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Forces
We all have a fairly deep intuitive understanding of what forces are and what effect they have on objects. We are constantly using our muscles to exert forces: we pull up on a coffee cup to get it to our mouths, we push against a car stuck in a ditch in order to get it moving and we exert a force to stop a basketball as we catch it. We know that sometimes, the force has the desired effect but sometimes it is inadequate (such as when the car is stuck too deep in mud or the basketball is thrown at us too fast). So a force is something whose purpose is to cause a change in motion. It can cause an object to increase its velocity, or to decrease its velocity depending on the circumstances. In fact, if the velocity is initially zero (eg car) or the force is in the same direction of the velocity, then the velocity will increase, whereas if the force and velocity are in opposite directions, then the velocity will decrease. Forces, like velocities, therefore have both a magnitude and a direction. The magnitude of the force (how hard we push) has something to do with how effective the force is in causing the desired motion, whereas the direction of the force will determine the direction of the resulting change in motion (if any).
Although many things can cause forces (our muscles, car engines, springs, the gravitational pull of the Earth), in Nature there are only four fundamentally distinct types of forces. In fact in our daily lives we only directly experience two of these forces: electromagnetic forces and gravitational forces. Electromagnetic forces we usually experience as two very different phenomena. Electric, or electrostatic forces, cause the sometimes painful sparks that we feel when touching a metal object after walking across a wool carpet in bare feet. Magnetic forces cause the magnet in a compass to point towards the North Pole. It was Maxwell who first showed that electrostatic and magnetic forces are two different manifestations of one and the same phenomenon, so they are now often referred to by their common name: electromagnetic forces. Electromagnetic forces hold the electrons in orbit around the nuclei in atoms, and, as the name suggests, they also provide the basis for many of the modern ``electronic'' gadgets (radios, TVs, microwaves, computers) that we currently take for granted. Our most direct experience with electromagnetic forces comes from the fact that they are responsible for holding together the molecules that make up solid objects. Thus electromagnetic forces are also the source of the ``repulsive'' contact forces we feel when we touch or hit solid objects. Electromagnetic forces keep you from falling through the chair that you are sitting on while you read this.
We all have a fairly deep intuitive understanding of what forces are and what effect they have on objects. We are constantly using our muscles to exert forces: we pull up on a coffee cup to get it to our mouths, we push against a car stuck in a ditch in order to get it moving and we exert a force to stop a basketball as we catch it. We know that sometimes, the force has the desired effect but sometimes it is inadequate (such as when the car is stuck too deep in mud or the basketball is thrown at us too fast). So a force is something whose purpose is to cause a change in motion. It can cause an object to increase its velocity, or to decrease its velocity depending on the circumstances. In fact, if the velocity is initially zero (eg car) or the force is in the same direction of the velocity, then the velocity will increase, whereas if the force and velocity are in opposite directions, then the velocity will decrease. Forces, like velocities, therefore have both a magnitude and a direction. The magnitude of the force (how hard we push) has something to do with how effective the force is in causing the desired motion, whereas the direction of the force will determine the direction of the resulting change in motion (if any).
Although many things can cause forces (our muscles, car engines, springs, the gravitational pull of the Earth), in Nature there are only four fundamentally distinct types of forces. In fact in our daily lives we only directly experience two of these forces: electromagnetic forces and gravitational forces. Electromagnetic forces we usually experience as two very different phenomena. Electric, or electrostatic forces, cause the sometimes painful sparks that we feel when touching a metal object after walking across a wool carpet in bare feet. Magnetic forces cause the magnet in a compass to point towards the North Pole. It was Maxwell who first showed that electrostatic and magnetic forces are two different manifestations of one and the same phenomenon, so they are now often referred to by their common name: electromagnetic forces. Electromagnetic forces hold the electrons in orbit around the nuclei in atoms, and, as the name suggests, they also provide the basis for many of the modern ``electronic'' gadgets (radios, TVs, microwaves, computers) that we currently take for granted. Our most direct experience with electromagnetic forces comes from the fact that they are responsible for holding together the molecules that make up solid objects. Thus electromagnetic forces are also the source of the ``repulsive'' contact forces we feel when we touch or hit solid objects. Electromagnetic forces keep you from falling through the chair that you are sitting on while you read this.