Sir Isaac Newton was a physicist, mathematician, astronomer, theologian and alchemist. In 1686 he introduced his three laws of motion.The motion of objects can be explained through Newton's Laws of Motion.
Newton's first law, the Law of Inertia, in simple terms, states that an object will remain at rest unless it is acted upon by an outside force. Basically, this means that if no force is pushing on an object, or if all the forces pushing on an object cancel each other out, then the object stays at a constant velocity. If the object is at rest, it will stay at rest because that is its natural tendency. So with no force there is no acceleration.
An example of the first law of motion would be the earth's gravitational force. An object falling through the atmosphere has zero velocity until it starts to fall. Once it starts falling through the atmosphere, the object begins to fall more rapidly starting to accelerate and it is acted upon by a force, the force of gravity. After a while the motion of the object is acted upon by air resistance which opposes its motion to slow it down.
Newton's second law explains on velocity of an object can change. This law states that force is equal to a change in momentum, and that momentum is equal to mass times velocity. The law also states that for any object whose mass is constant (called m) the force (F) is equivalent to the object's mass times its acceleration. This equation works in both directions, with an increase in force changing the acceleration of a constant mass and a change in velocity changing the force. In other words, acceleration occurs when a force acts on a mass.
Newton's second law of motion can be demonstrated with a large round object such as the Earth and a smaller round object, such as a satellite circling the Earth. The smaller satellite may attempt to move towards the larger object at its center, but because of the correct horizontal velocity, it only revolves around the larger mass in a circle, staying in its path.
Newton's third law of motion states that for every action, there is an opposite and equal reaction. Simply put, any force that is put on object 1 by object 2, the same force is put on object 2 by object 1.
Newton's third law can be explained by the thrust of a jet engine or the lift of an airplane wing. Another example of this law would be two ice skaters pushing against each other and then sliding apart in opposite directions.
A classic tug of war game can be used to explain Newton's three laws of motion. At the beginning of a tug of war, both sides are pulling at the rope and neither side is moving. The force on the rope is the same from both sides. This is an example of Newton's First Law. Once one side pulling on the rope exerts more force and therefore more pull, the other side has to increase their pull to try and get the rope back. This is Newton's second law of motion. Finally, although it is more subtle, the rope is resisting your pull and pulling back while you are pulling. This is Newton's third law of motion.
This is a simplistic view of Newton's three Laws of Motion. They are more complex and involve calculus, but this should give the reader a good idea of how the three laws work, and how important they are to Science, in particular the Science of Physics.