Force and Laws of Motion
About 7 min · Science · English
Time worked out from the lesson's 489 words at 150 words a minute, plus a minute for each question.
Force and Laws of Motion is a cornerstone topic in mechanics that appears in SOF NSO Class 9 and Class 10 papers. This chapter explains why objects move the way they do and introduces Newton's three laws of motion, which govern all motion in our everyday world—from a cricket ball being hit to a rocket launching into space.
Key ideas
- Force is a push or pull that changes or tends to change the state of rest or uniform motion of an object. SI unit: Newton (N). 1 N = 1 kg·m/s².
- Inertia is the tendency of an object to resist changes in its state of motion. Mass is the measure of inertia—heavier objects have more inertia.
- Newton's First Law (Law of Inertia): An object at rest stays at rest, and an object in uniform motion stays in uniform motion, unless acted upon by an external unbalanced force.
- Newton's Second Law: The rate of change of momentum is directly proportional to the applied force and occurs in the direction of the force. Mathematically: F = ma (when mass is constant).
- Newton's Third Law: For every action, there is an equal and opposite reaction. Forces always occur in pairs acting on different objects.
Watch out for
- Confusing mass and weight: Mass is the amount of matter (kg), weight is the force due to gravity (N). Weight = mg, not just m. Always use weight when asked for force.
- Ignoring direction in momentum problems: Momentum is a vector. In conservation problems, assign +/− signs to velocities carefully. Opposite directions must have opposite signs.
- Thinking Third Law pairs act on the same object: Action and reaction act on different objects. Example: When you push a wall, you exert force on the wall (action) and the wall exerts force on you (reaction)—not both on you.
Quick reference
- Inertia ∝ mass: More mass → more resistance to change in motion.
- F = ma: Force causes acceleration; zero net force → zero acceleration (uniform motion or rest).
- Momentum p = mv: Larger mass or higher velocity → larger momentum.
- Newton's Third Law: Forces exist in pairs on different objects; equal magnitude, opposite direction.
- Conservation of Momentum: Total momentum before = Total momentum after (isolated system, no external forces).
Check yourself: 3 questions
1. A cricket ball of mass 0.15 kg moving with a velocity of 20 m/s is stopped by a fielder in 0.1 seconds. What is the magnitude of the force applied by the fielder?
2. A passenger in a moving bus falls forward when the bus suddenly stops. Which of Newton's laws explains this phenomenon?
3. Two objects of masses 2 kg and 3 kg are moving in the same direction with velocities 4 m/s and 2 m/s respectively. They collide and stick together. What is their common velocity after collision?
This lesson is cut from the topic's note, not rewritten. Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain. The questions are from Shishya's validated question bank.
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