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CBSE Class 9  >  Class 9 Test  >  Science   >  MCQ : Force And Laws Of Motion - 2 - Class 9 MCQ

MCQ : Force And Laws Of Motion - 2 Class 9 Science MCQs & solutions - Free


MCQ Practice Test & Solutions: MCQ : Force And Laws Of Motion - 2 (20 Questions)

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Test Highlights:

  • - Format: Multiple Choice Questions (MCQ)
  • - Duration: 20 minutes
  • - Number of Questions: 20

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MCQ : Force And Laws Of Motion - 2 - Question 1
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An unbalanced force is necessary for an object to be

Detailed Solution: Question 1

Explanation:
An unbalanced force is necessary for an object to be accelerated. Here's a detailed explanation:
1. Balanced and Unbalanced Forces:
- Balanced forces: When the forces acting on an object are equal in magnitude and opposite in direction, the net force is zero. In this case, the object remains at rest or continues to move with a constant velocity.
- Unbalanced forces: When the forces acting on an object are unequal in magnitude or not opposite in direction, the net force is non-zero. In this case, the object experiences acceleration.
2. Acceleration:
- Acceleration is the rate at which an object changes its velocity. It can be a change in speed, direction, or both.
- When an unbalanced force acts on an object, it causes the object to accelerate, meaning there is a change in its velocity.
3. Newton's Second Law of Motion:
- Newton's second law of motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. It can be mathematically expressed as F = ma, where F is the net force, m is the mass, and a is the acceleration.
- This law shows that to accelerate an object, an unbalanced force must be applied to overcome the object's inertia.
4. Conclusion:
- An unbalanced force is necessary for an object to be accelerated because it is the net force that causes a change in the object's velocity.
- Therefore, the correct answer is option C: accelerated.

MCQ : Force And Laws Of Motion - 2 - Question 2
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Bags at the top of a school van are tied using a string to avoid the effect of

Detailed Solution: Question 2

Why are bags at the top of a school van tied using a string?
There are several reasons why bags at the top of a school van are tied using a string. These reasons are related to the physical concepts of inertia, momentum, force, and acceleration. Let's break it down:
1. Inertia: Inertia is the tendency of an object to resist changes in its state of motion. When the van accelerates, decelerates, or turns, the bags at the top will tend to continue moving in their original direction due to their inertia. Tying the bags with a string helps to minimize their movement and prevent them from falling off.
2. Momentum: Momentum is the product of an object's mass and velocity. The bags at the top of the van have both mass and velocity. If the van suddenly stops or changes direction, the bags will experience a change in momentum. Tying them with a string helps to reduce their momentum and keeps them in place.
3. Force: Force is a push or pull acting on an object. When the van accelerates or turns, the bags at the top may experience a force that tries to push them outwards or sideways. Tying the bags with a string provides an additional force that counteracts these external forces, keeping the bags secure.
4. Acceleration: Acceleration is the rate of change of velocity. When the van accelerates, the bags at the top may tend to move backwards due to the acceleration. Tying the bags with a string helps to prevent their backward movement and keeps them in position.
In conclusion, tying the bags at the top of a school van using a string helps to minimize the effects of inertia, momentum, force, and acceleration. It ensures that the bags remain secure and prevents them from falling off or moving excessively during the van's motion.

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MCQ : Force And Laws Of Motion - 2 - Question 3
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Inertia

Detailed Solution: Question 3

Inertia
Inertia is a fundamental concept in physics that describes an object's resistance to changes in its motion. It is directly related to the mass of an object and plays a crucial role in understanding the behavior of objects in motion.
Key Points:
- Sets objects in motion: Inertia is not responsible for setting objects in motion. It is the property of an object to resist changes in its state of motion. Once an external force is applied to an object, it overcomes its inertia and sets the object in motion.
- Causes changes in motion: Inertia is the reason why objects continue to move in a straight line at a constant speed in the absence of any external forces. However, if an external force is applied, the object's inertia resists the change, causing the object to accelerate, decelerate, or change direction.
- Is the result of a net force: Inertia is not directly the result of a net force. It is a property of matter and exists regardless of the presence or absence of external forces. However, when a net force acts on an object, it can overcome the object's inertia.
- Is directly proportional to the mass of an object: Inertia is directly related to the mass of an object. The greater the mass, the greater the inertia. This means that objects with larger masses are more resistant to changes in their motion compared to objects with smaller masses.
Therefore, the correct answer is option D: "is directly proportional to the mass of an object."

MCQ : Force And Laws Of Motion - 2 - Question 4
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A plate, a ball and a child all have the same mass. The one having more inertia is the

Detailed Solution: Question 4


To determine which object has more inertia, we need to understand what inertia is. Inertia is the property of an object that resists changes in its state of motion. It is directly related to an object's mass.
Mass of the plate, ball, and child:
Since the plate, ball, and child all have the same mass, we can assume that their masses are equal.
Inertia of the plate, ball, and child:
Inertia is directly proportional to mass. Therefore, the object with more mass will have more inertia.
Comparing the inertia of the plate, ball, and child:
Since all three objects have the same mass, they will have the same inertia.
Conclusion:
Therefore, the correct answer is option D: All have equal inertia. The plate, ball, and child all have the same mass and, therefore, the same inertia.

