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CBSE Questions for Class 9 Physics Force And Laws Of Motion Quiz 5 - MCQExams.com
CBSE
Class 9 Physics
Force And Laws Of Motion
Quiz 5
A particle is moving with a constant speed along a straight line path. A force is not required to:
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Increase its speed
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Decrease its momentum
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Change the direction
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Keep it moving with uniform speed
Explanation
Newton's first law states that, if a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed unless it is acted upon by a force. This postulate is known as the law of inertia.
Hence a force is not required to keep the particle moving with uniform speed.
Which of the following class of forces is different from others?
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Pulling of a cart
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Stretching of a coiled spring
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Kicking of a football
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Weight of the body
Explanation
Pulling of a cart, stretching of a coiled spring and kicking of a ball take place due to contact forces but weight of the body (gravitational force) is a non-contact force.
Hence option $$D$$ is correct.
Newton's third law of motion can be used to explain:
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Why the passengers in a bus tend to fall backward when its starts suddenly
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Swimming of a man
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Motion of a rocket
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Both (B) and (C)
Explanation
A man applies a force on water in the backward direction while swimming and according to Newton's third law, the water pushes the man with equal force in the forward direction and thus the man swims forwards.
Similarly, during the motion of a rocket, the gases exhausted in the backward direction gives a finite thrust to the rocket in the forward direction and thus helps it to move forwards.
Hence option D is correct.
When a magnet is dropped it falls to the ground. The force which makes this happen is an example of
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Magnitude force
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Balanced force
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Unbalanced force
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Zero force
Explanation
The only force acting on the magnet is the gravitational force in the downward direction and hence this is an example of an unbalanced force.
When a fire man directs a powerful stream of water on a fire from a hose pipe, the hose pipe tends to go backward. This is an example of Newton's:
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Law of gravitation
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First law of motion
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Second law of motion
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Third law of motion
Explanation
According to Newton's 3rd law of motion, every action has equal and opposite reaction.
Thus stream of water going in forward direction exerts a force on the hose pipe in the backward direction which makes hose pipe to go backwards.
The mass of object A is 5 kg, that of object B is 40 kg, that of object C is 38 kg, and that of object D is 56 kg. Which object has the greatest inertia?
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A
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B
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C
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D
Explanation
The object having the greatest mass has the greatest inertia because more force is required to change the initial state of the object.
Thus object D has the greatest inertia.
Newton's second law gives
a measure of
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Acceleration
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Force
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Momentum
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Angular momentum
Explanation
According to Newton's second, law, rate of change of momentum is directly proportional to applied force.
$$ F \space \alpha \space \dfrac {m(v-u)}{t} $$
Form this, we deduce that,
$$F= ma $$
So, Newton's second law gives us measure of force.
When a body is stationary :
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there is no force acting on it
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the combination of forces acting on it balance each other
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the force acting on it are not in contact with it
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the body is in vacuum
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.
If the net force acting on a body is doubled, the acceleration of the body is
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Halved
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Doubled
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Unchanged
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Quadrupled
Explanation
According to newton's
second law,
$$ F= \space ma $$
So, if force is doubled, a is also doubled.
Option B is correct.
A and B are two objects with masses 6 kg and
34 kg respectively. Then,
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A has more inertia than B
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B has more inertia than A
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A and B both have same inertia
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None of the above statements is true
Explanation
Mass is that quantity that is solely dependent upon the inertia of an object. The more inertia that an object has, the more mass that it has. A more massive object has a greater tendency to resist changes in its state of motion.
So option B is correct. Because B has more mass.
A stone is tied to the middle of a string and suspended from one end as shown in the given figure. Here S is the stone and O is the point of suspension. If you give a sharp jerk at P, the
string will break :
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below the stone
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at the point P itself
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from above the stone
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nothing can be decided
Explanation
When we give a quick downward jerk to the bottom string, we exceed the breaking tension in the bottom string in a very short time. Because of the inertia of the stone, we cannot accelerate it significantly, and the tension in the top string does not increase significantly above initial value
.
Therefore, the bottom string breaks, and the stone remains hanging from the top string.
Option A is correct.
