Unit 3: Dynamics
Simple English Study Guide - Physics Class 9
Introduction to Dynamics
Welcome, students! Today we are starting a very important chapter called Dynamics. In the previous unit, we only talked about how things move. But in this unit, we will learn the most important thing: Why do things move? The answer is "Force". We will study how forces make objects move or stop.
Dynamics is the branch of mechanics that deals with the study of the motion of an object and the cause of its motion. The main cause of motion is force. In this chapter, we will learn about Sir Isaac Newton’s famous laws. These laws help us understand everything from a ball rolling on the ground to a giant rocket flying into space.
We will keep the language very simple so that you can understand every concept easily. This unit covers topics like Momentum, Friction, and Centripetal Force. These topics are not just for exams; they are part of our daily lives. So, let’s dive into the world of Dynamics and see how forces rule the world!
Important Questions & Answers
Q1: Define Force and write its SI unit.
Answer: A force is a push or a pull upon an object resulting from the object's interaction with another object. It can move a stationary body or stop a moving body.
Unit: The SI unit of force is the Newton (N).
Q2: What is Inertia? On what factor does it depend?
Answer: Inertia is the property of a body due to which it resists any change in its state of rest or motion. Inertia depends directly on the Mass of the body. A heavier object has more inertia.
Q3: State Newton's Second Law of Motion.
Answer: When a net force acts on a body, it produces acceleration in the direction of the force. This acceleration is directly proportional to the force and inversely proportional to the mass.
Formula: $F = ma$
Q4: Differentiate between Mass and Weight.
Answer: This is a very important concept!
Mass: It is the quantity of matter in a body. It is a scalar quantity and remains constant everywhere.
Weight: It is the force of gravity acting on a body. It is a vector quantity and changes from place to place. ($W = mg$)
Q5: Define Momentum. Write its formula.
Answer: Momentum is the quantity of motion possessed by a body. It is the product of mass (m) and velocity (v).
Formula: $P = mv$. Its SI unit is $kgms^{-1}$ or $Ns$.
Q6: What is Friction? Name two types of friction.
Answer: Friction is a force that opposes the motion of one surface across another surface.
Types: 1. Static Friction (at rest) 2. Kinetic Friction (in motion).
Q7: Define Centripetal Force.
Answer: Centripetal force is the force that keeps an object moving in a circular path. It is always directed towards the center of the circle.
Formula: $F_c = \frac{mv^2}{r}$
📝 Unit 3: Test Your Knowledge (10 MCQs)
1. Newton's first law of motion is also known as the law of:
(A) Gravity (B) Inertia (C) Friction (D) Energy
▶ See Answer
(B) Inertia
2. Inertia of a body depends on its:
(A) Velocity (B) Acceleration (C) Mass (D) Volume
▶ See Answer
(C) Mass
3. The SI unit of momentum is:
(A) $Nm$ (B) $Ns$ (C) $Js$ (D) $Ws$
▶ See Answer
(B) $Ns$
4. A force of 20N moves a body with acceleration of $2ms^{-2}$. The mass is:
(A) 5kg (B) 10kg (C) 20kg (D) 40kg
▶ See Answer
(B) 10kg ($m = F/a$)
5. Which of the following is a vector quantity?
(A) Mass (B) Weight (C) Speed (D) Time
▶ See Answer
(B) Weight
6. Rolling friction is much _______ than sliding friction:
(A) Larger (B) Smaller (C) Equal (D) Double
▶ See Answer
(B) Smaller
7. Centripetal acceleration is produced by:
(A) Friction (B) Centripetal force (C) Mass (D) Inertia
▶ See Answer
(B) Centripetal force
8. Friction can be reduced by using ball bearings because:
(A) It increases area (B) It converts sliding friction into rolling friction (C) It increases speed (D) None
▶ See Answer
(B) It converts sliding friction into rolling friction
9. The weight of a body of mass 10kg on Earth is:
(A) 10N (B) 1N (C) 100N (D) 1000N
▶ See Answer
(C) 100N ($W = mg$)
10. Centripetal force always acts ________ the center:
(A) Away from (B) Towards (C) Parallel to (D) Below
▶ See Answer
(B) Towards
Conclusion
To conclude today's lesson, Dynamics is a fundamental part of Physics that explains why things around us move. By understanding force, mass, and acceleration, we can understand how everything works, from cars on the road to satellites in space. We hope these notes made the concept of Dynamics clear and easy for you. Keep studying hard!
UNIT 3 COMPLETE ✅
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