Case study ( 4 Marks )

Question 14 Marks

The sum of momentum of the two objects before collision is equal to the sum of momentum after the collision provided there is no external unbalanced force acting on them. This is known as the law of conservation of momentum. This statement can alternatively be given as the total momentum of the two objects is unchanged or conserved by the collision. Law of conservation of momentum is applicable to system of particle. Answer the following questions.
(i)Law of conservation of momentum is applicable to
$(a)$ A system of particles
$(b)$ Only for $2$ particles
$(c)$ Only for $1$ particle
$(d)$ None of the above
(ii) Law of conservation of momentum holds good provided that
$(a)$ There should be external unbalanced force acting on particles
$(b)$ There should not be any external unbalanced force acting on particles
$(c)$ No internal forces acting on particles
$(d)$ None of the above
(iii)The total momentum of the two objects when collision occurs is
$(a)$ Changed
$(b)$ Remains conserved
$(c)$ Become zero
$(d)$ None of the above
(iv) State law of conservation of momentum.
(v) If action and Reaction are equal in magnitude and opposite in direction then why they do not cancel each other?

Answer

$(i) a$
$(ii) b$
$(iii) b$
$(iv)$ The sum of momentum of the two objects before collision is equal to the sum of momentum after the collision provided there is no external unbalanced force acting on them. This is known as the law of conservation of momentum. This statement can alternatively be given as the total momentum of the two objects is unchanged or conserved by the collision.
$(v)$ Action and Reaction are equal in magnitude and opposite in direction but they do not cancel each other because they are not action on sane object. As these forces are acting on different object hence produces different acceleration and does not cancel each other.

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Question 24 Marks

The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first. These two forces are always equal in magnitude but opposite in direction. These forces act on different objects and neveron the same object. It is important to note that even though the action and reaction forces are always equal in magnitude; these forces may not produce accelerations of equal magnitudes, this is because each force acts on a different object that may have a different mass.The two opposing forces are also known as action and reaction forces. Answer the following questions.
(i) Action reaction forces are always
$(a)$ Equal and in same direction
$(b)$ Equal and in opposite direction
$(c)$ Unequal and in same direction
$(d)$ None of the above
(ii) Which of the following is correct about action reaction forces?
$(a)$ They act on different objects
$(b)$ They are equal in magnitude and opposite in direction
$(c)$ Both forces acted on different object simultaneously
$(d)$ All the above
(iii) State third law of motion
(iv) Give 5 examples of third law of motion
(v) Even though the action and reaction forces are always equal in magnitude; these forces may not produce accelerations of equal magnitudes. Give your justification on this statement

Answer

$(i) b$
$(ii) d$
$(iii)$ The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first. These two forces are always equal in magnitude but opposite in direction. These forces act on different objects and neveron the same object.
$(iv)$ Examples of third law of motion are

Swimming or rowing a boat.
•Static friction while pushing an object.
•Walking.
•Standing on the ground or sitting on a chair.
•The upward thrust of a rocket.
•Resting against a wall or tree.

$(v)$ Even though the action and reaction forces are always equal in magnitude; these forces may not produce accelerations of equal magnitudes because these action reaction forces are acting on two different objects having different masses that’s why they are acceleration with different magnitude.

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Question 34 Marks

The second law of motion is quantitative expression of force and it states that the rate of change of momentum of an object isproportional to the applied unbalanced force in the direction of force. Mathematically, $F$ = ma, the unit of force is $kg-m/s^2$ or Newton,which has the symbol $N$. The second law ofmotion gives us a method to measure the force acting on an object as a product of its mass and acceleration.Answer the following questions.
(i) SI unit of force is
$(a)$ Newton
$(b)$ $Kg-m/s^2$
$(c)$ Both $a$ and $b$
$(d)$ None of the above
(ii) The quantitative expression of force is given by
$(a)$ First law of motion
$(b)$ Second law of motion
$(c)$ Third law of motion
$(d)$ None of the above
(iii) Force is directly proportional to
$(a)$ Acceleration of object
$(b)$ Time for which force acts on object
$(c)$ Both a and b
$(d)$ None of the above
(iv) State second law of motion. State whether it is scalar or vector quantity
(v) Differentiate between first law and second law of motion.(give 3 points)

