Changes in the kinetic store as a measure of motion
Physics Narrative for 14-16
Changing velocity: changing energy in the kinetic store
So, momentum looks to be a useful measure of motion. What then of the energy associated with the motion of an object – and by this, of course, we mean the energy in the kinetic store? There is certainly a greater quantity of motion if an object is shown to have more energy in its kinetic store.
Again we need to take care in choosing our point of view. If you are moving alongside an object then you will record its velocity as zero. Act on the object – by exerting a force on it – and you will change its motion. From your point of view its velocity will have changed, so you will also measure a change in the energy in the kinetic store from the first situation (first snapshot) to the second situation (second snapshot). This change will depend on the force you exert, and on the distance from which you exert it (see more in the SPT: Forces topic and the SPT: Electricity and energy topic). Both the mass and the change in velocity will affect how much energy is shifted to or from the kinetic store by the action of your force. In many cases you'll not be providing the force but, given that physical substitution, you can see this in action in many everyday situations:
- A car pulls away from the traffic lights.
- A fast commuter train pulls away from your slower moving local service.
- A fisherman's catapult propels a bait ball into the river.
Of course, you can also run this kind of thought experiment backwards. Notice something moving relative to your point of view (first snapshot) and later notice that it has stopped (second snapshot). From this you can record a reduction in the energy in the kinetic store. Such a thing happens in many everyday situations:
- A cyclist coming to a halt.
- A cricketer catching a ball.
- A safety helmet bringing the wearer's head safely to rest.
In all of these cases it is pretty clear that there is a change in the motion of the object, and that this change is reflected in the energy added to or removed from the kinetic store associated with the object as motion is changed. As both the mass and the velocity (remember, this requires you to select a particular point of view) are essentially involved in the calculation of changes in energy, and both seem intuitively to be essential in measuring the quantity of motion associated with the object, the energy in the kinetic store is a second good candidate as a measure of the motion of an object.