  |
next planned revision: 2005-06-01 |
| Home Alphabetical home |
Physics -- Units and Concepts: Force |
This is a very familiar concept. When you pull open a door or push a supermarket cart you exert force. Your body's weight exerts a downward force on the seat of your chair while you are sitting in front of the screen reading this. That downward force is opposed by an equal upward force from the chair's seat, so not much happens. Forces cause things to move if they are not opposed. In the absence of friction, a force will cause an object to accelerate. You can best experiment with that by pushing floating objects in a swimming pool. Give them a kick and observe the speed with which they move when the push is over. There is little friction, but the acceleration depends on the mass of the object you push: an air mattres by itself is easier to accelerate than one with a person lying on it.
The unit of force is that force which causes 1 kg of mass to accelerate at 1 m/s2 (in the complete absence of friction). It is written as 1 kg.m/s2 but that is too difficult to pronounce. In honour of Sir Isaac Newton who first described these concepts with mathematical rigour, the unit of force is called a Newton and written N.
Everything is attracted by the gravitational field of the Earth. That gravitational force is proportional to the mass of the object, so the acceleration of a falling object is always the same, as Galileo proved by dropping things from the leaning tower of Pisa. The gravitational force is slightly dependent on where on Earth you are, but on average it is 9.8 m/s2 which means that if you jump off a building, you will be falling at 9.8 m/s after just the first second (over 35km/h!). If the Earth's gravity force accelerates 1kg at 9.8 m/s2 then the force acting on that kg of mass must be 9.8 N. The weight of 1 kg is 9.8 N. 1 N is also the force the Earth exerts on 1 kg/9.8=102 g of mass.
In the construction of buildings and bridges we must use materials that can cope with the forces of objects placed in them or rolling over them. Those forces are mainly weight, i.e. forces coming from gravity. For that reason, it is customary to give a second meaning to the word "kilogram", namely the force exerted as weight by one kilogram of mass. One kilogram-force is 9.8 Newton, 9.8 N. A kilogram-force is not a metric unit, but again a hybrid one, purely for convenience. It is written kgp to avoid confusion with kg as mass. The "p" is from "poids" the French word for weight.
Confused by the notion of a kilogram as mass but not as weight? Go to the Moon: the force of gravity there is roughly a sixth of the strength on the Earth because the Moon is much smaller. Take a hammer with you. This hammer will weigh a sixth of what it weighs on Earth, but it will be just as effective because its mass has not changed and it's the mass that is important. It will also be just as difficult to push someone on an air mattres in a pool on the Moon, though it will be much easier to lift her out of the water.