Laws of Motion Class 9 | Chapter 1 Notes | Important Questions

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Looking for Laws of Motion Class 9 notes? Get a clear breakdown of Newton’s Laws, momentum, and acceleration with practice problems and answers tailored for Class 9 Science.

Laws of Motion Class 9:

1. Fundamental Concepts

  • Motion: A relative concept; change in position of an object with respect to its surroundings.
  • Distance vs. Displacement: Distance is the total path length (Scalar). Displacement is the minimum distance between start and end points (Vector).
  • Speed: Distance covered per unit time. Unit: $m/s$.
  • Velocity: Displacement per unit time. Velocity changes if either speed or direction changes.

2. Acceleration ($a$)

The rate of change of velocity.

$$a = \frac{\text{Change in velocity}}{\text{Time}} = \frac{v – u}{t}$$

  • Positive: Velocity increases with time.
  • Negative (Retardation): Velocity decreases with time.
  • Zero: Velocity is constant.

3. Newton’s Laws of Motion

  1. First Law (Law of Inertia): An object stays at rest or in uniform motion unless acted upon by an unbalanced force. Inertia is the inherent resistance to change in state.
  2. Second Law: The rate of change of momentum is proportional to the applied force.
    • Formula: $F = ma$ (Force = mass $\times$ acceleration).
  3. Third Law: Every action force has an equal and opposite reaction force acting simultaneously.

4. Momentum ($P$) and Conservation

  • Momentum: $P = m \times v$. It is a vector quantity. Unit: $kg.m/s$.
  • Law of Conservation of Momentum: The total momentum of a system remains constant if no external unbalanced force acts on it ($m_1u_1 + m_2u_2 = m_1v_1 + m_2v_2$).
Laws of Motion Class 9

Question Bank: Laws of Motion Class 9th

Section A: Multiple Choice Questions (MCQs)

  1. If the displacement of an object is proportional to the square of time, then the object moves with:(a) Uniform velocity (b) Uniform acceleration (c) Increasing acceleration (d) Decreasing acceleration
  2. The product of mass and velocity is called:(a) Force (b) Acceleration (c) Momentum (d) Power
  3. $1\text{ Newton}$ is equal to:(a) $10^5\text{ dynes}$ (b) $10^3\text{ dynes}$ (c) $10^7\text{ dynes}$ (d) $10^4\text{ dynes}$
  4. Inertia of an object depends on its:(a) Velocity (b) Speed (c) Shape (d) Mass
  5. A passenger in a moving train tosses a coin which falls behind him. This means the motion of the train is:(a) Accelerated (b) Uniform (c) Retarded (d) Circular

Section B: Very Short Answer (1 Mark)

  1. Define ‘Uniform Circular Motion’.
  2. Give the S.I. unit of momentum.
  3. State the relation between force, mass, and acceleration.
  4. What is the displacement of a body moving in a semi-circle of radius $r$?
  5. If a body starts from rest, what is its initial velocity?

Section C: Short Notes & Definitions (2-3 Marks)

  1. Write a short note on Inertia of Rest with an example.
  2. Distinguish between Uniform and Non-uniform motion.
  3. Explain Negative Acceleration with a real-life example.
  4. Explain why a person falls backward when a bus starts suddenly.
  5. Derive the first equation of motion ($v = u + at$) using a velocity-time graph.

Section D: Scientific Reasons (2 Marks)

  1. Why is it difficult to stop a heavy truck than a light car moving at the same speed?
  2. Why does a gun recoil when a bullet is fired?
  3. Why is the motion of a stone tied to a string and whirled in a circle called accelerated motion?

Section E: Numerical Problems (3 Marks)

  1. A car increases its speed from $36\text{ km/h}$ to $72\text{ km/h}$ in $10\text{ seconds}$. Calculate its acceleration.
  2. A force of $50\text{ N}$ acts on a body of mass $5\text{ kg}$. What is the acceleration produced?

Laws of Motion Class 9 : Answers and Solutions

Section A: MCQs

  1. (b) Uniform acceleration.
  2. (c) Momentum.
  3. (a) $10^5\text{ dynes}$.
  4. (d) Mass (More mass = more inertia).
  5. (a) Accelerated.

Section B: Very Short Answer

  1. When an object moves with constant speed along a circular path, it is called Uniform Circular Motion.
  2. $kg.m/s$.
  3. $F = m \times a$.
  4. $2r$ (the diameter).
  5. $0\text{ m/s}$.

Section C: Short Notes

  1. Inertia of Rest: The tendency of an object to remain at rest. Example: When you shake a branch of a tree, the leaves fall because the branch moves but leaves try to stay at rest.
  2. Uniform Motion: Covering equal distances in equal intervals of time. Non-uniform: Unequal distances in equal intervals of time.
  3. Negative Acceleration: When velocity decreases. Example: Applying brakes to a moving car.
  4. Due to Inertia of Rest. The lower body moves with the bus, but the upper body tries to remain at rest.
  5. Hint: Acceleration $a = \text{Slope of v-t graph} = (v – u) / t$. Rearranging gives $v = u + at$.

Section D: Scientific Reasons

  1. Because the truck has more mass, it has more momentum ($P = mv$). Greater force or time is needed to stop it.
  2. Due to Newton’s 3rd Law. The gun exerts a forward force on the bullet (action), and the bullet exerts an equal backward force on the gun (reaction).
  3. Because its direction of motion changes at every point, and change in direction means a change in velocity, which is acceleration.

Section E: Numericals

  1. $u = 36\text{ km/h} = 10\text{ m/s}$; $v = 72\text{ km/h} = 20\text{ m/s}$; $t = 10\text{ s}$.$a = (20 – 10) / 10 = \mathbf{1\text{ m/s}^2}$.
  2. $F = 50\text{ N}$, $m = 5\text{ kg}$.$a = F / m = 50 / 5 = \mathbf{10\text{ m/s}^2}$.

#Laws of Motion Class 9

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