Where the Energy Goes When a Ball Falls to the Ground
Learn when work is done and why it is measured in joules, name the forms of energy with examples, follow the conversion of potential energy into kinetic energy, and trace energy changes in everyday devices.
Where does the energy go when a ball falls to the ground?
It changes form rather than disappearing. At the top the ball holds potential energy because of its height; as it falls that becomes kinetic energy of motion; on striking the ground it turns into sound and a little heat.
Following energy through changes like these is what the chapter is about. This page covers everything in the ICSE Class 7 Physics chapter on energy: work and its unit, the forms of energy, kinetic and potential energy, and the conversions inside everyday devices.
Following energy through changes like these is what the chapter is about. This page covers everything in the ICSE Class 7 Physics chapter on energy: work and its unit, the forms of energy, kinetic and potential energy, and the conversions inside everyday devices.
Formula
When is work done, and why is it measured in joules?
Work is done when a force moves a body through a distance in the direction of the force:
Energy is the capacity to do work, and both are measured in the joule (J). One joule is the work done when a force of one newton moves a body through one metre.
Worked example. A boy pushes a box with a force of through :
Lifting a school bag of weight onto a shelf high does
of work against gravity.
The everyday meaning of "work" is the trap here. Standing still holding a heavy bag feels exhausting, but in physics no work is done, because the bag is not displaced. Pushing hard against a wall that will not move is the same case: force but no displacement, so .
That also explains the phrasing in the direction of the force. Carrying a suitcase horizontally does no work against its weight, since the upward force and the forward motion are at right angles.
Energy is the capacity to do work, and both are measured in the joule (J). One joule is the work done when a force of one newton moves a body through one metre.
Worked example. A boy pushes a box with a force of through :
Lifting a school bag of weight onto a shelf high does
of work against gravity.
The everyday meaning of "work" is the trap here. Standing still holding a heavy bag feels exhausting, but in physics no work is done, because the bag is not displaced. Pushing hard against a wall that will not move is the same case: force but no displacement, so .
That also explains the phrasing in the direction of the force. Carrying a suitcase horizontally does no work against its weight, since the upward force and the forward motion are at right angles.
What are the different forms of energy?
Energy appears in several forms, and each has a familiar example.
- Mechanical — energy of position or motion, held by a stretched rubber band or a moving cricket ball. It divides into potential and kinetic energy.
- Heat (thermal) — energy that flows because of a temperature difference, as from burning coal in a stove.
- Light — energy that travels from a lamp or the Sun and lets us see.
- Sound — energy carried by vibrations, from a drum or a temple bell.
- Chemical — energy stored in the bonds of substances, in food, fuels and a dry cell.
- Electrical — energy carried by moving charges in a circuit, supplied to a home from the mains.
- Magnetic — energy stored in a magnet's field, used in an electric bell.
- Nuclear — energy locked in the nucleus of an atom, released in a nuclear reactor and in the Sun.
The Sun is the origin of most of them on Earth. Its light drives photosynthesis, which stores chemical energy in plants, which becomes the chemical energy in our food and in coal.
These forms are not separate substances. They are different accounts of the same quantity, which is exactly why one can be converted into another.
- Mechanical — energy of position or motion, held by a stretched rubber band or a moving cricket ball. It divides into potential and kinetic energy.
- Heat (thermal) — energy that flows because of a temperature difference, as from burning coal in a stove.
- Light — energy that travels from a lamp or the Sun and lets us see.
- Sound — energy carried by vibrations, from a drum or a temple bell.
- Chemical — energy stored in the bonds of substances, in food, fuels and a dry cell.
- Electrical — energy carried by moving charges in a circuit, supplied to a home from the mains.
- Magnetic — energy stored in a magnet's field, used in an electric bell.
- Nuclear — energy locked in the nucleus of an atom, released in a nuclear reactor and in the Sun.
The Sun is the origin of most of them on Earth. Its light drives photosynthesis, which stores chemical energy in plants, which becomes the chemical energy in our food and in coal.
These forms are not separate substances. They are different accounts of the same quantity, which is exactly why one can be converted into another.
How does potential energy turn into kinetic energy?
Potential energy is the energy a body has because of its position or condition; kinetic energy is the energy it has because of its motion.
A stone held on a rooftop has potential energy from its height. A stretched bowstring has potential energy from its condition. A flowing river and a moving bus have kinetic energy.
For a body of mass at height , the potential energy is , where is about . So a stone held above the ground has
As it falls, the height drops and the speed rises, so potential energy steadily becomes kinetic energy. At the halfway point, up, the remaining potential energy is
and the other has become kinetic energy. Just before impact the potential energy is nearly zero and almost the whole is kinetic.
The total stays at throughout, which is the point of the example. A swing shows the same exchange repeating: highest at the ends where it briefly stops, fastest at the bottom where its height is least.
A stone held on a rooftop has potential energy from its height. A stretched bowstring has potential energy from its condition. A flowing river and a moving bus have kinetic energy.
For a body of mass at height , the potential energy is , where is about . So a stone held above the ground has
As it falls, the height drops and the speed rises, so potential energy steadily becomes kinetic energy. At the halfway point, up, the remaining potential energy is
and the other has become kinetic energy. Just before impact the potential energy is nearly zero and almost the whole is kinetic.
