We cannot see energy and so it should not be surprising that the history of science has been one of observing and measuring the things we do see. We recognise matter in all its forms, forces and motions and explain their behaviours using laws and mathematics. In my first three articles I have tried to persuade you that everything in this universe of ours is either space or energy. If that is so then matter, forces and motions are all energy experiences. As far as we know energy is conserved so that the total energy of the universe remains constant.
I think my first learning about energy was at school about 65 years ago when I learnt of potential and kinetic energy and how they could be equated. It seemed the energy of an object was either related to its height above the ground, and called potential energy, or related to its speed of motion and called kinetic energy. My calculations of equating the two were good but my understanding poor.
When helping my granddaughter prepare for GCSE I found her course book saying “anything moving has energy in its kinetic energy store” and describing a gravitational potential energy store as “anything that has mass and is in a gravitational field”. I then found a Faraday Lecturer saying much the same. But do we really think structures have more energy when in motion and potential energy when at height?
Does it make sense that a book on the top shelf (or level with the top shelf) has more energy than the same book on a bottom shelf? When we are running do we really believe our body cells and their particles have more energy than when we are stood still? Potential energy is simply a calculation term. Kinetic energy is the interaction energy needed to bring about a change in the relative speed of a particle structure. It is not an energy transfer. We are wrong when we refer to either as energy stores. There is no motion energy store.
At school we learn that kinetic energy is 1/2 mv2 . On the right I show how that is derived and you will see it is about a steady acceleration of an object of mass m from rest up to a speed of v. The maths tells us kinetic energy is about acceleration or deceleration of an object. We are wrong to think of an object at steady speed as having kinetic energy. Kinetic energy is the interactive energy required to get an object up to speed or to bring a speeding object to rest. Kinetic energy does not add to a structure’s energy.
An object leaving the earth at speed has had an energy exchange with the earth or some earthly object. The particles of the object will have been pushed slightly closer together by the thrust; they will be exchanging slightly higher energies and giving them up as heat. The particles feel earth energy emissions and desire them. That desire is what acts to decelerate the object. The height an object reaches is related to the energy that delivered the motion and the strength of particle desire for energy (gravity). It is why we are able to equate potential and kinetic energies.
Our car’s engine provides the energy to accelerate our car and maintain its speed. It’s brakes provide the energy to slow from that speed. If we hit a tree at 5 miles per hour the interacting energy that brings us to a halt in the earth environment is related to 5 squared = 25. If our 5 m.p.h. car collides head on with a 95 m.p.h. train the interacting energy that brings us to a halt in the train environment is related to 100 squared = 10,000. That is some 400 times the energy that brought us to a halt in the land environment. Our car’s motion does not suddenly have 4,00 times as much energy.
Science seems so obsessed with the maths of equating potential energy with kinetic energy that it tells school children speed of motion is an energy store. Some of those children become the “experts” that repeat such things.
Many are fascinated by Newton’s cradle and so I will try to explain why it does what it does. It comprises a number of balls on wires in close contact. Most will know that if we draw back one ball and release it the ball at the other end of the cradle will respond with a near identical motion but in the opposite direction. If we raise and let two or three balls go the response involves two or three balls.
Lift and release ball 1 and the desire of its particles for earth energies come into play as restrained byenergy exchanges in its support wire cause it to approach ball 2 particles at speed.
At near contact the photon emissions of balls 1 and 2 start to conflict and their surface particles start to recoil in response. At light speed particles, throughout ball 1 and because they are in close contact throughout balls 2 to 5, start to reluctantly recoil and seriously add to the photon conflict.
In a collision kinetic energy interactions decelerate and then accelerate the colliding mass energies. The photon force interactions between the colliding masses act equally and oppositely on both objects during the common time of their interactions. When that time is over and the interactive forces are diminished each object will have a velocity different to that pre-collision. We say that momentum (mass x velocity) is conserved, and indeed in our cradle the momentum of ball 5 after collision was the same as that of ball 1 pre-collision. The above diagram tries to explain why that is so.
Ball 5 alone moves in a single ball release because it is the most efficient way to respond to the energy input. For the same reason two or three balls move when two or three balls are released. If balls 2,3,4 and 5 had been glued together, all of those balls would have moved away from the collision at a quarter of the speed of ball 1.
Our cradle does not transfer energy from particle to particle as perfectly as we describe. It is not what we call a perfectly elastic collision because some energy is expended pushing particles closer together as for a spring. Those energies act to increase mass energy and so do not diminish momentum. No collision is perfectly elastic. Inelastic collisions, like car crashes, are where substantial energy is used in changing particle structures and where mass energy is temporarily increased.
My hope is you now see and agree that kinetic energy is that which changes motion and is not the energy of motion.