Study guides › C/P › Physics
Work, Energy & Power
Energy bookkeeping solves problems that forces make messy. You will compute work with the dot product, apply the work-energy theorem, use conservation of energy with and without friction, handle springs, and read energy graphs.
Study this chapter in PrepMed (free)
What this chapter covers
- Work & the Dot Product. Work is force acting through a displacement, and only the part of the force along the motion counts.
- Work-Energy Theorem, Kinetic & Potential Energy. The net work on an object equals its change in kinetic energy, which skips the acceleration step entirely.
- Conservation of Energy. Mechanical energy is conserved only when conservative forces act; friction turns it into thermal energy but total energy always survives.
- Springs & Hooke's Law. A spring stores energy as ½kx², so doubling the stretch quadruples the stored energy.
- Power & Reading Energy Graphs. Power is the rate of energy transfer, and force and potential-energy graphs turn energy questions into geometry.
Key points to know
- W = Fd cosθ; work is a scalar in joules
- Force perpendicular to motion does zero work
- KE = ½mv²: doubling v quadruples KE
- W_net = ΔKE counts the work of every force, including friction and gravity
- Conservative forces only: KE + U is constant
- Friction converts mechanical energy to thermal energy; total energy is always conserved
- Hooke's law: F = −kx, with k in N/m
- U = ½kx² grows with the square of the displacement
Try a practice question
A waiter carries a tray horizontally at constant velocity across a room. What work does his upward force on the tray do?
- Positive, because he supports its weight
- Zero, because the force is perpendicular to the displacement
- Negative, because gravity opposes his force
- It depends on his walking speed
Show the answer
B. The upward force and the horizontal displacement are at 90°, and cos 90° = 0. Supporting a weight takes effort but transfers no energy without motion along the force.
Common questions
What does this chapter cover?
Work, Energy & Power covers Work & the Dot Product, Work-Energy Theorem, Kinetic & Potential Energy, Conservation of Energy, Springs & Hooke's Law, Power & Reading Energy Graphs.
What should I know first?
W = Fd cosθ; work is a scalar in joules Force perpendicular to motion does zero work KE = ½mv²: doubling v quadruples KE
24 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.