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CBSE Questions for Class 9 Physics Force And Laws Of Motion Quiz 1 - MCQExams.com
CBSE
Class 9 Physics
Force And Laws Of Motion
Quiz 1
A linear momentum of a body can be changed by:
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any force
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a net external force
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an internal force
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Both (2) and (3)
Explanation
Internal forces do not cause a change in momentum. Only external forces cause change in momentum.
Mark the correct option
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Force can be seen
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When an inflated balloon is pressed between the two hands,only its shape changes
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Force is a push,pull or squeeze
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All
Explanation
Force cannot be seen.we can 'see' or 'feel' the force through its effects.when an inflated balloon is pressed between the two hands,its shape and size changes.
Force acting on a body is a push,pull or a squeeze.
When a spring is pulled:
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Shape of the spring changes
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Size of the spring changes
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Size of the spring doesn't change
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Both $$A$$ and $$B$$
Explanation
Force is an external agent (push or pull) which changes the state of motion or rest of an object.
When spring is pulled, both the
shape and
size of the spring change.
The state of rest or state of motion of a body can be changed by using
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Pressure
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Force
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Heat
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None of these
Explanation
First law of motion:
A body in rest or a body in motion will continue to do that unless and until an external force is applied on it.
Force will accelerate or decelerate a body.
Use Newton's 2nd law of motion, find out the third missing quantity if mass and force on a body is given?
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Displacement
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Velocity
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Acceleration
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Linear momentum
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Kinetic energy
Explanation
From Newton's second law, $$force$$ $$(F)$$ $$=$$ $$mass (m)$$ $$\times$$ $$ acceleration (a)$$
As mass and force is given so missing quantity will be acceleration.
A gun recoils after firing to conserve:
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velocity
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momentum
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force
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speed
Explanation
According to law of conservation of momentum, initial momentum is equal to the final momentum.
So, in order to conserve momentum, the gun recoils. Velocity, force and speed are not conserved.
When a body is travelling at constant velocity, the net force on it is zero.
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True
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False
Explanation
F = ma
a is acceleration and is defined as rate of change of velocity w.r.t. time.
a = dv / dt
Since velocity is constant, dv/dt = 0
so a= 0, F = 0
So, given statement is true.
Inertia means resistance to the change of state.
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True
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False
Explanation
Inertia is defined as the property, by virtue of which, body tends to be in its state of rest or of uniform motion, until an external force is applied.
So, given statement is True.
When the net force acting on a system is zero the body may move with uniform acceleration.
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True
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False
Explanation
According to Newton's second law,
F = ma
So, if F is zero, then a is also zero.
So, given statement is false.
The inertia of a moving body makes it:
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stop due to friction
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increase its speed
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decrease its speed
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resist a change in its state of motion
Explanation
Inertia is a physical property that offers resistance to change in the state of motion of an object i.e an object tends to remain in its initial state of motion when no external force is applied to it.
A body is said to be under balanced forces when the resultant force acting on the body is
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unity
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zero
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infinite
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none of these
Explanation
Forces are said to be balanced if resultant of the forces is zero i.e. $$F_R = 0$$
Acceleration of the body $$a = \dfrac{F_R}{m} = 0$$
Thus a body under balanced forces has zero acceleration.
A vehicle is moving on a rough road in a straight line with uniform velocity. Then
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no force is acting on the vehicle
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an external force must act on the vehicle
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an acceleration is being produced in the vehicle
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no work is being done on the vehicle
Explanation
When a vehicle is moving on a rough road, friction acts on it in the opposite direction of its velocity which tends to retard its motion. To be able to move with uniform velocity, an external force must act on the vehicle which can overcome the friction.
As longer the interaction between two objects exist:
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The force remains
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The force stops after the initial interaction
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Both a and b
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The force remains for a specific time interval
Explanation
A force is a push or pull on an object. This push or pull comes from the objects interacting with one another and only from such interactions. Once the interaction stops, there is no longer any force.
When a force is applied on a rigid object
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It changes the inter spacing between its constituent particles.
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It changes the dimension of the object
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It only causes motion in it
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All
Explanation
A force, when applied to a rigid object, does not change the interspacing between its constituent particles and therefore it does not change the dimension of the object. It only causes motion in it.
A machine gun fires a bullet of mass $$40 \ g$$ with a velocity $$1200 \ ms^{-1}$$. The man holding it can exert a maximum force of $$144 \ N$$ on the gun. How many bullets can he fire per second at the most?
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$$One$$
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$$Four$$
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$$Two$$
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$$Three$$
Explanation
Mass of the bullet $$m=40 \ g=0.040 \ kg$$
Velocity with which the bullet leaves the gun $$v=1200 \ ms^{-1}$$
We know that
Force $$F=\dfrac{\Delta P}{\Delta t}$$
$$\implies \Delta P=F. \Delta t$$ . . . (i)
Change in momentum for each bullet $$\Delta p=p_f-p_i=mv-0=mv$$
Let there be $$n$$ bullets fired per second.
