Newton’s Second Law of Motion MCQs Quiz | Class 9

This quiz covers Class IX Science (Code 086), Unit III: Motion, Force and Work, specifically focusing on the statement and mathematical relation among force, mass, and acceleration (F=ma). Attempt all 10 questions, submit your answers to see your score, and download the solution PDF for offline revision.

Understanding Newton’s Second Law of Motion

Newton’s Second Law of Motion is often called the law of acceleration. It provides a quantitative measure of force. While the First Law defines force qualitatively (as something that changes the state of motion), the Second Law tells us how much force is needed to produce a certain acceleration.

The Statement

The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

Mathematical Formulation

If an object of mass m is moving with initial velocity u and accelerates to velocity v in time t due to a constant force F:

  • Initial Momentum (p1) = mu
  • Final Momentum (p2) = mv
  • Change in Momentum = p2 – p1 = m(v – u)
  • Rate of change of momentum = m(v – u) / t
  • Since (v – u) / t = acceleration (a), the rate of change is ma.

Thus, F = kma. In SI units, the constant of proportionality k is 1. Therefore, F = ma.

Key Concepts & Units

  • Force (F): A push or pull. SI Unit: Newton (N).
  • Mass (m): Measure of inertia. SI Unit: Kilogram (kg).
  • Acceleration (a): Rate of change of velocity. SI Unit: m/s^2.
  • 1 Newton: The force which produces an acceleration of 1 m/s^2 in a mass of 1 kg.

Real-life Applications

  • Catching a Cricket Ball: A fielder pulls their hands backward while catching a fast-moving ball. This increases the time interval for the change in momentum, thereby decreasing the rate of change of momentum and reducing the force exerted on the hands.
  • High Jump: Athletes land on cushioned beds or sand. This increases the time to stop, reducing the impact force.
  • Seatbelts: Seatbelts stretch slightly during a collision to increase the time taken for the passenger to come to rest, reducing the force of impact.

Extra Practice Questions

  1. Calculate the force needed to speed up a car with a rate of 5 m/s^2 if the mass of the car is 1000 kg. (Ans: 5000 N)
  2. Which has more momentum: a 3000 kg truck moving at 10 m/s or a 1000 kg car moving at 50 m/s? (Ans: The car)
  3. If the force applied on an object is doubled while mass remains constant, how does acceleration change? (Ans: Acceleration doubles)
  4. Why is it easier to push an empty shopping cart than a full one? (Ans: Lower mass means less force is required for the same acceleration)
  5. Define Impulse in terms of Newton’s Second Law. (Ans: Impulse is the product of Force and Time, equal to the change in momentum)