Monday, 19 January 2015

Newton's Laws of Motion


First law: the principle of inertia:

The first law of Newton states that the principle of inertia that an object retains its speed and direction of motion unless a force is applied to it. This principle may seem surprising: it is generally assumed that if no there is no force on a object then it must be at rest, that is to say still. The principle of inertia states that this is not the case!

It is difficult to verify this principle on Earth because the objects are usually subjected to forces. Slippery ice skater approaches this situation. If a bicycle is stopped after a few tens of meters when not pedal, it is because tire friction forces on the road and the bike with air. In space, there is no air is no friction. If an astronaut drops a object it away in a straight line at constant speed, never stopping, according to the principle of inertia, unless attracted by gravity or meet an obstacle.

Second Law: Fundamental principle of dynamics:

The second law has different names, but most often it is called the fundamental principle of dynamics. It's a tie between vectors, which states that, the acceleration of an object, {vec. a}, is proportional to the sum of the forces {vec F} _ {k} he suffers and inversely proportional to its mass.

m { vec a} =  sum _ {k} { vec F} _ {k}

This means for example that if we drop an object on Earth, its movement will be accelerated because of his weight. It will therefore fall faster and faster.

The acceleration vector {vec a} is the same direction as the vector sum of the forces {vec F} _ {k} applied to the object. So for an object to be at equilibrium (constant speed) requires that the sum of the force vectors is the zero vector:

{vec = {a} frac 1m}  sum _ {k} {vec F} _ {k} = {vec 0}

We can consider that the first law of Newton is a special case of the second: if no there is no force on an object then its acceleration is zero which means that its speed is constant.

Third law: the principle of reciprocal actions:

The third law is the principle of reciprocal actions. It states that if a body exerts a force on one another, while the opposite is true. In other words, if an object A exerts a force on object B, then B also exerts a force on A in the same direction and with the same intensity but in the opposite direction (toward A).

This is the principle of jet propulsion.

For example.

A rocket ejects gas by its reactor nozzles. It exerts a force on these gases to eject. In response, the gases exert a force on the rocket: it is this force that off the ground and propelled into space.