Force And Laws Of Motion - Olympiad Level MCQ, Class 9 Science - Class 9 MCQ

The cricket ball comes to rest after covering a short distance, because there is a force on the ball, opposing the motion. This force is due to resistance of air and also due to friction between the ball and the ground.

The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object's mass.

Law of Conservation of Momentum

In general, the law of conservation of momentum or principle of momentum conservation states that the momentum of an isolated system is a constant. The vector sum of the momenta (momentum is equal to the mass of an object multiplied by its velocity) of all the objects of a system cannot be changed by interactions within the system. In classical mechanics, this law is implied by Newton’s laws. This principle is a direct consequence of Newton’s third law.

Action-reaction forces:
Definition: Action-reaction forces, also known as Newton's third law of motion, state that for every action, there is an equal and opposite reaction. This means that whenever an object exerts a force on another object, the second object exerts a force of equal magnitude but in the opposite direction on the first object.
Characteristics of action-reaction forces:
1. Act on different bodies: The action force and the reaction force always act on different bodies. One body exerts a force on another body, and the second body exerts an equal and opposite force on the first body.
2. Act along the same line: The action and reaction forces always act along the same line of action. This means that the forces have the same direction, but they act in opposite directions.
3. Have equal magnitudes: The action and reaction forces have equal magnitudes. This is a fundamental principle of action-reaction forces.
4. Act simultaneously: The action and reaction forces always act simultaneously. They occur at the same time and are instantaneous.
Example:
To understand action-reaction forces, consider the example of a person pushing a wall.
- The person exerts a force on the wall, which is the action force.
- In response to the person's force, the wall exerts an equal and opposite force on the person, which is the reaction force.
- Both forces act along the same line, which is the direction of the push.
- The magnitudes of the forces are equal, ensuring that the person and the wall experience an equal amount of force.
In summary, action-reaction forces always act on different bodies, along the same line, have equal magnitudes, and act simultaneously. Understanding these characteristics is crucial in analyzing the motion and interactions of objects in various physical scenarios.

The reaction force is exerted by the earth on the porter. To understand this, we can break down the scenario using Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. In this case:
Action: The force exerted by the porter on the weight (200 N).
Reaction: The force exerted by the weight on the porter.
Now let's analyze the forces involved:
The force exerted by the porter on the weight:
- This force is due to the porter carrying the weight on his head.
- It is directed upwards, opposing the force of gravity.
The force exerted by the weight on the porter:
- This force is a reaction to the force applied by the porter on the weight.
- According to Newton's third law, this force is equal in magnitude but opposite in direction to the force exerted by the porter.
- It is directed downwards, towards the earth.
The force exerted by the earth on the porter:
- This is the reaction force mentioned in the question.
- It is a gravitational force exerted by the earth on the porter.
- It is equal in magnitude but opposite in direction to the force exerted by the weight on the porter.
- It is directed upwards, opposing the force of gravity.
Therefore, the reaction force is exerted by the earth on the porter (option C) as per Newton's third law of motion.

Explanation:
To determine whether a quantity is a vector or not, we need to consider its properties.
Momentum:
- Momentum is the product of an object's mass and its velocity.
- It is a vector quantity because it has both magnitude and direction.
- The direction of momentum is the same as the direction of velocity.
Velocity:
- Velocity is the rate of change of displacement with respect to time.
- It is a vector quantity because it has both magnitude and direction.
- The direction of velocity indicates the object's motion.
Force:
- Force is a push or pull that can cause an object to accelerate.
- It is a vector quantity because it has both magnitude and direction.
- The direction of force determines the direction of acceleration.
All of the above:
- Momentum, velocity, and force are all vector quantities because they have both magnitude and direction.
- They can be represented by arrows, where the length of the arrow represents the magnitude and the direction of the arrow represents the direction.
In conclusion, the correct answer is D: all of the above.

In a inelastic collision only momentum is conserved. Kinetic energy is not conserved Loss of heat may takes place so temperature is not conserved. Since Kinetic energy is not conserved, velocity is not conserved.

In a collision, an object experiences a force for a specific amount of time that results in a change in momentum. The result of the force acting for the given amount of time is that the object's mass either speeds up or slows down (or changes direction). The impulse experienced by the object equals the change in momentum of the object. In equation form, F • t = m • Δ v.

Rocket works on the conservation of momentum. In a rocket, the fuel burns and produces gas at high temperature. These gases are ejected out of the rocket from a nozzle at the back side of the rocket. The ejecting gas exerts a forward force on the rocket which help in accelerating. Through the mass of gases escaping per second is very small and their momentum is very large due to their tremendous velocity of escape. An equal and opposite momentum is imparted to the rocket which despite its large mass builds up a high velocity.

Explanation:
When a speeding car takes a sharp turn, the passengers inside the car experience an outward pull. This happens due to the inertia of direction.
Inertia of direction:
Inertia is the tendency of an object to resist changes in its motion. The inertia of direction refers to an object's resistance to changes in its direction of motion. When a car takes a sharp turn, it changes its direction of motion, causing the passengers to experience an outward pull.
Reasons behind the outward pull:
1. The passengers inside the car tend to continue moving in a straight line due to their inertia.
2. When the car takes a sharp turn, it changes its direction, but the passengers still tend to move in their original straight line path.
3. As a result, the passengers experience an outward pull towards the outside of the turn.
Alternative options:
- Change in momentum: This is not the correct answer because the outward pull experienced by the passengers is not directly related to a change in momentum.
- Change in acceleration: This is not the correct answer because the outward pull experienced by the passengers is not directly related to a change in acceleration.
- None of the above: This is not the correct answer because the correct answer is the inertia of direction.
Therefore, the correct answer to the question is A: inertia of direction.

