Work, Energy & Power
Section: Motion, Forces & Energy | Syllabus: Cambridge AS Level Physics 9702
Energy Stores & Transfers Energy is a fundamental concept in physics that describes the capacity to do work. Energy The ability to do work. It cannot be created or destroyed, only transferred from one store to another.
Energy Stores Kinetic: In moving objects Gravitational potential: Due to height Elastic potential: In stretched/squashed objects Thermal: In hot objects Chemical: In fuels, food, batteries Nuclear: In atomic nuclei Energy transfers occur through forces, heating, electrical currents, or radiation.
Key Example Be able to describe the energy store changes in a process, e.g.: Ball falling → gravitational potential → kinetic energy. Energy Transfers Example Flow diagram showing a ball falling: Gravitational Potential Energy (GPE) at the top converts to Kinetic Energy (KE) as it falls.
At ground level: maximum KE, zero GPE. Some energy dissipated as heat and sound on impact. Total energy remains constant. Kinetic Energy Kinetic energy is the energy possessed by moving objects. Kinetic Energy The energy a moving object has because of its motion.
KE = 1/2mv^2 Where: \(KE\) = kinetic energy (J), \(m\) = mass (kg), \(v\) = velocity (m/s) Memory Aid "Half m v squared keeps motion prepared!" → KE = ½mv² Kinetic Energy vs Speed Graph X-axis: Speed (m/s), Y-axis: Kinetic Energy (J).
A curved line (parabola) through the origin, rising steeply. Because KE = ½mv², doubling speed quadruples KE. The curve shows KE increases rapidly at higher speeds. Gravitational Potential Energy Gravitational potential energy depends on an object's position in a gravitational field.
Gravitational Potential Energy The energy stored in an object due to its height above the ground. GPE = mgh Where: \(GPE\) = gravitational potential energy (J), \(m\) = mass (kg), \(g 10\) m/s², \(h\) = height (m) Common Error Always measure height vertically from the reference level, not along the slope.
Energy Changes in a Falling Object A vertical diagram showing an object at three heights. Top: Maximum GPE, zero KE. Middle: Some GPE converted to KE. Bottom: Zero GPE, maximum KE. Total mechanical energy remains constant (GPE + KE = constant) if air resistance is ignored.
Conservation of Energy Energy transformations follow a fundamental law of physics. Law of Conservation of Energy Energy cannot be created or destroyed, only changed from one form to another. Total energy input = total energy output (though some may be wasted as heat or sound).
Example: Energy Transformation in a Pendulum A pendulum converts GPE ↔ KE repeatedly as it swings, with some energy lost as heat and sound due to friction. Work Done Work describes the energy transfer when forces cause movement.
Work Done The energy transferred when a force moves an object through a distance. W = Fd Where: \(W\) = work done (J), \(F\) = force (N), \(d\) = distance in direction of force (m) Key Point If the object doesn't move, no work is done even if a force acts.
Work Done Diagram A box being pushed horizontally across a surface. Force arrow (F) pointing in direction of motion. Distance (d) marked along the path. Work Done = Force × Distance in direction of force.
If F = 50N and d = 2m, then W = 100J. Power Power measures how quickly energy is transferred or work is done. Power The rate of doing work or transferring energy. P = W/t or P = E/t Where: \(P\) = power (W), \(W/E\) = work done or energy (J), \(t\) = time (s) Quick Trick 1 watt = 1 joule per second.
Efficiency Efficiency quantifies how effectively energy is converted from one form to another. Efficiency A measure of how much of the input energy is usefully transferred. Efficiency = Useful OutputTotal Input × 100\% Can use energy or power in the formula Important Efficiency is always less than 100% due to wasted energy (usually heat or sound).
Energy Flow Diagram (Sankey Diagram) Input arrow (100J electrical) entering a light bulb. Two output arrows: thin arrow for useful light energy (20J), thicker arrow for wasted heat energy (80J). Width of arrows proportional to energy.
Efficiency = 20/100 × 100% = 20%. Exam Tip In Sankey diagrams, the thicker the arrow, the more energy it represents.
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