Fundamental Principles of Forces and Motion

Understanding Physical Forces

A force is the interaction of a body with something external to it and is capable of:

• Modifying its state of motion or rest (we will focus on it).
• Causing deformation.

The Normal Force

The normal force (N) is a reaction force; it is a force resulting from the application of Newton’s third law, which must have exactly the value required for the net force along its axis to be null.

Newton’s Laws of Motion

Newton’s First Law

Newton’s first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This tendency to resist changes in a state of motion is inertia.

Newton’s Second Law

Newton's second law of motion can be formally stated as follows: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

Newton’s Third Law

Newton's third law states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction. The third law is also known as the law of action and reaction.

Tension, Gravity, and Friction

The tension force is a reaction force that is transmitted through a string when it is in tension, with the same value in both directions. The gravitational force is a force related to the attraction of two bodies due to their own mass. In fact, weight is a particular case of this force. The friction force acts when a body moves on another in such a way that, due to the contact of both surfaces, the movement is braked.

Concepts of Motion and Speed

Motion is the change in position of an object with respect to another one. Average speed is a scalar quantity. The average speed in an interval of time is the distance traveled divided by the duration of this interval. Speed is the rate of change of distance with respect to time. The symbol for speed is v. Speed is a scalar quantity. Velocity is the physical quantity that describes how a moving object's position changes.

Displacement and Distance

The displacement is simply the difference in the position of the two marks and is independent of the path taken in traveling between the two marks. The distance traveled is the total length of the path taken between the two marks.