Answer

$(i) a$
$(ii) b$
$(iii) a$
$(iv)$ The second law of motion is quantitative expression of force and it states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force. Mathematically, F = ma, the unit of force is $kg-m/s^2$ or Newton.
$(v)$ Difference between first law and second law of motion is given by

No	first law	second law
$1$	It is also known as the law of inertia	It is also known as the law of force and acceleration.
$2$	It is qualitative law	It is quantitative law
$3$	Newton’s first law states that an object at rest will stay at rest and an object in motion will stay in motion with the same speed and direction unless and until an external force is applied to the object.	Newton’s second law states that the rate of change of momentum of a body is directly proportional to the force applied, and this change in momentum takes place in the direction of the applied force.

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Question 44 Marks

Two strings $X$ and $Y$ are tied to the two opposite faces of the block as shown in figure. If we apply a force by pulling the string $X$, the block begins to move to the right. Similarly, if we pull the string$ Y$, the block moves to the left. But, if the block is pulled from both the sides with equal forces, the block will not move. Such forces are called balanced forces and do not change the state of rest or of motion of an object. Now, let us consider a situation in which two opposite forces of different magnitudes pull the block. In this case, the block would begin to move in the direction ofthe greater force. Thus, the two forces are not balanced and the unbalanced force acts in the direction the block moves. This suggests that an unbalanced force acting on an object brings it in motion. Force is push or pull.
$(i)$ Force is nothing but
$(a)$push
$(b)$pull
$(c)$both push or pulls
$(d)$none of the above
$(ii)$ When balanced forces acting on moving object then
$(a)$ Object continue to move with same speed
$(b)$ Object will change its direction of motion
$(c)$ Object will come to rest
$(d)$ None of the above
$(iii)$ When unbalanced force acts on moving object opposite to direction of motion then
$(a)$ Object continue to move with same speed
$(b)$ Object will come to rest
$(c)$ Both can be possible
$(d)$ None of the above
(iv) Differentiate between balanced and unbalanced force. give 3 points each .
(v) From above diagram if one person pull from Y rope with 10N force and another person pull from X rope with 5N force. In which direction box will move? Is this a case of unbalanced force or balance force?

Answer

$(i) c$
$(ii) a$
$(iii) b$
$(iv)$ Difference between balanced and unbalanced force

No.	Balanced force	Unbalanced force
$1$	The forces are equal in magnitude	The forces are unequal in magnitude
$2$	It does not cause any change in the state of motion of the object.	Causes change in the state of motion of the object.
$3$	Balanced force means the sum of all the forces(or net force) acting on a body is zero.	Unbalanced force means the sum of all the forces acting on a body is non-zero.

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Question 54 Marks

Newton’s first law of motion states that a body at rest will remain at rest position only and a body which is in motion continues to be in motion unless otherwise they are acted upon by an external force. In other words, all objects resist a changein their state of motion. In a qualitative way, the tendency of undisturbed objects to stayat rest or to keep moving with the same velocity is called inertia. This is why, the firstlaw of motion is also known as the law of inertia. Answer the following questions.
(i) The first law of motion is also known as
$(a)$law of inertia/p>
$(b)$law of thermodynamics
$(c)$both $a$ and $b$
$(d)$none of these
(ii) If no external force acts on object which is at rest. it will
$(a)$remain at rest
$(b)$start to move
$(c)$both a and b can possible
$(d)$none of these
(iii) If no external force acts on moving object. it will
$(a)$stop moving
$(b)$continue to move with same speed in same direction
$(c)$changes its direction of motion
$(d)$none of these
(iv) State Newton’s first law of motion.
(v) why Newton’s first law of motion is called law of inertia

Answer

$(i) a$
$(ii) a$
$(iii) b$
$(iv)$ Newton’s first law of motion states that a body at rest will remain at rest position only and a body which is in motion continues to be in motion unless otherwise they are acted upon by an external force.
$(v)$ All objects resist a change in their state of motion. In a qualitative way, the tendency of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia. This is why, the first law of motion is also known as the law of inertia.

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Science Case study ( 4 Marks )

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