In physics, one of the most fundamental ideas needed to understand the universe is that of force. Understanding this concept is the first step to begin understanding the more complex ideas in physics and also getting a handle on its immense power of prediction. In everyday usage the meaning of “force” can range from coercion to applying pressure, but in physics force has developed an exact definition.
The definition of force as it is used today was developed by Issac Newton over four hundred years ago. When he observed the moon orbit the earth, an apple fall to the ground, or horse pull a cart – basically anything that tended to move – he wanted an abstract idea to describe the push or pull that made the object move. He called this abstract idea “force” which implied that the originally motionless object was forced to move, otherwise the object would remained “at rest”. In fact, this idea is expressed in Newton’s first law of motion: An object at rest, and not subject to outside forces, remains at rest. (This also applies to object under constant motion because no force is needed maintain constant motion.)
Newton now had an simple starting idea upon which he could now test and refine. For any idea to be truly accurate, precise, and have predictive power it must eventually be defined mathematically. After exploring the ideas of velocity (for example, how many meters an object cover in one second), mass, and acceleration (how much your velocity changes in a given amount of time), Newton found his famous mathematical relationship: Force equals Mass times Acceleration. He found it from watching his experiments and noticing two things. One, more force was needed to move massive objects to bring them to a particular acceleration than less massive objects. Two, for a given object of constant mass, more force means more acceleration. He could find nothing else but mass and acceleration was needed to determine the amount of force needed to bring that mass to that acceleration. This finding is know as Newton’s second law of motion, which is by far the most influential idea in science. One might go as far to say modern science would be nonexistent without it.
So finally we have our mathematical (hence, precise, accurate, and useful for predictions) definition of force and this concept has survived the test of time. It is important to note that every definition has limited areas to which it applies. Newton’s ideas of force applies to describe objects that are not moving too fast, and of constant finite mass. This is typical of our everyday world and so Newton’s ideas work well within them. When we get to objects traveling near the speed of light however we find that mass rapidly increases with increased speed, and we are forced – pun aside – to develop other ideas. Next time, we will explore how Newton and other scientists used this concept of force to develop various force laws. If you want a hint, consider this: mass according to Newton is defined to be constant, but acceleration is not, and there are a lot of accelerating objects out there. Is there a pattern to how objects move and interact with each other, and cause each other to accelerate in certain perdictable ways? Answer: Yes!