Force and Laws of Motion Class 9 Science 20 Important Questions with answer

20 important questions and answers on the topic of Force and Laws of Motion for Class 9 Science:

1. What is Force?

Answer: Force is a vector quantity that causes an object to accelerate, change its direction, or change its shape. It is defined as a push or pull upon an object resulting from its interaction with another object. The unit of force in the International System of Units (SI) is the Newton (N).

2. What are the different types of forces?

Answer: Forces can be classified into several types:

• Contact Forces: Forces that act when objects are in physical contact, such as frictional force, normal force, and tension.
• Non-contact Forces: Forces that act at a distance, such as gravitational force, electrostatic force, and magnetic force.

3. State Newton’s First Law of Motion.

Answer: Newton’s First Law of Motion states that "An object will remain at rest or continue to move in a straight line at a constant speed unless acted upon by an external force." This law is also known as the Law of Inertia.

4. Explain Newton’s Second Law of Motion.

Answer: Newton’s Second Law of Motion states that "The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to its mass." Mathematically, it is expressed as $F = ma$, where $F$ is the force, $m$ is the mass, and $a$ is the acceleration.

5. What does Newton’s Third Law of Motion state?

Answer: Newton’s Third Law of Motion states that "For every action, there is an equal and opposite reaction." This means that if one body exerts a force on a second body, the second body exerts an equal and opposite force on the first body.

6. What is inertia?

Answer: Inertia is the property of an object to resist changes in its state of motion. It is directly related to the mass of the object: the greater the mass, the greater the inertia. Inertia is described by Newton’s First Law of Motion.

7. How is the concept of force related to acceleration?

Answer: According to Newton’s Second Law of Motion, force is directly related to acceleration. If the force applied to an object increases, the acceleration of the object also increases, provided the mass remains constant. Similarly, for a constant force, if the mass increases, the acceleration decreases.

8. What is frictional force?

Answer: Frictional force is a force that opposes the relative motion between two surfaces in contact. It acts parallel to the surfaces and can be categorized into static friction (preventing motion) and kinetic friction (opposing motion in progress).

9. Differentiate between static friction and kinetic friction.

Answer:

• Static Friction: Acts on an object at rest and prevents it from starting to move. It is generally greater than kinetic friction.
• Kinetic Friction: Acts on an object in motion and opposes its movement. It is usually less than static friction.

10. What is meant by the term “net force”?

Answer: Net force is the vector sum of all the individual forces acting on an object. It determines the object's acceleration according to Newton’s Second Law of Motion. If the net force is zero, the object either remains at rest or continues moving at a constant velocity.

11. What is momentum?

Answer: Momentum is the quantity of motion an object possesses and is defined as the product of its mass and velocity. Mathematically, it is given by $p = mv$, where $p$ is momentum, $m$ is mass, and $v$ is velocity. Momentum is a vector quantity and is conserved in isolated systems.

12. State the Law of Conservation of Momentum.

Answer: The Law of Conservation of Momentum states that in an isolated system (one not subject to external forces), the total momentum remains constant. This means that the total momentum before an interaction (like a collision) is equal to the total momentum after the interaction.

13. Explain how force and acceleration are related using an example.

Answer: Consider pushing a car and a bicycle with the same force. The car, having a larger mass, will accelerate less compared to the bicycle. This illustrates that for a given force, the acceleration is inversely proportional to the mass of the object. This relationship is described by $a = \frac{F}{m}$.

14. What is the role of a force in changing the shape of an object?

Answer: Force can change the shape of an object by applying pressure or tension. For example, when you squeeze a rubber ball, the shape changes because the force is altering its structure. This change in shape occurs due to the deformation of the material in response to the applied force.

15. What is a balanced force?

Answer: Balanced forces occur when two or more forces acting on an object are equal in magnitude and opposite in direction, resulting in no change in the object's state of motion. The net force on the object is zero.

16. What is an unbalanced force?

Answer: Unbalanced forces occur when the forces acting on an object are not equal in magnitude or direction, resulting in a net force. This causes a change in the object's state of motion, either accelerating or decelerating it.

17. Give an example of Newton’s Third Law of Motion in everyday life.

Answer: A common example is walking. When you walk, your foot exerts a backward force on the ground, and in response, the ground exerts an equal and opposite force forward on your foot, propelling you forward.

18. What is the significance of the coefficient of friction?

Answer: The coefficient of friction is a measure of how much frictional force exists between two surfaces. It is a dimensionless number that depends on the materials in contact and their surface roughness. A higher coefficient indicates more frictional force.

19. Describe the difference between mass and weight.

Answer:

• Mass: The quantity of matter in an object, measured in kilograms (kg). It is constant regardless of location.
• Weight: The force exerted on an object due to gravity, measured in Newtons (N). It is calculated as $W = mg$, where $g$ is the acceleration due to gravity. Weight varies depending on the strength of the gravitational field.

20. How does friction affect motion?

Answer: Friction opposes the relative motion between surfaces in contact, causing objects to slow down or stop. It converts kinetic energy into heat. While friction can be useful (e.g., providing grip), it can also be a hindrance, such as in machinery where it can cause wear and reduce efficiency.

These questions cover the fundamental concepts related to force and motion as outlined in Class 9 Science. Let me know if you need further elaboration or have more questions!