4-2:Newton’s First Law Objectives: Explain the relationship between the motion of an object and the net external force acting on it. Determine the net external force on an object Calculate the force required to bring an object into equilibrium

A ball at rest in the middle of a flat field is in equilibrium. No net force acts on it. If you saw it begin to move across the ground, you ’ d look for forces that don ’ t balance to zero. We don ’ t believe that changes in motion occur without cause.

Galileo, the foremost scientist of late-Renaissance Italy, was outspoken in his support of Copernicus. One of Galileo ’ s great contributions to physics was demolishing the notion that a force is necessary to keep an object moving. Galileo on Motion

Friction is the name given to the force that acts between materials that touch as they move past each other. Friction is caused by the irregularities in the surfaces of objects that are touching. Even very smooth surfaces have microscopic irregularities that obstruct motion. If friction were absent, a moving object would need no force whatever to remain in motion. Galileo on Motion

Galileo tested his idea by rolling balls along plane surfaces tilted at different angles. A ball rolling down an inclined plane speeds up. A ball rolling up an inclined plane—in a direction opposed by gravity—slows down. A ball rolling on a smooth horizontal plane has almost constant velocity. Galileo on Motion

Galileo stated that if friction were entirely absent, a ball moving horizontally would move forever. No push or pull would be required to keep it moving once it is set in motion. Galileo on Motion

Galileo stated that this tendency of a moving body to keep moving is natural and that every material object resists changes to its state of motion. The tendency of an object to maintain its state of motion is called inertia. Galileo on Motion

Newton further developed this thought in 1687 and has become Newton’s First Law of Motion. Newton ’ s first law states: An object at rest remains at rest, and an object in motion continues in motion with constant velocity (that is, constant speed in a straight line) unless it experiences a net external force. 4-2 Newton’s Law First Law (Law of Inertia)

The amount of inertia an object has depends on its mass—which is roughly the amount of material present in the object. Mass is a measure of the inertia of an object. Mass—A Measure of Inertia

You can tell how much matter is in a can when you kick it. Kick an empty can and it moves. Kick a can filled with sand and it doesn ’ t move as much. Mass—A Measure of Inertia

Mass Is Not Volume Do not confuse mass and volume. Volume is a measure of space and is measured in units such as cubic centimeters, cubic meters, and liters. Mass is measured in the fundamental unit of kilograms. Mass—A Measure of Inertia

Which has more mass, a feather pillow or a common automobile battery? Clearly an automobile battery is more difficult to set into motion. This is evidence of the battery ’ s greater inertia and hence its greater mass. Mass—A Measure of Inertia

The pillow has a larger size (volume) but a smaller mass than the battery. Mass—A Measure of Inertia

Mass Is Not Weight Mass is often confused with weight. We often determine the amount of matter in an object by measuring its gravitational attraction to Earth. However, mass is more fundamental than weight. Mass is a measure of the amount of material in an object. Weight, on the other hand, is a measure of the gravitational force acting on the object. Mass—A Measure of Inertia

Mass Is Inertia The amount of material in a particular stone is the same whether the stone is located on Earth, on the moon, or in outer space. The mass of the stone is the same in all of these locations. The weight of the stone would be very different on Earth and on the moon, and still different in outer space. Mass—A Measure of Inertia

The stone ’ s inertia, or mass, is a property of the stone and not its location. The same force would be required to shake the stone with the same rhythm whether the stone was on Earth, on the moon, or in a force-free region of outer space. Mass—A Measure of Inertia

It ’ s just as difficult to shake a stone in its weightless state in space as it is in its weighted state on Earth. Mass—A Measure of Inertia

We can define mass and weight as follows: Mass is the quantity of matter in an object. More specifically, mass is a measure of the inertia, or “ laziness, ” that an object exhibits in response to any effort made to start it, stop it, or otherwise change its state of motion. Weight is the force of gravity on an object. Mass—A Measure of Inertia

Mass and weight are proportional to each other in a given place: In the same location, twice the mass weighs twice as much. Mass and weight are proportional to each other, but they are not equal to each other. Mass—A Measure of Inertia

Equilibrium- The state of a body in which there is no change in its motion. The net forces will equal 0 in all directions.

One Kilogram Weighs 10 Newtons(9.81) It is common to describe the amount of matter in an object by its gravitational pull to Earth, that is, by its weight. In the United States, the traditional unit of weight is the pound. In most parts of the world, however, the measure of matter is commonly expressed in units of mass, the kilogram (kg). At Earth ’ s surface, 1 kilogram has a weight of 2.2 pounds. Mass—A Measure of Inertia

Newton’s Law of Inertia

Net external force can be determined by a change in motion Look at the car on page 131 figure 4-6. This shows all the forces acting upon the car. F resistance =Force of resistance on the car F ground-on-car = Force applied from the ground to the car F forward = Force of the car moving forward F gravity = Force of the car being pulled down by gravity

External Force vs. Net External Force External force is a single force that acts on an object as a result of the interaction between the object and its environment. Net external force is the total force resulting from a combination of external forces on an object; sometimes called the resultant force. – The sum ( ) of these forces will give you the resultant force.

Tug-of-war Figure 4-7 shows a tug of war. In figure (a) it shows two equal but opposite forces on a rope. In this case the knot in the middle does not move. In figure (b) the force on the right is more, therefore, the knot has accelerates to the right.

Derek left his physics book on top of a drafting table, as shown. The table is inclined at a 35° angle. Find the net external force acting on the book, and determine whether the book will remain at rest in this position. Given: F gravity-on-book = 22N F friction = 11N F table-on-book = 18N Unknown: F net = ? F table-on-book = 18N F friction = 11N F gravity-on-book = 22N Θ=35°

To Solve 1.Select an x and y axis F table-on-book = 18N F friction = 11N F gravity-on-book = 22N Θ=35° Y-axis X-axis

2. Find the X and Y components of all the vectors. Friction: We know that the X component of friction is 11 and Y is 0. Table-on-book: X=0 and Y=18 Gravity-on-book: Not sure about x and y. We need to solve it. F table-on-book = 18N F friction = 11N F gravity-on-book = 22N Θ=35° Y-axis X-axis

Solve the X and Y components Y component – Sin 55°= – y= 18N X component – Cos 55°= – x= 13N – The x component is what is pulling on the book. – 13N-11N = 2N down – The book will slide off the table – In the y direction they are both 18, so book stays on the table. Θ=35° 22N Book Θ=35° 22N Book X Component Y Component Θ= 55°

Page 133 Questions 1-3