Newton's Laws of Motion

Learning Objectives After completing this lesson you will be able to: Define the following terms: inertia, free body diagram, unbalanced force, net force, inertial mass. Explain what inertia means and state that mass is a measure of inertia. State Newton's laws of motion and provide various applications of these laws. Explain what is meant by an unbalanced force. Analyze situations involving forces with the aid of a free body diagram. Transfer an understanding of vector addition to applications involving Newton's laws of motion. Solve problems involving Newton's laws of motion. Predict the direction of acceleration on an object, given the direction of the unbalanced force and vise versa. Interpret direct and inverse relationships, as they occur in Newton's second law. Demonstrate an understanding of the relationship between the SI unit of force (Newton) and the corresponding fundamental units. Explain how the inertial mass of an object can be determined.

Key Concepts Inertia is the property of an object that resists changes in its state of rest or motion. Mass is a measure of the inertia of an object. Newton's first law of motion (Galileo's Principle of Inertia) describes what Galileo had discovered about inertia in his "thought experiments". A net force is the resultant of all forces acting on an object. Equilibrium exists if the net force is zero. (There must also be no resultant torque.) An unbalanced force exists when the net force does not equal zero. If no external unbalanced force acts on an object, its velocity will remain constant (i.e., it will remain at rest it it was initially at rest, or continue moving in a straight line at a constant speed, if it was initially doing so). An object at rest on a table has its weight opposed by a normal force acting on the object by the table. Many practical applications of Newton's laws are evident in common occurrences. Free body diagrams are useful to analyze situations involving more than one force acting on a body. Vector addition can be used to determine the net force. When a net force acts on an object, it accelerates in the direction of the net force. (Newton's second law) Acceleration is directly proportional to the force for a constant mass and inversely proportional to the mass for a constant force. The relationship between the SI unit for force and the corresponding fundamental units can be illustrated from Newton's second law. Newton's third law states that for every action force a reaction force exits which is equal in magnitude but opposite in direction. These action-reaction forces act on different objects. To solve problems relating to force and motion, geometric methods using vector diagrams, trigonometry, or vector component methods are some different ways to arrive at similar results. Architects, engineers, and people involved in a wide variety of other related disciplines require a thorough understanding of Newton's laws of motion.