The total stays at throughout, which is the point of the example. A swing shows the same exchange repeating: highest at the ends where it briefly stops, fastest at the bottom where its height is least.
What energy conversions happen inside everyday devices?
Every device is a converter, and questions ask you to name the input form, the useful output, and any waste.
- Electric bulb — electrical energy into light energy and heat energy.
- Loudspeaker — electrical energy into sound energy.
- Dry cell — chemical energy into electrical energy.
- Photosynthesis — light energy into chemical energy stored in the plant.
- Electric fan — electrical energy into mechanical energy of the blades.
- Microphone — sound energy into electrical energy, the reverse of a loudspeaker.
Some processes need a chain of conversions. In a hydroelectric power station:
potential energy of the stored water, then kinetic energy of the falling water, then mechanical energy of the spinning turbine, then electrical energy from the generator.
The same chain continues in your home, where that electrical energy becomes light in a bulb or mechanical energy in a fan.
The detail to include for full marks is the waste output. A bulb's heat is not a mistake in the list — it is why a bulb becomes hot to touch, and why only part of the electrical energy supplied ends up as light.
- Electric bulb — electrical energy into light energy and heat energy.
- Loudspeaker — electrical energy into sound energy.
- Dry cell — chemical energy into electrical energy.
- Photosynthesis — light energy into chemical energy stored in the plant.
- Electric fan — electrical energy into mechanical energy of the blades.
- Microphone — sound energy into electrical energy, the reverse of a loudspeaker.
Some processes need a chain of conversions. In a hydroelectric power station:
potential energy of the stored water, then kinetic energy of the falling water, then mechanical energy of the spinning turbine, then electrical energy from the generator.
The same chain continues in your home, where that electrical energy becomes light in a bulb or mechanical energy in a fan.
The detail to include for full marks is the waste output. A bulb's heat is not a mistake in the list — it is why a bulb becomes hot to touch, and why only part of the electrical energy supplied ends up as light.
Exam tip
Exam tip: naming every form in a conversion chain
Energy-conversion questions are marked form by form, so leaving one out costs a mark even when the rest is right.
Write the chain as a sequence of named forms with the device at each step, in order. For a hydroelectric station, four forms must appear: potential, kinetic, mechanical, electrical.
Include the unwanted output where it exists. An electric bulb gives light and heat; a working motor gives mechanical energy and heat and sound.
For work numericals, check the displacement is in the direction of the force before multiplying. If the body has not moved, write and say why — that is the complete answer, not an incomplete one.
And keep the units SI. Force in newtons times distance in metres gives joules; using centimetres gives an answer a hundred times too small.
Write the chain as a sequence of named forms with the device at each step, in order. For a hydroelectric station, four forms must appear: potential, kinetic, mechanical, electrical.
Include the unwanted output where it exists. An electric bulb gives light and heat; a working motor gives mechanical energy and heat and sound.
For work numericals, check the displacement is in the direction of the force before multiplying. If the body has not moved, write and say why — that is the complete answer, not an incomplete one.
And keep the units SI. Force in newtons times distance in metres gives joules; using centimetres gives an answer a hundred times too small.
Did you know
Why does a swing move fastest at the bottom of its arc?
Because that is where its height, and so its potential energy, is least — and the missing potential energy has become speed.
At the two ends of the arc the swing stops for an instant. Its energy is then entirely potential. Sweeping down, every centimetre of lost height is paid out as kinetic energy, and the bottom of the arc is where the payout is complete.
The exchange then runs backwards up the other side, which is why a swing left alone keeps almost the same height each time — until air resistance and friction quietly convert the total into heat.
At the two ends of the arc the swing stops for an instant. Its energy is then entirely potential. Sweeping down, every centimetre of lost height is paid out as kinetic energy, and the bottom of the arc is where the payout is complete.
The exchange then runs backwards up the other side, which is why a swing left alone keeps almost the same height each time — until air resistance and friction quietly convert the total into heat.
Key takeaways
Work and energy: quick revision
- Work is done only when a force displaces a body in the direction of the force: , measured in joules.
- Holding a heavy object still does no work, because there is no displacement.
- Energy is the capacity to do work, and its forms are mechanical, heat, light, sound, chemical, electrical, magnetic and nuclear.
- Potential energy comes from position or condition and equals ; kinetic energy comes from motion.
- A falling body converts potential into kinetic energy while the total stays constant — a stone at carries throughout its fall.
- Devices convert forms: a bulb gives light and heat, a cell gives electrical energy from chemical, and a hydroelectric station runs potential to kinetic to mechanical to electrical.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Holding a heavy object still does no work, because there is no displacement.
- Energy is the capacity to do work, and its forms are mechanical, heat, light, sound, chemical, electrical, magnetic and nuclear.
- Potential energy comes from position or condition and equals ; kinetic energy comes from motion.
- A falling body converts potential into kinetic energy while the total stays constant — a stone at carries throughout its fall.
- Devices convert forms: a bulb gives light and heat, a cell gives electrical energy from chemical, and a hydroelectric station runs potential to kinetic to mechanical to electrical.
You will remember all of this far better after answering five questions on it than after reading it twice.