HiSET Science: Laws of Motion

Newton's Laws

Newton's First Law
Before Newton formulated his laws of mechanics, it was generally assumed that some force had to act on an object continuously in order to make the object move at a constant velocity. Newton, however, determined that unless some other force acted on the object (most notably friction or air resistance), it would continue in the direction it was pushed at the same velocity forever. In this light, a body at rest and a body in motion are not all that different, and Newton's first law makes little distinction. It states that a body at rest will tend to remain at rest, while a body in motion will tend to remain in motion. This phenomenon is commonly referred to as inertia, the tendency of a body to remain in its present state of motion. In order for the body's state of motion to change, it must be acted on by a non-zero net force. Net force is the vector sum of all forces acting on a body. If this vector sum is zero, then there is no unbalanced force, and the body will remain in its present state of motion. It is important to remember that this law only holds in inertial reference frames.

Newton's Second Law
Newton's second law states that an object's acceleration is directly proportional to the net force acting on the object, and inversely proportional to the object's mass. It is generally written in equation form F= ma, where F is the net force acting on a body, m is the mass of the body, and a is its acceleration. It is important to note from this equation that since the mass is always a positive quantity, the acceleration vector is always pointed in the same direction as the net force vector. Of course, in order to apply this equation correctly, one must clearly identify the body to which it is being applied. Once this is done, we may say that F is the vector sum of all forces acting on that body, or the net force. This measure includes only those forces that are external to the body; any internal forces, in which one part of the body exerts force on another, are discounted. Newton's second law somewhat encapsulates his first, because it includes the principle that if no net force is acting on a body, the body will not accelerate. As was the case with his first law, Newton's second law may only be applied in inertial reference frames.

Newton's Third Law
Newton's third law of motion is quite simple: for every force, there is an equal and opposite force. When a hammer strikes a nail, the nail hits the hammer just as hard. If we consider two objects, A and B, then we may express any contact between these two bodies with the equation FAB = -FBA. It is important to note in this kind of equation that the order of the subscripts denotes which body is exerting the force. Although the two forces are often referred to as the action and reaction forces, in physics there is really no such thing. There is no implication of cause and effect in the equation for Newton's third law. At first glance, this law might seem to forbid any movement at all. We must remember, however, that these equal, opposite forces are exerted on different bodies with different masses, so they will not cancel each other out.