So total mass of bullets fired per sec $$M=n \times m$$
Total change in momentum for one second $$\Delta P=Mv=(n \times m)v . . . (ii) =mnv$$
Now,
Force exerted by man: $$F=144 N$$
Time interval to accomplish the change in momentum $$\Delta t = 1 \ sec$$
Using (i) and (ii),
$$F. \Delta t=mnv$$
$$\Rightarrow 144 \times 1=n(0.040)(1200)$$
So, number of bullets fired per second
$$n=\dfrac{144}{0.040 \times 1200} = 3$$
Option D is the answer.
Cleaning of dust from carpet is due to _____.
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Inertia of motion
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Inertia of rest
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Inertia of direction
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Momentum
Explanation
Cleaning of dust from the carpet is due to the inertia of rest that the particle tends to remain at rest if no external force is applied to it.
When we beat a carpet with the help of a stick, the carpet comes into a motion whereas the dust particle remain at rest due to their inertia of rest and hence get separated out of the carpet.
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Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
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Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
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Assertion is correct but Reason is incorrect
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Assertion is incorrect but Reason is correct
Explanation
Inertia is the resistance of any physical object to any change in its state of motion. It is the parameter used to describe the motion of the object and how it is affected by the applied force. And the mass(m) is the measure of it. Again $$\vec{F}=m \frac{d \vec{v}}{dt} $$. So force is proportional to $$mass$$. So, greater the mass, greater is the force required to change its state of rest or of uniform motion.
A man is stranded in the middle of a perfectly smooth island of ice, where there is no friction between the ground and his feet. Under these circumstances,
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he can reach the desired corner by throwing any object in the same direction
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he can reach the desired corner by throwing any object in the opposite direction
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he has no chance of reaching any corner of the island
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he can reach the desired corner by walking on the ground in that direction
Explanation
Since the surface is frictionless, there would be no external force and hence no external impulse. Hence, linear momentum would be conserved. So if he throws an object in a particular direction, he would get an impulse in the opposite direction and would acquire a constant velocity.
If action force acting on a body is gravitational in nature, the reaction force:
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may be a contact force
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must be gravitational too
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may be a gravitational or contact force
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may be a force of any origin
Explanation
$$\textbf {Hint :}$$ Use Newton's third law of motion
$${\textbf{Explanation:}}$$
Gravitational force is the force of attraction between two bodies with some mass, which is a pulling force in nature.
$$\bullet$$ According to Newton's third law, we can state that if any force P is applied on a body, it brings a reaction force that will be equal to the magnitude of force P but opposite in direction. Also, the nature of the reaction force and applied force P will be the same.
$$\bullet$$ It is given the nature of action force acting on a body is gravitational. Therefore, the reaction force's nature must be a gravitational force.
$${\textbf{Correct option: B}}$$
A force of 12 N gives an object an acceleration of 4m/s$$^{2}$$. The force required to give it an acceleration of 10m/s$$^{2}$$ is
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15 N
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20 N
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25 N
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30 N
Explanation
The force $$F=ma$$.
Using this, the mass of the object will be $$m=12/4 =3Kg$$.
So, the force required to give it an acceleration of $$10 m/s^2$$ is $$30N$$.
Which of the following is the most significant law of motion given by Newton?
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First law of motion
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Second law of motion
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Third law of motion
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Zeroth law of motion
Explanation
Newton's second law of motion states that the force on a body is equal to the rate of change of momentum of the body and its direction is the same as that of the change in momentum. Alternatively, this law defines force as mass times acceleration, i.e. $$F=m\times a.$$
Since this law gives a quantitative definition of force, it is the most significant law of motion out of the three. The other two laws can be explained using the second law of motion.
Which of following Newton's laws reveals the underlying symmetry in the forces that occur in nature?
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First law
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Second law
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Third law
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Law of gravitation
Explanation
Newton's third law of motion states that all the forces in nature have an equal and opposite reaction force present. This explains the underlying symmetry in the forces that occur in nature.
A bullet of mass $$20 \ gm$$ is fired from a rifle of mass $$8 \ kg$$ with a velocity of $$100 \ m/s$$. The magnitude of velocity of recoil of the rifle is:
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$$0.25 \ m/s$$
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$$25 \ m/s$$
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$$2.5 \ m/s$$
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$$250 \ m/s$$
Explanation
mass of bullet $$ m_1 = 20 gm = \dfrac{20}{1000} \ kg = 0.020 kg $$
velocity of bullet $$ v_1 = 100 \ m/s$$
mass of rifle $$m_2 = 8 \ kg $$
Let velocity of recoil of rifle be $$v$$
Using the law of momentum conservation,
$$ m_1u_1+m_2u_2=m_1v_1+m_2v_2$$
Since, initially the bullet and rifle are at rest.
