Sir Isaac Newton put the fundamental of classical physic through his famous publication Principia.
In his book he wrote three important laws. We know them as Newton’s three laws of motion. They are
1. Everybody continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed on it.
2. The change of motion is proportional to the motive force impressed, and is made in the direction of the straight line in which that force is impressed.
3. To every action there is always opposed an equal reaction or, the mutual actions of two bodies upon each other are always equal and directed to contrary part.
Let me start with the first law, which is called the law of inertia. This first law states if there is no resultant of force applied to an object, the object will maintain its motion state. The motion state here means wheatear the object move or stay still. If the object is at rest, it will stay at rest. Moving object without any resultant force applied on it will continue to move with constant velocity.
Velocity is a vector quantity. It bears both magnitude and direction. Constant velocity means both the magnitude and direction is unchanged during the motion. Therefore, an object without resultant forces acting on it will move with unchanged distance travelled in a particular time interval and direction.
We can simply say that an object tends to be “lazy” or hard to change its motion state. The degree of laziness or hardness to change the motion state is represented by the mass of the object. The greater of the mass of an object the harder it change the motion state. This is what the second law wants to tell you.
The first law tells about the absence of resultant force. What will happen if the resultant forces exist on an object? Well, as you guess, the motion state will change. The object now will be accelerated in the direction of the resultant forces. Acceleration is the positive change of velocity per unit time. If the object is at rest, it will start to move. If the object is moving in the same direction to the resultant forces, it will move faster. But if the object is moving in the opposite direction with the resultant forces, it will move slower or decelerated. In the next discussion, I just write acceleration because deceleration is simply negative acceleration.
The measure of acceleration depends on the measure of resultant of forces and mass. The acceleration is directly proportional to the resultant of forces and inversely proportional to the mass of the object. The greater the resultant of forces applied on the object the greater acceleration will be. On the contrary, the greater the mass of the object the less acceleration will be.
The third law discusses about action and reaction. When you push your friend away from you, you feel that your friend pull you to away from him. This is an example of the third law. Many phenomenons demonstrate this third law. The attractive force between two unlike charges, the attractive force between moon and the Earth makes the moon orbits the Earth as proposed by Newton Law of Gravitation.
Two forces are called an action – reaction pair provided that they have equal magnitude but work in opposite direction. If one force is eliminated, the pair also disappears instantaneously. I want to give an example about it. Imagine a box lies on a table. The box exerts a force (name it N1) on the table and the table also exerts an equal but opposite force to the box (name it N2). N1 and N2 are action – reaction pair forces. But the weight of the box is not an action-reaction pair with N2, this is a pitfall. When I raise the box, the force N1 and N2 will disappear but the weight is still there.
Actually the weight of the box is come from the mutual attractive forces between the box and the Earth. They are pulling each other according to the Newton Law of Gravitation.
