Assertion & Reason

Question 11 Mark

Assertion : Impulse and momentum have different dimensions.
Reason : From Newton's second law of motion, impulse is equal to change in momentum.

Answer

(e) From Newton's second law lmpulse = Change of momentum.So they have equal dimensions

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Question 21 Mark

Assertion : A body subjected to three concurrent forces cannot be in equilibrium.
Reason : If large number of concurrent forces acting on the same point, then the point will be in equilibrium, if sum of all the forces is equal to zero.

Answer

(e) A body subjected to three concurrent forces is found to in equilibrium if sum of these force is equal to zero.$\text { i.e. } \vec{F}_1+\vec{F}_2+\vec{F}_3+\ldots . .=0 \text {.$

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Question 31 Mark

Assertion : A table cloth can be pulled from a table without dislodging the dishes.
Reason : To every action there is an equal and opposite reaction.

Answer

(b) According to law of inertia (Newton's first law), when cloth is pulled from a table, the cloth come in state of motion but dishes remains stationary due to inertia. Therefore when we pull the cloth from table the dishes remains stationary.

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Question 41 Mark

Assertion : A reference frame attached to earth is an inertial frame of reference.
Reason : The reference frame which has zero acceleration is called a non inertial frame of reference.

Answer

(d) An inertial frame of reference is one which has zero acceleration and inwhichlawofinertia hold good i.e. Newton's law of motion are applicable equally. Since earth is revolving around the sun and earth is rotating about its own axis also, the forces are acting on the earth and hence there will be acceleration of earth due to these factors. That is whyearthcannotbetaken as inertial frame of reference.

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Question 51 Mark

Assertion : Newton's third law of motion is applicable only when bodies are in motion.
Reason : Newton's third law applies to all types of forces, e.g. gravitational, electric or magnetic forces etc.

Answer

(e) According to third law of motion it is impossibletohaveasingleforceoutofmutualinteraction between two bodies, whether they are moving or at rest. While, Newton's third lawisapplicable for all types of forces.

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Question 61 Mark

Assertion : Linear momentum of a body changes even when it is moving uniformly in a circle.
Reason : In uniform circular motion velocity remain constant.

Answer

(c) In uniform circular motion, the direction of motion changes, thereforevelocitychanges.As$P=m v$ therefore momentum of a body also changes in uniform circular motion.

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Question 71 Mark

Assertion : The acceleration produced by a force in the motion of a body depends only upon its mass.
Reason : Larger is the mass of the body, lesser will be the acceleration produced.

Answer

(b) According to Newton's second law,$F=m a \Rightarrow a=F /m$Forconstant(F$,acceleration is inversely proportional to mass i.e. acceleration produced by a force depends only upon the mass of the body and for larger mass acceleration will be less.

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Question 81 Mark

Assertion : A player lowers his hands while catching a cricket ball and suffers less reaction force.
Reason : The time of catch increases when cricketer lowers its hand while catching a ball.

Answer

(a) By lowering his hand player increases the time of catch, by doing so he experience less force on his hand because $F \propto 1 / d t$.

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Question 91 Mark

Assertion : In the case of free fall of the lift, the man will feel weightlessness.
Reason : In free fall, acceleration of lift is equal to acceleration due to gravity.

Answer

(a)

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Question 101 Mark

Assertion : When the lift moves with uniform velocity the man in the lift will feel weightlessness.
Reason : In downward accelerated motion of lift, apparent weight of a body decreases.

Answer

(e) For uniform motion apparent weight $=$ActualweightFordownwardacceleratedmotion,Apparent weight $<$ Actual weight

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Question 111 Mark

Assertion : The apparent weight of a body in an elevator moving with some downward acceleration is less than the actual weight of body.
Reason : The part of the weight is spent in producing downward acceleration, when body is in elevator.

Answer

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Question 121 Mark

Assertion : A rocket works on the principle of conservation of linear momentum.
Reason : Whenever there is a change in momentum of one body, the same change occurs in the momentum of the second body of the same system but in the opposite direction.

Answer

(a) As the fuel in rocket undergoes combustion, the gases so produced leavethebodyoftherocket with large velocity and give upthrust to the rocket. If weassumethatthefuelisburntataconstant rate, then the rate of change of momentum of the rocket will be constant. As morand more fuel gets burnt, the mass of the rocket goes on decreasing and it leads to increaseofthe velocity of rocket more and more rapidly.

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Question 131 Mark

Assertion : A bullet is fired from a rifle. If the rifle recoils freely, the kinetic energy of rifle is more than that of the bullet.
Reason : In the case of rifle bullet system the law of conservation of momentum violates.