Hence, $$ u_1 = 0, \ \ \ \ u_2 = 0$$
On substituting the values, we get
$$0.020\times 100=8\times -v $$
$$v =- 0.25m/s$$
Here. negative sign shows that the direction is opposite to motion of bullet.
Action and reaction forces never balance out. Because,
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they are equal but not opposite always
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they are unequal in magnitude even though opposite in direction
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they act on different bodies
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they are unequal in magnitudes
Explanation
Action and reaction exist in pair and are always equal in magnitude and opposite in direction. But they act on different bodies, hence they cannot balance each other.
Match list (I) with List (II)
List I
List II
a) Recoil of gun
e) Newtons first law
b) Acceleration in one dimension and motion in two dimensions
f) Newtons second law
c) Definition of force
g) Projectile motion
d) Measurement of force
h) Newton’s third law
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$$a\rightarrow e, b\rightarrow g,c\rightarrow h, d\rightarrow f.$$
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$$a\rightarrow h, b\rightarrow g,c\rightarrow e, d\rightarrow f.$$
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$$a\rightarrow e, b\rightarrow h,c\rightarrow g, d\rightarrow f.$$
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$$a\rightarrow h, b\rightarrow e,c\rightarrow g, d\rightarrow f.$$
Explanation
Recoil is caused due to the force exerted by the bullet on the gun which in turn is felt by the hand i.e. the force exerted by the gun on the bullet and that by the bullet on the gun form an action-reaction pair and follow Newton's 3rd law.
In projectile motion, the body has acceleration only in the y-direction but there is motion in both x and y-directions.
Definition of force comes from the Newton's 1st law which defines force as the physical quantity responsible for changing the state of motion of a body.
Newton's 2nd law of motion quantifies force as the rate of change of momentum pet unit time. By this we get,
$$\vec{F}=\dfrac{d\vec{P}}{dt}=m\dfrac{d\vec{v}}{dt}=ma$$
Newton's second law of motion gives
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magnitude of force
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velocity of the object
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concept of inertia
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all the above
Explanation
According to Newton's second law if a body is acceleration with acceleration $$a$$ and mass of a body is $$M$$ then the force will on the body equal to $$F=ma$$.
Hence Option A
In above figure, two boys A and B are shown applying force on a block. If the block moves towards the right, which one of the following statements is correct?
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Magnitude of force applied by A is greater than that of B.
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Magnitude of force applied by A is smaller than that of B.
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Net force on the block is towards A.
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Magnitude of force applied by A is equal to that of B.
Explanation
In right direction total force $$= A-B$$
Since, block is moving in right direction, net force in right direction must be positive.
therefore, $$A-B > 0$$
$$A> B$$
A girl is pushing a box towards east direction. In which direction should her friend push the box so that it moves faster in the same direction?
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North
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South
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East
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West
Explanation
Towards East,
As both of them would be applying the force in the same direction, so that resultant force will be more.
A $$500 \ gm$$ gun fires a $$2 \ gm$$ bullet with a muzzle velocity of $$1600 \ ms^{-1}$$. The velocity of the gun is:
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$$-0.64 \ ms^{-1}$$
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$$-6.4 \ ms^{-1}$$
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$$6.4 \ ms^{-1}$$
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$$0.64 \ ms^{-1}$$
Explanation
Given , mass of gun $$m_g = 500 \ gm = 0.5 \ kg$$
mass of bullet $$m_b = 2 \ gm = 0.002 \ kg$$
final velocity of bullet $$v_b=1600 \ ms^-1$$
Let $$v_g$$ be the final velocity of gun
Since, the gun and bullet are initially at rest, their initial velocities are $$u_b=u_g=0$$.
Using linear momentum conservation:
$$m_bu_b+m_gu_g=m_bv_b +m_gv_g$$
$$0=0.002 \times 1600 + 0.5 \times v_g$$
$$\Rightarrow v_g = -6.4\ m/s$$
A boy of mass 40 kg jumps off a boat with a velocity of $$3ms^{-1}$$ . With what momentum does
the boat move ?
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$$-210kg.ms^{-1}$$
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$$-120kg.ms^{-1}$$
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$$-125kg.ms^{-1}$$
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$$-215kg.ms^{-1}$$
Explanation
Using momentum conservation,
$$m_1 u_1 + m_2 u_2 = m_1 v_1 + m_2 v_2 $$
$$m_1=mass$$ $$of$$ $$boy$$ $$m_2=mass $$ $$of $$ $$boat$$
$$m_1=40kg$$
$$,v_1=3\ m/s$$ $$u_1=0$$ $$u_2=0$$
$$\Rightarrow 40\times 0+m_2 \times 0= 40 \times 3+m_2\times v_2$$
$$\Rightarrow 0+0= 120+P\:boat$$
$$0 = P\:boat + 120$$
$$P\:boat = -120\ kgm/s$$
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