ICSE Class 10 Physics: Force — Important Questions with Answers 2026

Short Answer Questions (2-3 Marks)

• Q: Define force and state its SI unit.
  Ans: Force is a push or pull acting on a body that changes or tends to change its state of rest or uniform motion. SI unit: Newton (N). One Newton is the force that produces an acceleration of 1 m/s² in a body of mass 1 kg.
• Q: State Newton's First Law of Motion. Why is it called the law of inertia?
  Ans: Newton's First Law states: A body at rest remains at rest, and a body in uniform motion continues in uniform motion in a straight line unless acted upon by an external unbalanced force. It is called the law of inertia because it defines inertia — the tendency of a body to resist any change in its state of rest or motion.
• Q: Define momentum. A body of mass 5 kg is moving with a velocity of 10 m/s. Find its momentum.
  Ans: Momentum is the product of mass and velocity of a body. p = mv. SI unit: kg⋅m/s. Given: m = 5 kg, v = 10 m/s. Momentum p = 5 × 10 = 50 kg⋅m/s.
• Q: State Newton's Second Law of Motion and derive F = ma.
  Ans: Newton's Second Law states: The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force. Derivation: F ∝ dp/dt = d(mv)/dt. For constant mass: F ∝ m(dv/dt) = ma. In SI units, the constant of proportionality is 1, so F = ma.
• Q: State Newton's Third Law. Give two examples.
  Ans: Newton's Third Law states: To every action, there is an equal and opposite reaction. Examples: (1) When a gun is fired, the bullet moves forward (action) and the gun recoils backward (reaction). (2) A swimmer pushes water backward with hands (action) and water pushes the swimmer forward (reaction).

Long Answer / Application Questions (4-6 Marks)

• Q: Differentiate between mass and weight.
  Ans: Mass is the quantity of matter in a body, measured in kg, is constant everywhere, and is a scalar. Weight is the gravitational force acting on a body, measured in Newton (N), varies with location (g changes), and is a vector quantity. Relation: W = mg.
• Q: A force of 20 N acts on a body of mass 4 kg at rest. Find the velocity after 5 seconds.
  Ans: Given: F = 20 N, m = 4 kg, u = 0, t = 5 s. Acceleration a = F/m = 20/4 = 5 m/s². Using v = u + at: v = 0 + 5 × 5 = 25 m/s.
• Q: State the law of conservation of momentum. A bullet of 50 g is fired from a gun of mass 5 kg with velocity 300 m/s. Find the recoil velocity.
  Ans: The law of conservation of momentum states: In an isolated system (no external force), the total momentum before and after an event remains constant. Solution: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂. Before firing: total momentum = 0. After: 0.05 × 300 + 5 × v = 0. v = -15/5 = -3 m/s. The gun recoils at 3 m/s.
• Q: What is centripetal force? Why is it necessary for circular motion?
  Ans: Centripetal force is the force that acts on a body moving in a circular path, directed towards the centre of the circle. Formula: F = mv²/r. It is necessary because without it, the body would move in a straight line (Newton's First Law). It continuously changes the direction of velocity, keeping the body in circular path.
• Q: Define impulse. How is it related to momentum?
  Ans: Impulse is the product of force and the time for which it acts. Impulse = F × t. By Newton's Second Law: F = (mv - mu)/t, so F × t = mv - mu = change in momentum. Thus, impulse equals the change in momentum. SI unit: N⋅s or kg⋅m/s.

Exam Tips for This Chapter

• Revise all definitions and laws from Force — they are commonly asked as 1-2 mark questions
• Practice diagrams related to Force — neat labelled diagrams carry 2-3 marks
• For numericals, always show formula → substitution → answer with correct units
• Previous year analysis shows Force carries 8-12 marks in the board exam