— CHAPTER MASTERY · CLASS 9

Force and Laws of Motion Important Questions.

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Key Concepts in Force and Laws of Motion

Newton's first, second and third laws of motionInertia and massMomentum and its conservationFriction: static, sliding, rollingPractical applications of Newton's laws

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Force and Laws of Motion — Important Questions with Answers

Practice these Force and Laws of Motion questions for Class 9 Science, each with the correct answer and a step-by-step explanation. Sign up free to practice all 116+ questions with adaptive difficulty.

  1. Q1Easy

    What is the SI unit of force?

    • A.Joule (J)
    • B.Pascal (Pa)
    • C.Newton (N)
    • D.Watt (W)
    Answer: Newton (N)

    Explanation: The SI unit of force is the newton (N), defined as the force required to accelerate a mass of 1 kg at a rate of 1 m/s².

  2. Q2Easy

    What is the momentum of an object of mass m moving with velocity v?

    • A.(mv)²
    • B.mv²
    • C.½mv²
    • D.mv
    Answer: mv

    Explanation: Momentum is defined as the product of mass and velocity. The correct formula is momentum (p) = mass (m) × velocity (v).

  3. Q3Easy

    Which of the following is an example of balanced forces?

    • A.A block being pulled by a single force.
    • B.A block being pushed with unequal forces from opposite sides.
    • C.A block being pulled equally from both sides with two strings.
    • D.A block experiencing friction while moving.
    Answer: A block being pulled equally from both sides with two strings.

    Explanation: Balanced forces are equal in magnitude and opposite in direction, resulting in no change in the object's state of motion. This matches the description of the block being pulled equally from both sides.

  4. Q4Easy

    Which of the following is an example of an unbalanced force?

    • A.Pushing a box with equal forces from opposite sides.
    • B.Pushing a box with a force equal to the frictional force.
    • C.Pushing a box across a rough floor with a force greater than the frictional force.
    • D.Lifting a box with a force equal to its weight.
    Answer: Pushing a box across a rough floor with a force greater than the frictional force.

    Explanation: An unbalanced force is one that causes a change in the object's state of motion. Pushing a box with a force greater than friction results in motion.

  5. Q5Medium

    An engine pulls a train with 5 wagons, each of 2000 kg. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, what is the net accelerating force?

    • A.25000 N
    • B.35000 N
    • C.45000 N
    • D.50000 N
    Answer: 35000 N

    Explanation: The net accelerating force is the difference between the force exerted by the engine and the friction force. Net force = 40000 N (engine force) - 5000 N (friction) = 35000 N.

  6. Q6Medium

    What is the acceleration of a 1500 kg automobile vehicle if it is stopped with a negative acceleration of 1.7 m/s²?

    • A.1.7 m/s² in the forward direction
    • B.1.7 m/s² in the backward direction
    • C.0 m/s²
    • D.1.7 m/s² (magnitude)
    Answer: 1.7 m/s² (magnitude)

    Explanation: The negative sign indicates the direction (deceleration), but the magnitude of acceleration is 1.7 m/s². The question asks for the acceleration, so we consider the magnitude.

  7. Q7Medium

    Which of the following correctly represents the momentum of an object with mass m and velocity v?

    • A.(mv)²
    • B.mv²
    • C.½mv²
    • D.mv
    Answer: mv

    Explanation: Momentum is defined as the product of mass and velocity. The correct formula is p = mv, where p is momentum.

  8. Q8Medium

    A wooden cabinet is moved across a floor with a constant velocity using a horizontal force of 200 N. What can be concluded about the frictional force acting on the cabinet?

    • A.The frictional force is less than 200 N
    • B.The frictional force is greater than 200 N
    • C.The frictional force is equal and opposite to the applied force of 200 N
    • D.The frictional force is zero
    Answer: The frictional force is equal and opposite to the applied force of 200 N

    Explanation: Since the cabinet moves with constant velocity, the net force is zero. This means the frictional force must be equal in magnitude but opposite in direction to the applied force.

  9. Q9Hard

    A 10 kg object is initially at rest. A force of 20 N is applied to it for 5 seconds. What will be its final velocity?

    • A.5 m/s
    • B.10 m/s
    • C.15 m/s
    • D.20 m/s
    Answer: 10 m/s

    Explanation: Using Newton’s Second Law, a = F/m = 20 N / 10 kg = 2 m/s². Using the equation v = u + at, where u = 0 m/s, v = 0 + (2 m/s² * 5 s) = 10 m/s.

  10. Q10Hard

    A 10 kg box is initially at rest. A force of 50 N is applied to it for 4 seconds. What will be its final velocity?

    • A.10 m/s
    • B.15 m/s
    • C.20 m/s
    • D.25 m/s
    Answer: 20 m/s

    Explanation: Using Newton’s Second Law, a = F/m = 50 N / 10 kg = 5 m/s². Using the equation v = u + at, where u = 0 m/s, v = 0 + (5 m/s² * 4 s) = 20 m/s.

  11. Q11Hard

    A 5 kg object is moving at 8 m/s. If it experiences a force of 20 N for 3 seconds, what will be its final velocity?

    • A.10 m/s
    • B.14 m/s
    • C.16 m/s
    • D.20 m/s
    Answer: 20 m/s

    Explanation: Using Newton’s Second Law, a = F/m = 20 N / 5 kg = 4 m/s². Using the equation v = u + at, where u = 8 m/s, v = 8 + (4 m/s² * 3 s) = 20 m/s. However, since the question involves a net increase in velocity, the correct calculation should be: v = 8 + (4 * 3) = 20 m/s. This question might have a typo as the correct answer should be 20 m/s.

  12. Q12Hard

    Based on the distance-time table provided, what can be concluded about the acceleration of the object?

    • A.The acceleration is increasing.
    • B.The acceleration is decreasing.
    • C.The acceleration is constant and equal to 7 m/s².
    • D.The acceleration is zero.
    Answer: The acceleration is constant and equal to 7 m/s².

    Explanation: The distance-time table shows a pattern of distances being cubes of time (e.g., 1³ = 1, 2³ = 8, 3³ = 27, etc.). This implies the distance is proportional to the cube of time, indicating that the object's velocity is proportional to time squared, which suggests constant acceleration. The velocity at time t is given by v = 7t, so acceleration is 7 m/s².

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