Aristotle believed that the natural tendency of objects is to rest, unless there is some external push on them to keep them moving. e.g. if you give a push to a swing, it continues for some time and gradually slows down, swinging less and less, until eventually coming to a halt. Another example is that of a car, it needs the energy from the fuel to keep it moving on the road. If there is no fuel, the car won’t start. If the fuel exhausts when the car is moving, it will suddenly stop. So it does seem reasonable to infer that all objects are likely to maintain a state of rest, and a force is needed to make them move or to maintain their state of motion.

As much as it appeals to us intuitively, this theory is, in fact flawed. In the example of the car, when the fuel exhausts, we have assumed there is no force left to cause the motion of the car.That is true, but there is still the force of friction supplied by the road, trying to stop the car. But if friction weren’t acting to stop it, no force would be required to maintain the motion of the car, even if all fuel ran out.

Galileo Galilei is generally credited with proposing the law of inertia which states that “a body moving on a level surface will continue in the same direction at a constant speed unless disturbed”.

Newton incorporated the law into the three laws of motion, restating the law of inertia as the first law of motion:

Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.

In other words, if no net force acts on it, the object will continue to move in a straight line (if it were moving already) or it will continue to stand still (if it were standing still). A force is needed to change the velocity of the object. Hence the car in question will continue to move with a constant speed along a straight line if there are no obstacles in the way and no friction.

One can go as far as to say that an object moving in a straight line with uniform velocity and another object standing still, are equivalent. The laws of Physics are valid for both reference frames, one standing still and one moving with uniform velocity. One cannot distinguish between a uniform motion and state of rest, because there is no absolute fixed reference point in space (all stars and planets and galaxies are moving relative to each other). Unless there is acceleration, you can’t tell the difference.

One way to understand this proposition is to do a thought experiment: Imagine one day you wake up in a room with no windows and a locked door that looks like inside of a spaceship. There is a glass of water on a table that is lying still. Since the water is still, you think the spaceship may be stationary, may be parked on the surface of a planet. But you can’t look outside, and there are no jerks. Since you don’t “feel” any movement, can you say for sure that it is actually not moving at all? Or maybe, it is moving in space with a constant velocity? How can you be sure?

This is a fundamental problem which has puzzled scientists for over a thousand years. Maxwell’s equations showed that the speed of light is constant. If Newtonian mechanics is correct, we should be able to measure (i.e. to tell) the absolute velocity of an object using optical(and electromagnetic) experiments, even without a reference point. However, the famous Michelson-Morley experiment (1887) failed to measure the absolute velocity of the Earth. This was a major mystery until Einstein showed that the Newtonian equations of motion need to be modified, to take relativistic effects into account. The application of Special theory of Relativity proved that the absolute velocity cannot be defined. We cannot tell, in principle as well as experimentally, whether something is moving at a constant velocity or staying still without a reference (i.e. without looking outside so as to speak).

Law of Inertia: First law of motion

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