Newton’s 1st Law of Motion Inertia Objectives • Explain the relationship between the motion of an object and the net external force acting on the object. • Determine the net external force on an object. • Calculate the force required to bring an object into equilibrium. Newton’s 1st Law of Motion States: • An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. 1st Law 1st Law All objects resist changes in their state of motion – they tend to "keep on doing what they're doing." 1st Law 1st Law Inertia is the resistance an object has to a change in its state of motion. 1st Law 1st Law • Galileo, developed the concept of inertia. • Galileo reasoned that moving objects eventually stop because of a force called friction. 1st Law 1st Law • The tendency of an object to resist changes in its state of motion is dependent upon its mass. 1st Law • Mass is not volume. • Mass is measured in kilograms. 1st Law • Mass is not weight. • It is more fundamental than weight. • Mass is a measure of the actual material in an object. It depends only the number of atoms that compose it. • Weight is a measure of the gravitational force that acts on a material and depends on where the object is located. 1st Law 1st Law • Mass: the quantity of matter in a body. • The measure of inertia. • Weight: the force due to gravity upon a body. 1st Law • The SI unit of force is the NEWTON. • 1 kg = 9.8 N = 2.2 lbs. Check Your Understanding 1. Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will: A. gradually stop. B. continue in motion in the same direction at constant speed. Check Your Understanding • An 2-kg object is moving horizontally with a speed of 4 m/s. • How much net force is required to keep the object moving with the same speed and in the same direction? Check Your Understanding • Mac and Tosh are arguing in the cafeteria. Mac says that if he throws his jello with a greater speed it will have a greater inertia. • Tosh argues that inertia does not depend upon speed, but rather upon mass. • With whom do you agree? Why? Check Your Understanding • If you were in a weightless environment in space, would it require a force to set an object in motion? Check Your Understanding • Mr. Wegley spends most Sunday afternoons at rest on the sofa, watching pro football games and consuming large quantities of food. • What effect (if any) does this practice have upon his inertia? Explain. Check Your Understanding • Ben is being chased through the woods by a bull moose which he was attempting to photograph. • The enormous mass of the bull moose is extremely intimidating. • Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. • Explain this in terms of inertia and Newton's first law of motion. State of Motion • Inertia is the tendency of an object to resist changes in its velocity. State of Motion • We learned before that an object which is not changing its velocity is said to have an acceleration of 0 m/s2. • Thus, an alternate definition of inertia would be: • Inertia is the tendency of an object to resist accelerations. Check Your Understanding • Several physics teachers are taking some time off to play a little mini golf. The 15th hole has a large metal rim which putters must use to guide their ball towards the hole. • Mrs. Menken guides her golf ball around the metal rim. When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow? Check Your Understanding • A 4.0 kg object is moving across a frictionless surface with a constant velocity of 2 m/s. Which one of the following horizontal forces is necessary to maintain this state of motion? • 0N • 0.5 N • 2.0 N • 8.0 N Balanced and Unbalanced Forces • An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. • What is an unbalanced force? Balanced and Unbalanced Forces Balanced and Unbalanced Forces • Since these two forces are of equal magnitude and in opposite directions, they balance each other. • The book is said to be at equilibrium. Balanced and Unbalanced Forces Balanced and Unbalanced Forces Balanced and Unbalanced Forces • The book is not at equilibrium and it subsequently accelerates. • Unbalanced forces cause accelerations.
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