If you are asked to push an object so that the acceleration produced in it has to be twice as
before, then the force required will be :
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Twice as before
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Half as before
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Same as before
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Four times as before
Explanation
By Newton's second law,
$$ F= ma$$
So, for twice acceleration,
$$a' = 2a$$
$$\Rightarrow F' = ma'$$
$$\Rightarrow F' = 2ma$$
$$\Rightarrow F' = 2F$$
So required force is twice as before.
The law which gives a quantitative measurement
of force is :
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Newton's third law of motion
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Newton's first law of motion
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Newton's second law of motion
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Newton's law of gravitation
Explanation
Newton's second law states that rate of change of momentum is directly proportional to applied force.
We can further state that, $$ F = ma$$
From this, we can measure force acting on mass 'm', having acceleration a.
So, option C is orrect.
A cricket player while taking a catch lowers his hands to :
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increase time of impulse
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decrease time of impulse
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take catch safely
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increase the area of impact
Explanation
Force acting on the hand $$F = \dfrac{\Delta P}{\Delta t}$$
Thus a player, by lowering his hand, increases the time of impulse so as to decrease the force acting on his hand by the ball which prevents him from injury.
$$P$$ and $$Q$$ are two objects with masses $$5 kg$$ and $$30\ kg$$ respectively. Then:
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$$P$$ has more inertia than $$Q$$
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$$Q$$ has more inertia than $$P$$
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$$P$$ and $$Q$$ have the same inertia
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Neither $$P$$ nor $$Q$$ has any inertia
Explanation
The mass of an object is a measure of
its inertia.
Given,
Mass of the object $$P$$, $$m_p=5\ kg$$
Mass of the object $$Q$$, $$m_q=30\ kg$$
$$m_q \gt m_p$$
$$\therefore$$ $$Q$$ has more inertia than $$P$$
The process of walking involves:
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Newton's first law of motion
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Newton's second law of motion
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Newton's third law of motion
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None of these
Explanation
Newtons' third law of motion states that every action has an equal and opposite reaction.
While walking, the man applies a force on the ground in a backward direction, and thus the ground pushes the man by the same amount of force in the forwarding direction which makes the man move forward.
Which of the following represents the SI unit
of force?
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Dyne
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Gram-weight
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Newton
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Kilogram-weight
Explanation
Option C is correct.
$$ F= ma$$
In S.I unit system,
unit of force is $$kg\ m/s^2= Newton$$
S.I unit of Force is newton and it is represented by $$N$$.
When will a body under the action of several forces have 0 acceleration?
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When the vector sum of all the forces acting on the body is zero
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When a body is very heavy
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When the body is very light
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When the body is a point body
Explanation
Referring formula $$\displaystyle F =ma$$
Thus acceleration will be zero, when resultant force is zero. In option (d), the point body is to be considered. A point body does not mean a body of zero mass. A point body or a particle is ideally a piece or quantity of matter having practically no linear dimensions but only a position. The measure of this quantity of matter is the mass of the particle. It is a usual convention to imagine bodies as point objects when we are only concerned about their mass and not about the distribution of mass. Thus earth can be considered as a point body when we describe its
motion about sun.
The force needed to produce an acceleration of $$6 m/s^2$$ in a ball of mass $$ 4 kg$$ will be
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$$24 N$$
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$$30 N$$
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$$32 N$$
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$$36 N$$
Explanation
$$F=ma$$
$$F=6\times4$$
$$F=24N$$
So, the required force is $$F=24N$$.
A body is said to be underbalanced 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
If the net force acting on a body is zero, then the body is said to be in equilibrium, or a body is said to be underbalanced forces when the resultant force acting on the body is zero.
A driver accelerates his car first at the rate of $$1.8 {m}/{{s}^{2}}$$ and then at the rate of $$1.2 {m}/{{s}^{2}}$$. The ratio of the forces exerted by the engines will be respectively equal to
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2 : 3
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1 : 2
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2 : 1
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3 : 2
Explanation
Given,
$$a_1=1.8\ m/s^2\\a_2=1.2\ m/s^2$$
Let Mass of car is $$m$$, to accelerate car with acceleration $$a$$ force required $$F=ma$$
Ratio $$=\dfrac{F_1}{F_2}=\dfrac{ma_1}{ma_2}=\dfrac{1.8}{1.2}=\dfrac{3}{2}$$
A driver accelerates his car first at the rate of
1.8 m/s$$^2$$ and then at the rate of 1.2 m/s$$^2$$. The ratio of the forces exerted by the engines will be respectively equal to
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2 : 3
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1 : 2
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2 : 1
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3 : 2
Explanation
We have to find $$\dfrac{{F}_{1}}{{F}_{2}}$$
$$F\propto a$$
$$\dfrac{{F}_{1}}{{F}_{2}}=\dfrac{{a}_{1}}{{a}_{2}}$$.
