All questions of Force and Laws of Motion for Class 9 Exam

The concept of inertia is a fundamental concept in physics. It is defined as the tendency of an object to resist changes in its state of motion. In other words, it is the property of an object that causes it to remain at rest or in motion with a constant velocity unless acted upon by an external force. The correct option among the given options is option D, which states that inertia is directly proportional to the mass of an object.

Explanation:

Direct Proportionality to Mass:

Inertia is directly proportional to the mass of an object. This means that the greater the mass of an object, the greater its inertia. This can be explained using Newton's first law of motion, which states that an object at rest will remain at rest, and an object in motion will continue in motion with a constant velocity unless acted upon by an external force.

For example, consider a small ball and a large ball. If we push both balls with the same force, the small ball will move faster than the large ball because it has less mass. This is because the large ball has more inertia, and it requires more force to put it in motion.

The Importance of Inertia:

Inertia is an important concept in physics, and it has many practical applications. For example, it is the reason why seat belts are important in cars. When a car suddenly stops, the passengers continue to move forward due to their inertia. The seat belt applies an external force that stops the passengers from moving forward and prevents injuries.

Conclusion:

In conclusion, inertia is the tendency of an object to resist changes in its state of motion. It is directly proportional to the mass of an object. The greater the mass of an object, the greater its inertia. Inertia has many practical applications in everyday life and is a fundamental concept in physics.

Since momentum depends on impulse
I.e if momentum increase impulse increase vice versa

Yes the si unit of force is newton.

No external force is being applied on that moving object. If no force is applied then it will continue doing it's work. Just like in the question they have asked, it was moving in a straight line, so no external force is applied that's why it is moving in the same sped in a straight line.

Explanation:

The cushioned bed or mat used in high jump competition serves the purpose of decreasing the impact of the athlete's landing. When the athlete jumps over the bar, he gains a significant amount of momentum due to his speed and upward thrust. The cushioned bed helps to decrease this momentum gradually, thereby reducing the impact of the landing.

Increase the time to stop:
By decreasing the momentum gradually, the cushioned bed increases the time taken for the athlete to come to a complete stop. This is important because if the athlete were to land on a hard surface, the sudden stop could cause injuries to his joints and muscles. By increasing the time taken to stop, the cushioned bed reduces the risk of injury.

Conclusion:
Thus, option C, i.e., "Increase the time to stop" is the correct answer. The cushioned bed in high jump competition is designed to decrease the impact of the athlete's landing by reducing his momentum gradually and increasing the time taken to stop.

Here came the application of Newton's third law of motion.. A gun recoils when a shot is fired from it.. initially both the gun and the bullet are at rest so the total momentum of the system is constant ....
when a shot is fired the bullet moves forward pushing the gun backward this phenomenon is known as recoiling of gun...
Due to recoiling of gun a backward jerk is experience on the shoulder of the shooter..

The Correct Answer Is A

Newton's First Law of Motion

Newton's first law of motion is also known as the law of inertia. It states that a body will continue in its state of rest or uniform motion in a straight line unless acted upon by some external force. This means that if an object is at rest, it will stay at rest, and if it is moving at a constant speed in a straight line, it will continue to do so unless something else interferes.

Explanation

The first law of motion is based on the concept of inertia, which is the tendency of an object to resist a change in its state of motion. In other words, an object will continue to do what it is doing unless something else makes it change. This can be seen in everyday life when you try to push a stationary object, such as a heavy box. It will not move unless you apply a force to it.

Similarly, if an object is already in motion, it will keep moving in a straight line at a constant speed unless some external force acts upon it. For example, if you slide a hockey puck across the ice, it will keep moving until it hits something or is acted upon by another force, such as friction.

Conclusion

Newton's first law of motion is a fundamental concept in physics. It explains how objects behave when no external forces are acting upon them. By understanding this law, scientists and engineers can design better machines and devices, and also predict how objects will behave in various situations.

**Explanation:**

To understand why the correct answer is option 'D' (0 N), let's analyze the forces acting on the object:

1. **Weight:** The weight of the object is the force due to gravity acting vertically downwards. It can be calculated using the equation: Weight = mass x gravitational acceleration. In this case, the weight would be 2 kg x 9.8 m/s^2 = 19.6 N.

2. **Normal force:** The normal force is the force exerted by a surface to support the weight of an object resting on it. When an object is on a horizontal table, the normal force is equal in magnitude but opposite in direction to the weight of the object. So, the normal force in this case would also be 19.6 N.

3. **Frictional force:** The frictional force acts in the opposite direction to the motion of the object. It can be calculated using the equation: Frictional force = coefficient of friction x normal force. However, since the object is sliding with a constant velocity, we can conclude that the frictional force is equal in magnitude but opposite in direction to the force applied to keep the object moving with the same velocity. Therefore, the frictional force in this case is 0 N.

Now, let's consider the forces in the horizontal direction:

- Since the object is sliding with a constant velocity, the net force acting on it must be zero. This means that the force required to keep the object moving with the same velocity must be equal in magnitude but opposite in direction to the frictional force.
- As mentioned earlier, the frictional force is 0 N in this case, so the force required to keep the object moving with the same velocity is also 0 N.
- Hence, the correct answer is option 'D' (0 N).

Overall, the object is able to maintain a constant velocity of 4 m/s on the frictional horizontal table without any external force acting on it.

Action and Reaction Forces

Action and reaction forces are two forces that are always equal in magnitude but opposite in direction. These forces always act on different objects. Let's understand this concept in detail.

Action Force

An action force is a force that occurs when one object exerts force on another object. This force can be a push, pull, or any other force that one object exerts on another. For example, when you push a table, you are applying an action force on the table.

Reaction Force

A reaction force is a force that occurs when an object exerts force back on the object that applied the action force. This force is equal in magnitude but opposite in direction to the action force. For example, when you push a table, the table exerts a reaction force back on you.

Key Points

- Action and reaction forces always act on different objects.
- They are equal in magnitude but opposite in direction.
- They are always present in pairs.
- They occur simultaneously.

Incorrect Statement

The incorrect statement is option 'D', which says that action and reaction forces act on the same object. This statement is incorrect because action and reaction forces always act on different objects. When one object exerts force on another object, the second object exerts a reaction force back on the first object. For example, when you jump off a diving board, you exert a force on the board, and the board exerts a reaction force back on you. These forces always act on different objects.

Conclusion

In conclusion, action and reaction forces are equal in magnitude but opposite in direction. They always act on different objects and occur simultaneously. It is essential to understand this concept because it is a fundamental principle of physics that explains how objects move and interact with each other.

Because the dust particle is in rest position hence if we beat the carpet the inertia of rest will break and tey fall down .

S=ut+1/2at^2 Since the mass is constant, and i guess you dont mean above to be dealt in relativistic speed, and a constant force is applied then we will have constant acceleration. So distance covered will be only dependent on elapsed time t.So s ∝ t^2