (coefficient of friction is $\mu $) 

  1. If the apply breaks, he will come to rest after a distance $\dfrac { { { v }^{ 2 } } }{ 2\mu g } $

  2. If he turn the cart in a circle to avoid hitting, then radius of the circle is $\dfrac { { { v }^{ 2 } } }{ \mu g } $

  3. It is better to apply breaks rather than to turn the vehicle in a circle

  4. It is better to turn the vehicle in a circle than to apply brakes


Correct Option: B
Explanation:

If he turn the cart in a circle to avoid hitting then radius of the circle is $\dfrac{{{v^2}}}{{2\mu g}}$
Option $B$ is correct answer.

Statement 1 : There can be displacement of an object even in the absence of any force acting on it.
Statement 2 : An object in uniform motion in a straight line shows displacement even when the net force acting on it is zero.

  1. Both statement 1 and statement 2 are true and statement 2 is correct explanation of statement 1

  2. Both statement 1 and statement 2 are true, but statement 2 is not correct explanation of statement 1

  3. Statement 1 is true but statement 2 is false

  4. Statement 1 is false but statement 2 is true


Correct Option: A
Explanation:

Answer is A.

If any body is in the state of rest or in uniform motion, it remains in that state until an external force is applied on it. 
So, if a body is initially moving with some constant velocity, it can continue to move with same velocity even if no external force is acting on it.
In fact, the effect of force is to accelerate and not just displace. For example, an object moving straight in space far away from any celestial objects will continue to move forever, although there is no force acting on it. This is due to Newton's first law of motion or law of inertia.
Hence, 
Both statement 1 and statement 2 are true and statement 2 is correct explanation of statement 1.

Newton's law of motion were published in the year 1699:

  1. True

  2. False


Correct Option: B
Explanation:

Newton's laws of motion published in $1687$ by the scientist $Issac$ $Newton$ in his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy).

While dusting a carpet, we give it a sudden jerk or beat it with a stick because: 

  1. the inertia of rest keeps the dust in its position and the dust is removed when the carpet moves away.

  2. the inertia of motion removes dust.

  3. no inertia involved in process.

  4. jerk compensates for the force of adhesion between dust and carpet and dust is removed.


Correct Option: A
Explanation:

While dusting we give a sudden serk our beat with stick because when we do that stick because when we do that the particle of carpet move with our help and the dust particle stays at rest by inertia and eventually they separate from the carpet.

Physical independence of force is a consequence of:

  1. Newton's third law of motion

  2. Newton's second law of motion

  3. Newton's first law of motion

  4. All of these


Correct Option: C
Explanation:

Newton''s first law of motion is related to physical independence of force.

Hence,
option $C$ is correct answer.

A person sitting in an open car moving at constant velocity throws a ball vertically up into air. Where will the ball fall?

  1. Outside the car

  2. In the car ahead of the person

  3. In the car to the side of the person

  4. Exactly in the hand from which it was thrown up


Correct Option: D
Explanation:

The ball has forward velocity equal to the velocity of the car even when ball is given velocity upwards.

So, the ball will cover the same distance as that by the car, and hence it will land in the hands of the person.

Who among the following anticipated Newton by declaring that all things gravitate to the earth?

  1. Buddhagupta

  2. Varahamihira

  3. Aeyabhatta

  4. Brahmagupta


Correct Option: D
Explanation:

Brahmagupta  the  anticipated Newton by declaring that all things gravitate to the earth.

Hence,
option $D$ is correct answer.

Inertia is the property of a body which preserves its state of motion or uniform motion in a straight line. The following factors tell me about inertia.
I. Greater the mass of a body, greater is its inertia.
II. Greater the inertia of a body, the less will be the acceleration produced by a given force.
III. The law of inertia is the same as Newton's first law of motion. 
Which combination is true?

  1. I and III only

  2. I and II only

  3. I, II and III

  4. II and III only


Correct Option: C

A block is moved from rest through a distance of 4m along a straight line path.The mass of the block is 5 kg,and the  force acting on it is 20 N.If the kinetic energy acquired by the block be 40J,at what angle to the path the force is acting:

  1. $30^o$

  2. $60^o$

  3. $45^o$

  4. none of the above


Correct Option: B
Explanation:
From work Energy theorem 
$ \triangle w = \triangle KE $ 
$ F.S = K.E $
$ F.S cos\theta = KE  $
$ cos\theta = \frac{KE}{FS} $
$ cos\theta = 1/2 $
$ \theta = 60^{\circ} $ 
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