What is the difference between mass and weight?

Mass is the amount of matter in a body (scalar, SI unit: kg) — it is constant everywhere. Weight is the gravitational force on a body W = mg (vector, SI unit: N) — it varies with location (e.g., less on the Moon where g is smaller). Mass is measured by a balance; weight by a spring scale.

Why is Newton's First Law called the Law of Inertia?

Because it defines inertia — the natural tendency of a body to resist change in its state of motion. If no net external force acts, a resting body stays at rest and a moving body continues with constant velocity. The law quantifies that inertia is proportional to mass.

Why does a bullet fired from a gun cause the gun to recoil?

By Newton's Third Law, when the explosive gases push the bullet forward with a large force, the bullet exerts an equal and opposite force backward on the gun. This backward force causes the gun to recoil (kick back). By conservation of momentum: m_bullet × v_bullet = m_gun × v_recoil.

Newton's Three Laws of Motion

Laws of Motion • Class 11 Physics • NCERT • CBSE

Newton's First Law: A body continues in its state of rest or uniform motion unless acted upon by an external force (Inertia). Second Law: F = ma (Force = mass × acceleration). Third Law: Every action has an equal and opposite reaction.

Key Formulas

F = ma
F = dp/dt
Impulse J = F·Δt = Δp
fₖ = μₖN
fₛ(max) = μₛN

Frequently Asked Questions

What is the difference between mass and weight?: Mass is the amount of matter in a body (scalar, SI unit: kg) — it is constant everywhere. Weight is the gravitational force on a body W = mg (vector, SI unit: N) — it varies with location (e.g., less on the Moon where g is smaller). Mass is measured by a balance; weight by a spring scale.
Why is Newton's First Law called the Law of Inertia?: Because it defines inertia — the natural tendency of a body to resist change in its state of motion. If no net external force acts, a resting body stays at rest and a moving body continues with constant velocity. The law quantifies that inertia is proportional to mass.
Why does a bullet fired from a gun cause the gun to recoil?: By Newton's Third Law, when the explosive gases push the bullet forward with a large force, the bullet exerts an equal and opposite force backward on the gun. This backward force causes the gun to recoil (kick back). By conservation of momentum: m_bullet × v_bullet = m_gun × v_recoil.

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