Physics -- Newton's Laws of Motion
Sir Isaac Newton (a.k.a. "The Big Fig") learned a lot from his famous apple-on-the-head incident. If you're studying physics this year, you'll soon come to know his three laws of motion very well.
Newton's first law of motion, also called the law on inertia, states that an object continues in its state of rest or of uniform motion unless compelled to change that state by an external force. The law appears to contain two separate statements. The first statement a state of rest will continue unless a force is applied seems intuitively correct. The second statement an object will continue with a constant velocity unless compelled to change by an impressed force seems contrary to common experience. It is important to realize that objects observed to slow down are being compelled to change by a frictional force. Friction is a retarding force that is ever present in our everyday world. For the ideal the absence of outside forces acting on the object, as described by the law friction must be eliminated. The value of the law is the introduction of the concept of force as a push or pull that causes a body to change its state of motion.
Newton's second law of motion states that if a net force acts on an object, it will cause an acceleration of that object. The law addresses the cause and effect relationship between force and motion commonly stated as F = ma, where m is the proportionality constant (mass). Force is measured in SI units of newtons, abbreviated N.
Newton's third law of motion states that for every action there is an equal and opposite reaction. Therefore, if one object exerts a force on a second object, the second exerts an equal and oppositely directed force on the first one.