Introduction
The Turning Effect of Forces is an important chapter of Class 9 Physics that explains how forces produce rotational motion. In this chapter, students learn about the moment of force (torque), the principle of moments, center of mass, equilibrium, stability, and different classes of levers. These concepts help explain why objects rotate, remain balanced, or become unstable under the action of forces.
Students also study how the distance between the point of application of a force and the pivot affects the turning effect. The chapter introduces practical applications of rotational mechanics in everyday life, including beam balances, seesaws, wheelbarrows, fishing rods, and other simple machines. Understanding these principles helps students solve numerical problems and explain many real-world situations involving rotational motion.
On this page, you will find carefully selected Class 9 Physics Chapter 4 MCQs with answers and explanations, along with learning outcomes, quick revision notes, important definitions, important formulas, exam tips, and common mistakes to help you prepare effectively for your school and board examinations.
Learning Outcomes
After studying this chapter, students will be able to:
- Define the turning effect of a force and torque.
- Calculate the moment of a force.
- Apply the principle of moments to solve simple problems.
- Explain the concept of center of mass.
- Differentiate between stable, unstable, and neutral equilibrium.
- Identify first-class, second-class, and third-class levers.
- Describe the applications of moments and levers in everyday life.
Quick Notes – Chapter Summary
- The turning effect of a force is called the moment of force or torque.
- Torque depends on both the magnitude of the force and its perpendicular distance from the pivot.
- The SI unit of torque is the newton metre (N·m).
- The principle of moments states that clockwise moments equal anticlockwise moments in rotational equilibrium.
- The center of mass is the point at which the entire weight of a body appears to act.
- A body is in equilibrium when the resultant force and resultant moment acting on it are zero.
- Stable equilibrium returns an object to its original position after a small disturbance.
- Unstable equilibrium causes an object to move farther from its original position after disturbance.
- Neutral equilibrium allows an object to remain in its new position after being disturbed.
- A first-class lever has the fulcrum between the effort and the load.
- A second-class lever has the load between the fulcrum and the effort.
- A third-class lever has the effort between the fulcrum and the load.
- A couple consists of two equal, opposite, and parallel forces producing rotation without translation.
Important Definitions
Moment of Force (Torque): The turning effect produced by a force about a pivot or axis.
Pivot (Fulcrum): The fixed point about which a body rotates.
Principle of Moments: A body is in rotational equilibrium when the total clockwise moment equals the total anticlockwise moment.
Center of Mass: The point at which the entire weight of a body may be considered to act.
Equilibrium: The state in which the resultant force and resultant moment acting on a body are zero.
Couple: Two equal, opposite, and parallel forces acting on a body that produce rotation without translation.
Lever: A simple machine consisting of a rigid bar that rotates about a fixed point called the fulcrum.
Important Formulas
Moment of Force (Torque)
τ = F × d
Where:
- τ = Torque (Moment of Force)
- F = Applied Force
- d = Perpendicular distance from the pivot
Principle of Moments
Clockwise Moment = Anticlockwise Moment
or
F₁ × d₁ = F₂ × d₂ (for equilibrium)
Class 9 Physics Chapter 4 – Turning Effect of Forces MCQs
1. The turning effect of a force about a point is called:
- A. Moment of force ✅
- B. Torque
- C. Inertia
- D. Impulse
Explanation: Moment of force is the product of force and its perpendicular distance from the pivot point.
2. The SI unit of moment of force is:
- A. Newton meter ✅
- B. Newton second
- C. Joule
- D. Watt
Explanation: Moment of force is measured in newton meters (N·m).
3. Torque is defined as:
- A. Force × perpendicular distance from pivot ✅
- B. Mass × acceleration
- C. Force ÷ distance
- D. Weight × height
Explanation: Torque measures the tendency of a force to cause rotation.
4. The principle of moments states that:
- A. Clockwise moments = anticlockwise moments in equilibrium ✅
- B. Force = mass × acceleration
- C. Momentum is conserved
- D. Energy cannot be created or destroyed
Explanation: For rotational equilibrium, total clockwise moment equals total anticlockwise moment.
5. The point where the entire weight of a body appears to act is called:
- A. Center of mass ✅
- B. Pivot
- C. Axis of rotation
- D. Torque point
Explanation: The center of mass is the point of balanced mass distribution.
6. Which type of lever has the fulcrum between the load and the effort?
- A. First-class lever ✅
- B. Second-class lever
- C. Third-class lever
- D. None of these
Explanation: In first-class levers, the fulcrum is between load and effort (e.g., seesaw).
7. A seesaw is an example of:
- A. First-class lever ✅
- B. Second-class lever
- C. Third-class lever
- D. Inclined plane
Explanation: A seesaw has the pivot between effort and load.
8. Which lever has the load between the fulcrum and the effort?
- A. Second-class lever ✅
- B. First-class lever
- C. Third-class lever
- D. None
Explanation: Second-class levers provide mechanical advantage (e.g., wheelbarrow).
9. A fishing rod is an example of:
- A. Third-class lever ✅
- B. First-class lever
- C. Second-class lever
- D. None
Explanation: In a third-class lever, the effort is applied between the fulcrum and load.
10. A couple is a pair of forces which are:
- A. Equal, opposite, and parallel ✅
- B. Equal, opposite, and perpendicular
- C. Unequal and opposite
- D. Equal and in the same direction
Explanation: A couple causes rotation without translation.
11. The SI unit of torque is the same as:
- A. Work ✅
- B. Power
- C. Momentum
- D. Pressure
Explanation: Torque and work share the same unit (N·m), but they are different physical quantities.
12. Which factor does NOT affect the stability of an object?
- A. Color of the object ✅
- B. Height of center of mass
- C. Size of base
- D. Weight distribution
Explanation: Stability depends on center of mass and base area, not color.
13. The moment of a force increases if:
- A. Perpendicular distance from pivot increases ✅
- B. Force decreases
- C. Pivot is moved closer
- D. Force acts at pivot
Explanation: Greater distance from pivot increases the turning effect.
14. When the total clockwise moment equals the total anticlockwise moment, the body is in:
- A. Rotational equilibrium ✅
- B. Translational motion
- C. Acceleration
- D. Free fall
Explanation: This is the condition for rotational equilibrium.
15. Which device works on the principle of moments?
- A. Beam balance ✅
- B. Spring balance
- C. Thermometer
- D. Stopwatch
Explanation: Beam balances use moments to compare weights.
Exam Tips
- Memorize the formula for calculating the moment of force.
- Learn the differences between the three classes of levers.
- Understand the principle of moments and practice related numerical problems.
- Revise the characteristics of stable, unstable, and neutral equilibrium.
- Remember common real-life examples of levers and couples.
Common Mistakes
❌ Confusing force with the moment of force.
❌ Forgetting that the perpendicular distance affects torque.
❌ Mixing up the three classes of levers.
❌ Assuming torque and work are the same because they share the same SI unit.
❌ Ignoring the condition for rotational equilibrium.
Conclusion
The turning effect of forces explains how forces produce rotational motion and maintain the balance of objects. Understanding torque, the principle of moments, equilibrium, center of mass, and levers helps students explain many everyday mechanical systems and solve Physics problems confidently. Regular practice of these MCQs and revision of the important definitions and formulas will strengthen conceptual understanding and improve performance in school and board examinations.
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