MCQ : Force And Laws Of Motion - 2 - Question 5
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A passenger in a moving train tosses a coin which falls behind him. It means that the motion of the train is  

Detailed Solution: Question 5


To determine the motion of the train when a passenger tosses a coin and it falls behind him, we can analyze the situation using the concept of relative motion.
When the coin is thrown, it possesses the same velocity as the passenger since they are both in the same frame of reference. However, since the coin falls behind the passenger, it means that the horizontal velocity of the coin is less than the horizontal velocity of the train.
This indicates that the train is accelerating. Here's a detailed explanation:
- The coin is initially at rest relative to the passenger and the train.
- When the passenger tosses the coin, it acquires the same initial velocity as the passenger.
- However, as the coin moves horizontally through the air, it experiences a resistance force due to the air resistance.
- This air resistance slows down the horizontal velocity of the coin relative to the train.
- Since the coin falls behind the passenger, it means that the horizontal velocity of the coin is less than that of the train.
- Therefore, the train must be accelerating.
Hence, the correct answer is option A: accelerated.

MCQ : Force And Laws Of Motion - 2 - Question 6
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Two equal masses m each moving in the opposite direction with the same speed v collide and stick to each other. The velocity of the combined mass is

Detailed Solution: Question 6

Total initial momentum = Total final momentum = zero

MCQ : Force And Laws Of Motion - 2 - Question 7
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The rate of change of momentum with respect to time is measured in

Detailed Solution: Question 7

We know,  p = mv 
Units of p, ⇒ kg ms-1 
We have to measure, rate of change of momentum with respect to time ; 
⇒  kg m s-2

MCQ : Force And Laws Of Motion - 2 - Question 8
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The incorrect option regarding action-reaction pair is

Detailed Solution: Question 8

Since, action and reaction forces are equal in magnitude and opposite in directions. Therefore, there resultant is zero. 

MCQ : Force And Laws Of Motion - 2 - Question 9
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A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goalkeeper to

Detailed Solution: Question 9

A large force that acts in a very short duration of time is reffered to as impulsive force. This force which is large due to a high speed such as kick of a football may be disastrous as the momentum change is large and hence a goalkeeper holds his hand backwards in order to reduce the force exerted on his hands.

MCQ : Force And Laws Of Motion - 2 - Question 10
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If force, change in momentum and time are given by F, p and t respectively, then they are related b

Detailed Solution: Question 10

The rate at which an object’s momentum changes is equal to the force acting on the object. If a force F acts on an object for a time Δt, then the change in the object's momentum is Δp = FΔt.
 

MCQ : Force And Laws Of Motion - 2 - Question 11
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According to Newton’s third law of motion, action and reaction

Detailed Solution: Question 11

Newton's Third Law of Motion:
According to Newton's third law of motion, for every action, there is an equal and opposite reaction. This law states that when one object exerts a force on a second object, the second object exerts a force of equal magnitude in the opposite direction on the first object.
Explanation:
The statement in question asks for the correct interpretation of Newton's third law of motion. Let's break down each option and determine which one is correct:
A: Always act on the same body:
This statement is incorrect because Newton's third law of motion states that the action and reaction forces act on different bodies.
B: Always act on different bodies in opposite directions:
This statement is correct because Newton's third law of motion states that the action and reaction forces act on different bodies and in opposite directions.
C: Have the same magnitudes and direction:
This statement is incorrect because while the magnitudes of the action and reaction forces are equal, their directions are opposite.
D: Act on either body at right angles to each other:
This statement is incorrect because Newton's third law of motion does not specify that the action and reaction forces act at right angles to each other. The forces can act in any direction as long as they are opposite.
Conclusion:
Based on the explanations above, the correct answer is B: Always act on different bodies in opposite directions. This interpretation aligns with Newton's third law of motion, which states that every action has an equal and opposite reaction.

MCQ : Force And Laws Of Motion - 2 - Question 12
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The minimum number of unequal forces that can make zero resultant is

Detailed Solution: Question 12

Three vectors of unequal magnitude which can be represented by the three sides of a triangle taken in order, produce zero resultant.

MCQ : Force And Laws Of Motion - 2 - Question 13
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An object of mass 2 kg is sliding with a constant velocity of 4 ms-1 on a frictional horizontal table. The force required to keep the object moving with same velocity is

Detailed Solution: Question 13

From newton's Ist law of motion, since there is no external force acting on the object it will remain in its original state of motion. Hence, zero force is required to keep the object moving with constant velocity.