Answer

(d) Law of conservation of linear momentum is correct when no external forceacts.Whenbulletis fired from a rifle then bothshouldpossessequalmomentumbutdifferentkineticenergy.E(=\frac{P^2}{2 m} \therefore$ Kinetic energy of the rifle is less than thatofbulletbecause$E\propto1m$

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Question 141 Mark

Assertion : linear momentum of a body changes even when it is moving uniformly in a circle.
Reason : Force required to move a body uniformly along a straight line is zero.

Answer

(b) In uniform circular motion of a body thespeedremainsconstantbutvelocitychangesasdirection of motion changes.As linear momentum $=$ mass $\times$ velocity, therefore linearmomentum of a body changes in a circle.On the other hand, if the body is movinguniformlyalongastraight line then its velocity remains constant and hence acceleration is equal to zero. So force is equal to zero.

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Question 151 Mark

Assertion : The work done in bringing a body down from the top to the base along a frictionless incline plane is the same as the work done in bringing it down the vertical side.
Reason : The gravitational force on the body along the inclined plane is the same as that along the vertical side.

Answer

(c) Work done in moving an object against gravitational force (conservative force)dependsonly on the initial and final position of the object, not upon the pathtaken.Butgravitationalforce on the body along the inclined plane is not same as that along the vertical and it varieswith the angle of inclination.

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Question 161 Mark

Assertion : A cyclist always bends inwards while negotiating a curve.
Reason $\quad$ : By bending, cyclist lowers his centre of gravity.

Answer

(c) The purpose of bending is to acquire centripetalforceforcircularmotion.Bydoingsocomponent of normal reaction will counter balance the centrifugal force.

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Question 171 Mark

Assertion : The slope of momentum versus time curve give us the acceleration.
Reason : Acceleration is given by the rate of change of momentum.

Answer

(d) $F=\frac{d p}{d t}=$ Slope of momentum-time graphi.e. Rate of change of momentum =Slope of momentum- time graph $=$ force.

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Question 181 Mark

Assertion : Mass is a measure of inertia of the body in linear motion.
Reason : Greater the mass, greater is the force required to change its state of rest or of uniform motion.

Answer

(a) According to Newton's second law of motion $a=\frac{F}{m$i.e.magnitudeoftheacceleration produced by a given force is inversely proportional to the mass of the body. Higher isthemass of the body, lesser will be theaccelerationproducedi.e.massofthebodyisameasureoftheopposition offered by the body to change a state, when the force is applied i.e. mass of a body is the measure of its inertia.

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Question 191 Mark

Assertion : No force is required by the body to remain in any state.
Reason : In uniform linear motion, acceleration has a finite value.

Answer

(c) Force is required to change the state of the body. In uniform motionbodymoveswithconstant speed so acceleration should be zero.

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Question 201 Mark

Assertion : Aeroplanes always fly at low altitudes.
Reason : According to Newton's third law of motion, for every action there is an equal and opposite reaction.

Answer

(a) The wings of the aeroplane pushes the external air backward and the aeroplane moveforward by reaction of pushed air. At low altitudes. density of air is high and so the aeroplane gets sufficient force to move forward.

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Question 211 Mark

Assertion : If two objects of different masses have same momentum, the lighter body possess greater velocity.
Reason : For all bodies momentum always remains same.

Answer

(c) According to definition of momentum$P=m v$ if $P=$ constant then $m v=$ constant or $v \propto \frac{1}{m}$.As velocity is inversely proportional to mass, thereforelighterbody possess greater velocity.

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Question 221 Mark

Assertion : Force is required to move a body uniformly along a circle.
Reason : When the motion is uniform, acceleration is zero.

Answer

(b) When a body is moving in a circle, its speed remains same but velocity changesduetochange in the direction of motion of body. According to first lawofmotion,forceisrequiredtochangethe state of a body. As in circular motionthedirectionofvelocityofbodyischangingsotheacceleration cannot be zero. But for a uniform motion acceleration is zero (for rectilinear motion).

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Question 231 Mark

Assertion : Newton's second law of motion gives the measurement of force.
Reason : According to Newton's second law of motion, force is directly proportional to the rate of change of momentum.

Answer

(a) According to second law $F=\frac{d p}{d t}=m a$.If we know the values of $m$ and(a$, the force acting on the body can be calculated and hence second law gives that how much force is applied on the body.

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Question 241 Mark

Assertion : If the net external force on the body is zero, then its acceleration is zero.
Reason : Acceleration does not depend on force.

Answer

(c) According to Newton's second lawAcceleration $=\frac{\text { Force }}{\text { Mass }}$ i.e. if net external force on the body is zero then acceleration will be zero

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Question 251 Mark

Assertion : Inertia is the property by virtue of which the body is unable to change by itself the state of rest only.
Reason : The bodies do not change their state unless acted upon by an unbalanced external force.

Answer

(e) Inertia is the property by virtue of which the body is unable to change by itself notonly the state of rest, but also the state of motion.

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