Explanation:
The ratio of force and acceleration of a moving body is the measure of its mass. This is known as Newton's Second Law of Motion, which states that the force acting on an object is equal to its mass multiplied by its acceleration (F = ma). In this equation, force and acceleration are directly proportional, meaning that if the force acting on an object increases, its acceleration will also increase. Similarly, if the mass of the object increases, its acceleration will decrease for a given force. Therefore, the ratio of force and acceleration is a measure of the object's mass.
Key Points:
- The ratio of force and acceleration is a measure of the mass of a moving body.
- This is known as Newton's Second Law of Motion.
- The equation is F = ma, where F is the force, m is the mass, and a is the acceleration.
- Force and acceleration are directly proportional.
- If the force acting on an object increases, its acceleration will also increase.
- If the mass of the object increases, its acceleration will decrease for a given force.

This happens due to Newton's third law of motion.'To every action, there is always an equal and opposite reaction.'When you kick a stone, you apply some force on it. To maintain the Law of Conservation of momentum, the stone also exerts a force on the leg, which we feel as pain.

 

The car pushes the ground in the backward direction and according to the third law of motion, reaction force of the ground in the forward direction acts on the car.

Impulse has the same unit as that of momentum.
Explanation:
The unit of impulse and momentum is the same because impulse is defined as the change in momentum of an object. Both impulse and momentum are quantities related to the motion of an object and can be calculated using the same formula:
Impulse: Impulse is the product of the force applied to an object and the time interval over which the force acts. It is given by the equation:
Impulse = Force x Time
Momentum: Momentum is the product of an object's mass and its velocity. It is given by the equation:
Momentum = Mass x Velocity
Since impulse is defined as the change in momentum, both quantities have the same unit. The unit of momentum is kg·m/s, which represents the product of mass (kg) and velocity (m/s). Similarly, the unit of impulse is also kg·m/s.
Therefore, the correct answer is C: momentum.

Explanation:
To calculate a physical quantity based on Newton's second law, we need to know the magnitude of force on a given mass. The physical quantity that can be calculated is the acceleration of the object.
Newton's second law:
According to Newton's second law of motion, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Mathematically, it can be represented as:
F = m * a
Where:
F = Net force acting on the object
m = Mass of the object
a = Acceleration of the object
Explanation of options:
A: Velocity: Velocity is the rate at which an object changes its position with respect to time. It is not directly related to Newton's second law.
B: Momentum: Momentum is the product of an object's mass and velocity. While momentum can be calculated using Newton's second law, it is not the physical quantity that can be directly calculated if we only know the magnitude of force and mass.
C: Acceleration: Acceleration is the rate of change of velocity with respect to time. It is directly related to Newton's second law and can be calculated if we know the magnitude of force and mass.
D: None of the above: This option is incorrect as the correct answer is option C, acceleration.
Conclusion:
Based on Newton's second law, the physical quantity that can be calculated if we know the magnitude of force on a given mass is the acceleration of the object.

It will have a constant momentum, change in momentum would be zero. It has exception of non-inertial frame of reference, wherein the change in mass occurs at higher speeds. If the body moves with speed of light, it will have no mass and hence no momentum. It will have linear momentum since its velocity is not zero.

In order to conserve momentum the two parts move in the opposite direction and have same momentum. 

Heavier the bat, less will be velocity of recoil for the change in momentum and hence less strain on the hands. 

Given,

Mass of the vehicle, m = 1500 kg

Acceleration, a = – 1.7 m s–2

Force acting between the vehicle and road, F =?

We know that, F = m x a

Therefore, F = 1500 kg X 1.7 m s–2

Or, F = – 2550 N

Thus, force between vehicle and road = - 2550 N. Negative sign shows that force is acting in the opposite direction of the vehicle.

Given:
- Mass of the aeroplane (m) = 2.5 t = 2.5 * 1000 kg = 2500 kg
- Force developed by the engine (F) = 8750 N
To find:
Acceleration of the aeroplane at takeoff
Formula:
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.
- F = m * a
- a = F / m
Calculation:
- Substitute the given values in the formula:
a = 8750 N / 2500 kg
a = 3.5 m/s^2
Answer:
The acceleration of the aeroplane at takeoff is 3.50 m/s^2. Therefore, the correct option is C.

Momentum = mass × velocity
mass = momentum/velocity
= 3000/15 =200 kg

The truck starts from rest, so initial velocity = 0

Distance travelled = 400 m

Time taken = 20 s

We know the equation of motion, S = ut + ½ at²

400 = 0 + ½ a * (20)²

a = 2 m/s²

Now, mass = 7 metric tonnes

= 7 * 1000

= 7000 kg

So force = mass * acceleration

F = 7000 * 2

= 14000 N

The acceleration acting on the bullet can be calculated using the formula:
Acceleration = (Final velocity - Initial velocity) / Time taken
Given:
Mass of the bullet, m = 0.01 kg
Time taken, t = 0.003 s
Final velocity, v = 300 m/s
Step 1:
Calculate the initial velocity of the bullet.
Initial velocity = 0 m/s (since the bullet starts from rest)
Step 2:
Substitute the values into the formula to calculate the acceleration.
Acceleration = (300 m/s - 0 m/s) / 0.003 s
Acceleration = 300 m/s / 0.003 s
Acceleration = 100,000 m/s^2
Therefore, the acceleration acting on the bullet is 100,000 m/s^2. So, the correct answer is B.

Given, force of engine = 40000 N

Force of friction = 5000 N
Mass of engine = 8000 kg
Total weight of wagons = 5 x 2000 kg = 10000 kg

(a) The net accelerating force 
= Force exerted by engine – Force of fricition
= 40000 N – 5000 N = 35000 N