MCQ : Force And Laws Of Motion - 2 - Question 14
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A mass M breaks into two pieces in the ratio 1 : 3 while at rest. If the heavier has a speed of v, the speed of the lighter is

Detailed Solution: Question 14


Given:
- Mass of the object, M
- The mass breaks into two pieces in the ratio 1:3
- Speed of the heavier piece, v
To find:
- The speed of the lighter piece
Explanation:
When a mass breaks into two pieces, the total momentum before and after the breakup remains the same. Therefore, we can use the law of conservation of momentum to solve this problem.
Let's assume the mass of the lighter piece is m and the mass of the heavier piece is 3m.
Before the breakup:
- Total mass = M
- Total momentum = 0 (as the mass is at rest)
After the breakup:
- Mass of the lighter piece = m
- Mass of the heavier piece = 3m
- Total mass = m + 3m = 4m
- Total momentum = (3m * v) + (m * v) = 4mv
Since the total momentum before and after the breakup is the same, we can equate the two values:
0 = 4mv
Simplifying the equation, we get:
4mv = 0
Dividing both sides by 4m, we get:
v = 0
Therefore, the speed of the lighter piece is 0.
Answer: A. 0

MCQ : Force And Laws Of Motion - 2 - Question 15
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An object of mass of 2 kg is sliding with a velocity of 4 ms-1 on a frictional horizontal surface. The retarding force necessary to stop the object in 1 second is

Detailed Solution: Question 15

Here, u = 0 m /s    
a  = v- u / t 
=>  0-4/1 = > -4. 
Formula of regarding force remains same. 
F = - ma. ( negative sign comes due to  restarting  acceleration.) 
Therefore,  F = - 2* (-4) 
F  =  8 N 

MCQ : Force And Laws Of Motion - 2 - Question 16
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The two states of motion treated alike by the Newton’s first law, among A, B, C and D are
A : Rest
B : Uniform motion
C : Uniformly accelerated
D : Non-uniformly accelerated

Detailed Solution: Question 16

Newton's First Law:
According to Newton's first law of motion, an object will remain at rest or continue to move with a constant velocity unless acted upon by an external force.
States of Motion:
There are four states of motion that can be considered:
A) Rest
B) Uniform motion
C) Uniformly accelerated
D) Non-uniformly accelerated
Analysis:
We need to determine which states of motion are treated alike by Newton's first law.
- Rest: An object at rest will remain at rest unless acted upon by an external force.
- Uniform motion: An object in uniform motion will continue to move in a straight line with a constant speed unless acted upon by an external force.
- Uniformly accelerated: An object undergoing uniform acceleration will continue to accelerate at a constant rate unless acted upon by an external force.
- Non-uniformly accelerated: An object undergoing non-uniform acceleration will change its acceleration over time, and its motion will not be treated alike by Newton's first law.
Conclusion:
Based on the analysis, the states of motion that are treated alike by Newton's first law are:
- Rest (A)
- Uniform motion (B)
Therefore, the correct answer is B: A, B.

MCQ : Force And Laws Of Motion - 2 - Question 17
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Passengers standing in a bus are thrown outwards when the bus takes a turn suddenly. This happens because of

Detailed Solution: Question 17

Explanation:
The correct answer is B: inertia.
Inertia is the tendency of an object to resist changes in its motion. When a bus takes a turn suddenly, the passengers inside the bus continue to move in a straight line due to their inertia. As a result, they are thrown outwards.
Here is a detailed explanation:
1. Inertia:
- Inertia is a property of matter that describes its resistance to changes in motion.
- An object at rest tends to stay at rest, and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an external force.
2. Bus turning suddenly:
- When a bus takes a turn suddenly, it changes its direction of motion.
- However, the passengers inside the bus still have the same velocity and want to continue moving in a straight line due to their inertia.
3. Inward force:
- As the bus turns, the passengers experience an inward force due to the centripetal force acting on the bus.
- This inward force tries to change the direction of the passengers' motion and make them move along with the bus.
4. Outward force:
- However, due to their inertia, the passengers resist the change in their motion and continue to move in a straight line.
- As a result, they are thrown outwards, away from the center of the turn.
5. Newton's first law of motion:
- The phenomenon observed in this situation is in accordance with Newton's first law of motion, also known as the law of inertia.
- Newton's first law states that an object at rest or in motion will remain at rest or in motion with a constant velocity unless acted upon by an external force.
In conclusion, when a bus takes a turn suddenly, the passengers inside the bus are thrown outwards due to their inertia.

MCQ : Force And Laws Of Motion - 2 - Question 18
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 When a 12 N force acts on 3 kg mass for a second, the change in velocity is (in m/s)

Detailed Solution: Question 18

MCQ : Force And Laws Of Motion - 2 - Question 19
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A force of ‘P* N acts on a particle so as to accelerate it from rest to a velocity 'v' m/s. The force 'P’ is then replaced by ‘Q’ N which decelerates it to rest.

Detailed Solution: Question 19

To decelerate a body, a force must be applied opposite to the direction of velocity, irrespective of its magnitude.
According to the condition of motion under two forces P and Q must be acting in opposite directions and may be equal, not necessarily must be equal.

MCQ : Force And Laws Of Motion - 2 - Question 20
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Which of the following does not happen? As an automobile accelerates suddenly,

Detailed Solution: Question 20

When an automobile accelerates suddenly, due to inertia, all objects and people inside tend to remain in their state of rest. As the vehicle moves forward, they appear to move backward relative to the car. This is why the toy monkey, standing person, and bag seem to move back. However, a person in the seat is not pushed in front; instead, they also appear to move backward. So, option d is the one that does not happen.

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