Energy stores: everyday examples and changes

Principles of energy • Energy stores and changes

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What happens to energy when an object is thrown upward?

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Kinetic energy decreases and transfers into gravitational potential energy as height increases; the process reverses during descent .

Key concepts

What you'll likely be quizzed about

Object projected upwards - kinetic to gravitational potential

An object thrown upward has an initial kinetic energy store determined by its mass and speed. Gravity acts opposite to the motion and does negative work on the object, so kinetic energy decreases as height increases. Energy transfers from the object’s kinetic energy store into its gravitational potential energy store as height increases, and the energy transfer reverses during the fall. This relationship follows conservation principles when resistive forces are negligible . Limiting factors: air resistance and energy transferred to the air reduce mechanical energy conservation; when non-negligible, some kinetic energy converts into thermal energy in the air and the object.

Moving object hits an obstacle - kinetic to thermal, sound and elastic stores

A moving object colliding with an obstacle transfers energy from its kinetic energy store into other stores nearly instantaneously. Deformation of the object and obstacle stores energy elastically while internal friction and molecular vibrations convert part of the kinetic energy into internal (thermal) energy. Sound waves carry away some energy as acoustic transfer. The exact distribution depends on elasticity, mass, speed and material properties; a perfectly elastic collision returns most energy to kinetic stores, while an inelastic collision converts more into thermal and permanent deformation . Limiting factors: material stiffness, contact time and surface area alter how much energy becomes elastic potential versus internal energy and sound.

Object accelerated by a constant force - work increases kinetic store

A constant force acting over a displacement does work on an object equal to force multiplied by displacement. That work increases the kinetic energy store of the object when the force has a component along the motion. The increase in kinetic energy equals the net work done on the object, following the work–energy principle. The relationship links Newton’s laws and energy calculations: net force causes acceleration and the resulting change in speed changes the kinetic energy store . Limiting factors: opposing forces such as friction or air resistance reduce net work available to increase kinetic energy; if constant force does positive work while resistive forces do negative work, the net change equals the algebraic sum of these works.

Vehicle slowing down - kinetic to internal (thermal) via braking

When a vehicle slows, the kinetic energy store decreases. Braking systems apply forces that do negative work on the vehicle; that work transfers kinetic energy into the internal energy stores of the brakes and tyres, raising their temperature. Some energy also dissipates into the surroundings as heat and sound. Braking converts useful kinetic energy into less useful thermal energy, which is often described as energy dissipation. Calculations of braking distance and energy changes use work done by braking force and the initial kinetic energy of the vehicle . Limiting factors: brake efficiency, tyre–road friction and aerodynamic drag affect how rapidly kinetic energy converts to internal energy and how much is lost as heat or sound.

Water boiled in an electric kettle - electrical work to internal and latent stores

An electric kettle supplies electrical energy to a heating element; the element does work on water molecules so thermal (internal) energy of the water increases. Temperature rises according to the water’s specific heat capacity until the boiling point. At boiling point further energy input does not increase temperature but supplies the latent heat of vaporisation, increasing the internal energy of water as it becomes steam. The total energy input equals the increase in thermal/internal energy plus any energy lost to the kettle body and surroundings . Limiting factors: kettle power rating, heat losses to the surroundings and heating element efficiency determine how much electrical energy appears as increased internal energy of the water; boiling requires specific latent heat per kilogram.

Key notes

Important points to keep in mind

Energy stores: kinetic, gravitational potential, elastic potential, internal (thermal), chemical, magnetic and others.

Work by a force transfers energy between stores; W = F · d for constant force in the direction of motion .

Thrown object: kinetic → gravitational potential; air resistance causes additional thermal losses .

Collision: kinetic → elastic potential (temporary) or internal/thermal and sound (permanent losses) .

Acceleration by constant force increases kinetic energy equal to the net work done on the object.

Braking converts kinetic energy into internal energy of brakes and tyres and dissipated heat; braking distance links kinetic energy and work by braking force .

Heating water raises internal energy; during boiling added energy becomes latent heat and temperature remains constant until phase change completes .

Specific heat capacity and latent heat quantify energy needed for temperature change and phase change respectively .