Isaac Newton’s three laws of motion were first published in his Philosophiae Naturalis Principia Mathematica in 1687. The laws form the basis for classical mechanics and Newton used them to explain many results concerning the motion of physical objects.
Newton’s Three Laws
First Law: Bodies move in a straight line with a uniform speed, or remain stationary, unless a force acts to change their speed of direction.
Second Law: Forces produce accelerations that are in proportion to the mass of a body (F=ma)
Third Law: Every action of a force produces an equal and opposite reaction.
Forces
Newton used Galileo’s principle of inertia as the basis for his first law. It states that bodies do not move or change their speed unless a force acts. Effectively, bodies that are stationary will not move unless a force is applied to them while bodies that are moving at a constant speed will keep moving at that speed unless a force acts upon them. A force (such as a push) supplies an acceleration that changes the velocity of the body.
This first law can be hard to appreciate in the everyday world. While a push may start an object in motion, the object will eventually slow down due to friction with the surface on which it is travelling. Friction causes a force which makes the object slow down. However, Newton’s law can be seen to operate in special situations where friction is absent. The nearest such situation would be events in space but even there forces such as gravity are at work.
Nevertheless, Newton’s first law provides the groundwork from which an understanding of forces and motion comes.
Acceleration
Newton’s second law of motion relates the size of the force to the acceleration it produces. The force needed to accelerate an object is proportional to the object’s mass. Therefore, heavy objects need greater force to accelerate them than lighter objects.
This second law can be expressed algebraically as F=ma where force (F) equals mass (m) times acceleration (a).
By turning this definition around, the second law could be expressed so that acceleration is equal to force per unit mass. For a constant acceleration, force per unit mass is also unchanged. So the same amount of force is needed to move a kilogram mass whether it is part of a small or large body.
Action Equals Reaction
Newton’s third law states that any force applied to a body produces an equal and opposite reaction force in that body. Effectively, for every action there is a reaction. The opposing force is felt as recoil and the recoil force is equal in size to that originally expressed.
Sources:
Baker, Joanne (2007) 50 Physics Ideas You Really Need to Know (Quercus Publishing)
Holzner, Steve (2005) Physics for Dummies (John Wiley & Sons)
Kuhn, Karl (1996) Basic Physics: A Self-Teaching Guide (John Wiley & Sons)
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