Newton's Laws Introduction
From MyMCAT
Contents |
Introduction
In the previous section, translational motion was examined in the form of displacement, velocity, and acceleration. These concepts provide enough information to describe how an objects position and speed changed with time in relation to either no acceleration or a constant acceleration but they do not answer what is causing the acceleration? Sir Isaac Newton formulated the theory of force, relating the concepts of acceleration, mass, and force together, to help describe the previous observations made of displacement, velocity, and acceleration. His work can be summarized in what is now known as Newton's laws of motion.
Force
Force acts on any object with mass and causes acceleration. It is a vector quanitity in the same direction as the acceleration it causes. While we often do not know what the force is, it is easily determined by observing the objects acceleration and calculating backwards.
The SI units of force are the Newton [N] and 1N is equivalent to 1[kg][m/s2].
Newton's First Law: The Law of Inertia
Newton's first law says that an object at rest will stay at rest unless a force is applied to it, or similarly, an object in motion will not change in velocity unless a force is applied. To be more specific, this "force" must act on the object such that it is nonzero in tis vector direction. That is to say, if there were a block on a table and two people pushed it with equal force from both sides, then it would clearly NOT move, but if one were to push slightly harder, than the net force is nonzero and the block would be accelerated.
Newton's Second Law: Law of Resultant Force
While already glanced at, Newton's second simply describes how an object behaves when forces are applied to it.
If there are multiple forces acting on an object, then the net sum of these forces will be equal to ma, thus the acceleration can be found for the object regardless of how many forces act upon it.
Newton's Third Law: Law of Reciprocal Actions
Newton's third law is stated as the following: All forces occur in pairs, and these two forces are equal in magnitude and opposite in direction. Consider what happens when two ice skaters stand next to each other and one of them pushes the other. Assuming the ice provides minimal friction, both skaters would slide apart, this is because while only one skater pushed, both experienced a force as a result of the third law. Where the example done on ground friction would play a role and alter the outcome.
For a person standing in an elevator moving with constant velocity (including stopped), the normal force on the person's feet balances the person's weight. In an elevator that is accelerating upward, the normal force is greater than the weight and so the person's apparent weight increases (making the person feel heavier). In an elevator that is accelerating downward, the normal force is less than the weight and so a user's apparent weight decreases. If a user were to stand on a "weight scale", such as a conventional bathroom scale, onto the elevator, the scale would read either more or less than the person's actual weight when the elevator is accelerating up or down (respectively) because weight scales measure normal force (which varies as the lift accelerates), not gravitational force (which does not).