Which gives the ratio $$3:2$$.
When a force of 1 newton acts on a mass of 1 kg that is able to move freely, the object
moves with a/an
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speed of $$\displaystyle 1{ ms }^{ -1 }$$
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acceleration of $$\displaystyle 1{ ms }^{ -2 }$$
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speed of $$\displaystyle 1{ kms }^{ -1 }$$
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acceleration of $$\displaystyle 10{ ms }^{ -2 }$$
Explanation
According to Newton's second law of motion
$$ F= ma$$
Here,
Force $$F= 1 \ N$$
Mass $$m=1 \ kg$$
So,
$$ 1 \ N = (1 \ kg)\ \times a$$
$$\Rightarrow a =1 \ m/s^2 $$
Option B is correct.
A ball of mass $$0.2 kg$$ moves with a velocity of $$20 m/s$$ and it stops in $$0.1 s$$, then the force on the ball is
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$$40 N$$
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$$20 N$$
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$$4 N$$
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$$2 N$$
Explanation
$$\displaystyle F=ma=m\left( \frac { \upsilon -u }{ t } \right) =\frac { -mu }{ t } $$
Substituting the values and neglecting the negative sign (because
none of the options is negative)
$$\displaystyle F=\frac { 0.2\times 20 }{ 0.1 } =40$$N
When a body is in translatory equilibrium
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The body is definitely at rest
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The body is definitely in the state of uniform motion
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The body will be either at rest or in the state or uniform motion
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None of these
Explanation
When a body is in translatory equilibrium , the net force acting on a body is zero, and the body is said to be in translational equilibrium. In translatory equilibrium, the center of mass of the body remains either at rest or moves rectilinearly with constant velocity. Hence, the body is definitely in the state of uniform motion or in rest.
A cannon after firing recoils due to
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Conservation of energy
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Backward thrust of gases produced
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Newton's first law of motion
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Newton's third law of motion
Explanation
Correct Option is D.
Explanation for the correct:
$$\bullet$$ Newton's Third Law of Motion states that to every Action there is equal and opposite Reaction. Thus firing of bullet from cannon is an Action and there is opposite Reaction on the cannon due to which it recoils. Option D is correct.
$$\bullet$$ Conservation of Energy states that Energy can neither be created nor destroyed. It can only change it forms.
$$\bullet$$ Backward thrust of gases is not a Law. It is also one of the consequences of Third Law of Motion.
$$\bullet$$ First Law of Motion states that if a body is at rest/motion it will continue to remain at rest/motion until any external force is applied.
Thus, Only Option D is correct.
Identify the action that can cause motion of a book placed on a table.
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Pulling
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Pushing
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Lifting
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All of the above
Explanation
The action involved in moving a book placed on a table can be pulling, pushing or lifting for which force is required.
Identify the type of force involved when loaded cart is moved.
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Lifting
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Pulling
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Running
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Reading
Explanation
To move a loaded cart we have to pull it . Hence action involved is as pulling.
If a cricket ball hits you, it will hurt much more than a tennis ball would, because :
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Cricket ball is bigger
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Cricket ball has more mass
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Cricket ball has less density
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None of the above
Explanation
A moving balls have momentum that is equal to, $$p=mv$$
A ball with higher mass will have higher momentum at a constant velocity. Therefor to stop higher momentum objects, high forces are required. A cricket ball has more mass hence more momentum and will hurts more.
Identify the type of force involved in opening the door of refrigerator.
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Pulling
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Pushing
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Heating
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Cooling
Explanation
The action involved to open the door of the refrigerator is pulling because the door is moving towards us. F
orce is applied by our arm to PULL the door of the refrigerator.
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