When is work done zero even if a force is applied?

Work done is zero (W = Fdcosθ = 0) when: (1) θ = 90° — force perpendicular to displacement (e.g., a coolie carrying a load on his head while walking horizontally — the load's weight is vertical but displacement is horizontal); (2) displacement = 0 — pushing a wall; (3) force = 0.

What is the difference between elastic and inelastic collisions?

In an elastic collision, both kinetic energy and linear momentum are conserved (e.g., billiard balls, gas molecules). In an inelastic collision, only momentum is conserved — some kinetic energy is lost as heat/sound/deformation. A perfectly inelastic collision sees maximum KE loss with the objects sticking together after collision.

What is the relationship between kinetic energy and momentum?

Momentum p = mv and KE = ½mv². Therefore KE = p²/(2m). This means: (1) if two objects have same momentum, the lighter one has greater KE; (2) if two objects have same KE, the heavier one has greater momentum.

Work-Energy Theorem

Work, Energy and Power • Class 11 Physics • NCERT • CBSE

The Work-Energy Theorem states: The net work done on a body equals the change in its kinetic energy. W_net = ΔKE = ½mv² − ½mu². Work done W = F·d·cosθ where θ is the angle between force and displacement.

Key Formulas

W = F·d·cosθ
KE = ½mv²
W_net = ½mv² − ½mu²
PE (gravity) = mgh
PE (spring) = ½kx²
P = W/t = F·v

Frequently Asked Questions

When is work done zero even if a force is applied?: Work done is zero (W = Fdcosθ = 0) when: (1) θ = 90° — force perpendicular to displacement (e.g., a coolie carrying a load on his head while walking horizontally — the load's weight is vertical but displacement is horizontal); (2) displacement = 0 — pushing a wall; (3) force = 0.
What is the difference between elastic and inelastic collisions?: In an elastic collision, both kinetic energy and linear momentum are conserved (e.g., billiard balls, gas molecules). In an inelastic collision, only momentum is conserved — some kinetic energy is lost as heat/sound/deformation. A perfectly inelastic collision sees maximum KE loss with the objects sticking together after collision.
What is the relationship between kinetic energy and momentum?: Momentum p = mv and KE = ½mv². Therefore KE = p²/(2m). This means: (1) if two objects have same momentum, the lighter one has greater KE; (2) if two objects have same KE, the heavier one has greater momentum.

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