Work-Energy Theorem
Work Energy Power
45
JEE Qs
8%
Hard
60
min
Always identify ALL forces acting on the body and calculate the net work done by them for the given displacement to correctly apply the Work-Energy Theorem.
🧮 Key Formulas
✅ Key Points for JEE
- 1The Work-Energy Theorem states that the net work done by ALL forces acting on a body is equal to the change in its kinetic energy.
- 2This theorem is a scalar equation, making it powerful for problems where only initial and final states (velocities, positions) are given, and time or acceleration is not directly required.
- 3W_net includes work done by conservative forces (gravity, spring), non-conservative forces (friction, air resistance), and external applied forces.
- 4It can be applied to a point mass or to the center of mass of a rigid body (translational kinetic energy). For rotational motion, there's a rotational analogue.
- 5When only conservative forces do work, the Work-Energy Theorem simplifies to the principle of conservation of mechanical energy.
⚠️ Common Mistakes
- ✕Including work done by only some forces instead of the net work done by ALL forces acting on the system/body.
- ✕Confusing initial and final kinetic energies or making sign errors in the change in kinetic energy (ΔK = K_final - K_initial).
- ✕Incorrectly calculating work done, especially for variable forces requiring integration or for forces acting at an angle to displacement.
📝 Practice Questions
See allQ37.A body of mass 100 g is moving in circular path of radius 2 m on vertical plane as shown in figure. The velocity of the body at point A is 10 m/s. The ratio of its kinetic energies at point B and C is : (Take acceleration due to gravity as 10 m/s2 ) (1) 2+√2 (2) 2+√3 3 3 (3) 3+√3 (4) 3−√2 2 2
Q34.A force F = α + βx2 acts on an object in the x -direction. The work done by the force is 5 J when the object is displaced by 1 m . If the constant α = 1 N then β will be (1) 15 N/m2 (2) 12 N/m2 (3) 8 N/m2 (4) 10 N/m2
Q31.A bob of mass m is suspended at a point O by a light string of length l and left to perform vertical motion (circular) as shown in figure. Initially, by applying horizontal velocity v0 at the point ' A ', the string becomes slack when, the bob reaches at the point ' D '. The ratio of the kinetic energy of the bob at the points B and C is ______ -. (1) 1 (2) 2 (3) 4 (4) 3
Q36.A ball of mass 100 g is projected with velocity 20 m/s at 60∘ with horizontal. The decrease in kinetic energy of the ball during the motion from point of projection to highest point is (1) 5 J (2) 15 J (3) 20 J (4) zero
Q45.A force F = 2^i + b^j + ^k is applied on a particle and it undergoes a displacement ^i −2^j −^k. What will be the value of b, if work done on the particle is zero. (1) 0 (2) 1 2 (3) 2 (4) 1 3
Q30.A particle of mass ' m ' and charge ' q ' is fastened to one end ' A ' of a massless string having equilibrium length l, whose other end is fixed at point ' O '. The whole system is placed on a frictionless horizontal plane and is initially at rest. If uniform electric field is switched on along the direction as shown in figure, then the speed of the particle when it crosses the x -axis is (1) √qEl2 m (2) √qElm (3) √qEl4 m (4) √2qElm
NCERT Chapters
- Class 11 Physics Ch 6: Work, Energy and Power