Energy, Work & Power — Physics Flashcards

What is energy?

Energy is the capacity to do work or cause change. It comes in many forms (kinetic, potential, thermal, chemical, and more) and is measured in joules (J). Example: the food you eat stores chemical energy that your body converts into the motion energy you use to run.

What is kinetic energy? (and its formula)

Kinetic energy is the energy an object has because of its motion: KE = ½mv², where m is mass in kilograms and v is speed in m/s. It grows with the square of speed, so doubling the speed quadruples the kinetic energy. Example: a 2 kg ball moving at 3 m/s has KE = ½ × 2 × 3² = 9 J.

What is gravitational potential energy? (and its formula)

Gravitational potential energy is the stored energy an object has because of its height in a gravitational field: PE = mgh, where m is mass (kg), g ≈ 9.8 m/s², and h is height (m). Example: a 2 kg book on a shelf 1.5 m high has PE = 2 × 9.8 × 1.5 = 29.4 J, ready to be released if it falls.

What is the law of conservation of energy?

Energy cannot be created or destroyed, only transformed from one form to another; the total energy of an isolated system stays constant. Example: a falling ball converts gravitational potential energy into kinetic energy as it drops, but the total energy stays the same (ignoring air resistance).

What is work? (and its formula)

Work is done when a force moves an object through a distance in the direction of the force: W = F × d (force in newtons, distance in meters). Work transfers energy and is measured in joules (J). Example: pushing a box with 10 N of force for 3 m does W = 10 × 3 = 30 J of work. If nothing moves, no work is done.

What is the joule (J)?

The joule is the SI unit of work and energy. One joule is the work done when a force of 1 newton moves an object 1 meter: 1 J = 1 N·m = 1 kg·m²/s². Example: lifting a small 100 g apple about 1 m up takes roughly 1 J of work.

What is power? (and its formula)

Power is the rate at which work is done or energy is transferred: P = W / t, where W is work in joules and t is time in seconds. It's measured in watts (W). Example: doing 100 J of work in 5 s gives a power of P = 100/5 = 20 W. Doing the same work faster means more power.

What is the watt (W)?

The watt is the SI unit of power. One watt equals one joule of energy transferred per second: 1 W = 1 J/s. Example: a 60 W light bulb converts 60 joules of electrical energy into light and heat every second.

What is mechanical energy?

Mechanical energy is the sum of an object's kinetic energy and potential energy: ME = KE + PE. It's the total energy associated with an object's motion and position. Example: a swinging pendulum continuously trades potential energy (at the top) for kinetic energy (at the bottom), while their sum, the mechanical energy, stays roughly constant.

What is thermal (heat) energy?

Thermal energy is the total energy of the random motion of the particles that make up a substance; it flows from hotter objects to colder ones as heat. The faster the particles move, the higher the temperature. Example: rubbing your hands together converts motion energy into thermal energy, warming your palms.

What is chemical energy?

Chemical energy is energy stored in the bonds between atoms and molecules, released or absorbed during chemical reactions. Example: burning wood or digesting food breaks and reforms chemical bonds, releasing stored chemical energy as heat, light, or motion.

What is energy transformation? Give an example.

Energy transformation is the process of energy changing from one form to another while the total amount is conserved. Example: in a flashlight, chemical energy in the battery becomes electrical energy, which the bulb turns into light energy and some thermal energy. In a hydroelectric dam, gravitational potential energy of water becomes kinetic energy and then electrical energy.

What is efficiency? (and its formula)

Efficiency measures how much of the input energy is converted into useful output energy, usually as a percentage: efficiency = (useful energy output / total energy input) × 100%. No real machine is 100% efficient because some energy is always lost, often as heat. Example: a motor given 100 J that delivers 80 J of useful work is 80% efficient, with 20 J lost to heat and friction.

What is the difference between work and power?

Work is the total energy transferred when a force moves something (W = F × d, in joules), while power is how quickly that work is done (P = W / t, in watts). Work tells you how much; power tells you how fast. Example: two people who each lift the same box up the same stairs do equal work, but the one who runs up does it with more power because they finish in less time.