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Chapter 3: Newton’s First Law of Motion – Inertia 1 You see a ball in the middle of a ball field. It suddenly starts moving. Why? Historical views on motion: Aristotle: 4th century BC Copernicus (1473-1543) Divided motion into two types: Simplest way to interpret astronomical observations was to assume that Earth and the other planets move around the sun. Natural motion – straight up or straight down o Boulder falling down, smoke rising up o Objects seek their natural resting places Circular motion was natural for the heavens (no beginning, no end) Planets and stars moved in perfect circles around Earth Violent motion – imposed motion o Result of forces that pushed or pulled o Carts pulled by horses, ships pushed by wind o External cause of violent motion – motion was imparted to objects Objects in their natural resting places could not move by themselves – must be pushed or pulled. Believed that the proper state of objects was one of rest, unless they were being pulled or pushed, or were moving toward their natural resting place. Earth is in its natural resting place o No force is large enough to move it o Earth did not move Galileo (1564-1642) Supporter of Copernicus If friction were absent, a ball moving horizontally would move forever. o No push or pull required once it is set in motion. Was tried and sentenced to house arrest One of his greatest contributions to physics was to demolish the idea that a force is necessary to keep an object moving. Friction: name given to the force that acts between materials in contact c. Galileo found that a ball rolling on a smooth horizontal plane has almost constant velocity. Very controversial idea o People preferred to believe that the Earth was the center of the universe Worked on his ideas in secret to avoid persecution. Caused by irregularities in the surfaces of objects Microscopic irregularities exist in very smooth objects that obstruct motion Without friction, a moving object would need no force to remain in motion o Only when friction is present is a force needed to keep an object moving. Tested idea by rolling balls along plane surfaces inclined at different angles. Chapter 3: Newton’s First Law of Motion – Inertia 2 Galileo (continued) Galileo used 2 inclined planes facing each other. A ball released to roll down one plane would roll up the other to reach nearly the same height o Smoother planes had better results o Final height would be nearly equal to initial height If the second plane was longer and had a smaller angle, the ball reached the same height. If the angle of the incline were 0° (horizontal), the ball would roll forever if friction were not present. Disputed Aristotle. It was not the nature of a ball to come to rest. Stated that it is the tendency of a moving body to keep moving; it is natural for an object to resist changes in its motion Inertia – property of a body to resist changes to its state of motion Interested in how things move, not why! Experimentation, not logic, is the best test of knowledge. Findings about motion and concept of inertia discredited Aristotle’s theory of motion. Chapter 3: Newton’s First Law of Motion – Inertia 3 Newton’s Law of Inertia – First Law of Motion Isaac Newton was born December 25, 1642 (the year Galileo died). By the age of 24, he had developed his laws of motion Laws of motion replaced Aristotelian ideas that had dominated science for 2000 years! Newton’s first law of motion, also known as the law of inertia, restates Galileo’s idea that a force is not needed to keep an object moving. “Every object continues in a state of rest, or of uniform speed in a straight line, unless acted on by a nonzero net force.” Objects tend to keep on doing what they’re already doing. Objects in a state of rest tend to remain at rest. o Only a force will change that state Objects in motion Ex. A hockey puck sliding … On the road soon comes to a stop On ice, slides much farther o Friction is very small On an air table, friction is virtually absent o Puck slides with no apparent loss of speed. In the absence of forces, a moving object tends to move in a straight line indefinitely. In space, it will travel forever! Chapter 3: Newton’s First Law of Motion – Inertia 4 Inertia Objects move (or don’t) by virtue of their own inertia. Law of inertia provides a different way of viewing motion. Ancients thought continual forces were needed to maintain motion Now, we know objects continue to move by themselves o Forces are needed to overcome friction and to set the object in motion o Once in motion in a force-free environment, the object will continue to move in a straight line indefinitely. Mass – A Measure of Inertia Mass – the amount of material present in an object The more mass an object has, the greater its inertia and the more force it takes to change its state of motion. Mass is the measure of inertia of an object Mass ≠ Volume o Mass is the amount of material and is measured in kilograms o Volume is a measure of space and is measured in cm3, m3, and liters. o Large mass does not necessarily mean a large volume Mass is not weight. Mass is amount of material in an object Weight is the gravitational force acting on the object Mass is more fundamental than weight weight can change because it depends on location mass depends only on the number and kinds of atoms that make up an object o doesn’t change Mass is inertia. The amount of material in a particular object is the same regardless of its location – Earth, moon, space. Therefore, mass is the same. Same force is required to shake the object back-and-forth, in all 3 locations. Inertia is a property of the object, not its location. Chapter 3: Newton’s First Law of Motion – Inertia Weight – Depends on location because it depends on the force of gravity exerted on the object. Definitions Weight – force of gravity on an object Mass – quantity of matter in an object; measure of the inertia, or “laziness” that an object exhibits in response to any effort to start, stop, or otherwise change its state of motion. Mass and weight are different, but are proportional in any location. Large mass, large weight Little mass, little weight In the same location, twice the mass = twice the weight 5 Units of weight vs. mass. In the U.S., we describe matter by its gravitational pull to the Earth, its weight. Weight is measured in pounds. In the rest of the world, matter is described by its mass. The unit of mass is the kilogram. 1 kg has a weight of 2.2 pounds SI unit of force is the newton 1 kg weighs 10 newtons You will use these facts when converting. The Moving Earth When Copernicus announced that the Earth was moving around the Sun, opponents to the idea argued that the Earth couldn’t be moving. One of their arguments went like this: Bird at rest in a tree sees a worm on the ground below. The bird drops down and catches the worm. This action would not be possible if the Earth were in motion. If Copernicus was correct, the Earth would have to move at 107,000 km/h to circle the sun in one year. That’s 30 km/s! Birds do catch worms, so is Copernicus right? How would you defend his idea? Can you give any examples to support his claim? Objects move with the Earth The Earth moves at 30 km/s. So do the tree, the bird, the worm, even the air! Everything on Earth moves at 30 km/s. The law of inertia states that objects in motion remain in motion if no unbalanced forces act on them. Objects on Earth move with Earth as the Earth orbits the sun. The horizontal motion of the Earth has no bearing on the bird’s motion as it drops to the ground. Try this! Stand next to a wall. Jump up so your feet no longer touch the floor. Does the wall (moving at 30 km/s) slam into you? Why not? The 30 km/s is the speed of the earth relative to the sun, not the speed of the wall relative to you. Objects move with vehicles Flip a coin while riding in a moving car or while flying in an airplane. The coin behaves as if the car and the plane were at rest.