Chapter 4: Work, Energy and Power

The Principle

The Work-Energy Theorem states that the net work done on an object equals its change in kinetic energy.

W_net = ΔKE = KE_f − KE_i

Breakdown of the Diagram

• Initial State: The block starts with velocity v_i, giving it Initial Kinetic Energy (KE_i = ½mv²). This is shown by the full energy bar on the left.
• The Work Done: As the block slides, Friction (f_k) acts opposite to the motion. It does negative work (W = −f_k · d), effectively draining energy from the system.
• Final State: When the block stops, its velocity is zero, meaning KE_f = 0.

Conclusion

The energy didn't disappear. The graphic shows that the Kinetic Energy lost is exactly equal to the Work done by friction (converted into heat and sound).