When one considers the variety of motions in the world one might wonder: what causes such variety? Why do some bodies remain unchanging in motion, altering neither speed nor direction, while others undergo changes now and again?
The box can be moved by giving it a push, and the cart by pulling it. We affect the motions of objects by imposing direct or indirect pushes and pull. Pushes and pulls are what we call forces.
Except for our own muscular experience of exertion, forces are invisible. Whenever we observe acceleration (change in the state of motion), we say that some force has come into play. Thus forces are hypothetical entities subsisting in the roots of perceived reality, in terms of which can describe and explain non-uniform motion.
What this means is that every time we observe non-uniform motion we say (in the world view of physics) that some force has come into play. Force, in other words, is the cause of acceleration, NOT of motion itself. It is, however, important to realize that if there is uniform motion it does not mean that there is no force at all. You may push a box in one direction and a friend may push it in the opposite direction, keeping the box stationary.
In other words, forces can balance out and cancel each other’s effect. When two forces come into play, as in a tug of war, the stronger overcomes the effect of the weaker. What we call strength and weakness are in effect relative measures of forces.
The stronger a force is, the greater will be the acceleration it causes on a given body. Give a greater push to the hockey puck, and the faster it takes off. On the other hand, the larger the body on which the force is applied, the smaller the acceleration it suffers: the toy cart is pulled more easily than the truck by the same exertion. These two facts are formulated as a simple formula: F = ma.
This is one of the most sweeping formulas in all of physics. It couches every instance of motion, actual and conceivable, that occurs in the universe. That all the wondrous variety can be encapsulated in such beautiful brevity is a marvel in itself. With the aid of this simple-looking mantra of modern science, we have been able to explain and understand much of the complexity in the world.
When you push an object, the push is the force; you are the pusher or source of the force; and the object is the pushed or target. The child pulls the string: the pulling is the force, the child is the source, and the string is the target. In every instance we can distinguish between these three entities: force, its source, and its target. Observations reveal a most interesting symmetry in the roots of perceived reality: whenever a force comes into play, another force of equal magnitude appears also for which source and target are precisely the reverse of the first force.
As a Shakespearean character said: I should kick, being kicked. The effects of celestial bodies on things terrestrial had been suspected since ancient times. Effects there sure are, but not of the kind imagined by planetary fortune-tellers. Rather, they are related to the cosmic force we call it gravitation: Every mass in the universe attracts every other mass with a force whose strength is determined by the masses of the bodies and the distance between them. In one grand sweep this law accounts for planetary motion, extends to the moon and to other satellites, as also to distant stars and planetary systems. Newton, the proponent of the law, believed that chemical combinations are also due to gravitational attractions between the atoms of substances, because he used to term to signify any force between bodies. After all, this was the only kind of universal force then known. As astronomical observations extended the role of gravitation became clear in the harmony of the universe. In the words of Samuel Rogers,
“That very law which moulds a tear / And bids it trickle from its source, / That law preserves the earth a sphere, / And guides the planets in their course.”
Try moving something without touching it directly or indirectly, and you will fail. Yet, this is happening all the time around us, and we are not surprised. When the vase falls from the table, there is no contact between earth and vase. We never wonder about how the ground pulls the falling body. We speak of the sun whirling the planets as if it had ropes tied to them. This is no mean feat of a puppeteer that the Sun accomplishes: pulling bodies millions of miles across space. If we stop to think, there seems to be something spooky about this gravitation bit. So it is with the other fundamental forces that make the world tick: whether weak or strong to electromagnetic, all invisible except in their effects. We are, indeed, forced (pun intended) to live in a world where things happen.