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Chapter 4: The Nature of Force and Motion Notes-pages 116- 121 What is a Force? A. A _________________ is a push or a pull. When one object pushes or pulls another object, the ___________________ object is exerting a force on the _________________ object. You exert a ________________ on a pencil when you write, on a book when you lift it, and on a doorknob when you pull on it. Forces like ____________________ & _____________, are described not only by how strong they are, but also by the __________________ in which they act. B. When _____________ forces act in the same direction, you ____________ them together. The arrow or vector points in the ________________ of a force. The width of each arrow tells you the ______________ of a force. A wider arrow shows a __________________ force. Two forces can combine so that they __________ together (top), or _______________from each other (center). They can also _______________ each other out (bottom). When forces act in _______________ directions, they also add together. If one force is _________________ than the other force, the overall force is in the _____________________of the greater force. C. The overall force on an object after all the forces acting on it are added together is called the ______________________. Forces give ________________ to an object. Forces and Motion A. When a net force acts on an object, the forces are ___________________. ____________________ forces can cause an object to start ________________, stop __________________, or change ____________________. Unbalanced forces acting on an object will change the object’s _____________________ or cause an object to _______________________. For example, if two unequal ___________________ acting in opposite directions are applied to a box, the box will accelerate in the __________________ of the greater force. B. Equal forces acting on one object in opposite directions are called ___________________ forces. One force is exactly ________________ by one or more other forces. Balanced forces acting on an object will ________ change the object’s motion. When you add equal forces exerted in opposite directions, the ______________________ is zero. Newton’s First Law of Motion A. The ancient Greeks observed that objects have natural _________________. A rock ______________to the ground. A puff of smoke ________________ into the air. Once an object is in its natural resting place, it ______________ move by itself. B. In the early 1600s, the Italian astronomer Galileo suggested that once an object is in _____________, no _____________ or ___________ is needed to keep it __________. C. Force is needed only to _____________ the motion of an object. However, whether it is moving or at rest, every object _________________ any change to its motion. This resistance is called ________________. D. ______________________is the tendency of an object to resist change in its motion. A hockey puck that rides on a cushion of air in an “air-hockey” game glides along quite freely once you push it. A tennis ball flies through the air once you hit it with a racket. In both cases, the object continues to move even after you remove the force. E. The English mathematician, Sir Isaac Newton discovered the three basic ____________________________________ in the late 1600s. Newton’s ________________ law of motion states that an object at ______________ will remain at _______________and an object that is _________________ at constant velocity will continue __________________ at constant velocity unless acted upon by an ________________________ force. An object will not start moving unless a __________ acts on it. An object will not ________________ moving unless a force acts on it. An object will not change ________________ unless a force acts on it. An object will not change ________________unless a force acts on it. Newton’s first law of motion is also called the __________________________. Newton was the first scientist to use the term inertia to describe the tendency of objects to remain in ________ or _____________at rest. Inertia comes from the Latin word iners, which means “______________.” EXAMPLE: If you are in a car that stops suddenly, _______________ causes you to continue moving forward. Crash-test dummies don’t stop when the car does. A _________ is required to change their motion. That force is exerted by the safety belt. Mass and Inertia A. ____________ is the amount of matter in an object. (unit of mass is kg) B. _________________ is the amount of space an object occupies. C. The object with the _______________ mass has the greater ______________. D. The amount of inertia an object has depends on its ___________. A car’s mass is greater than the bicycle’s, so the car’s inertia is greater. E. Mass can also be defined as a measure of the ____________ of an object. Page 125 Force = _____________ X _________________ Force is measured in _________________. (N) Chapter 4.3 Friction and Gravity Notes-pages 127-133 I. Forces and Motion A. According to Newton’s first law of motion, a book’s motion changes only if an ________________ acts on it. A force should not be necessary to keep the book moving at a __________________. B. When the book slides, the force of _____________ causes it to slow to a stop. When the book falls, the force of the _______________causes it to accelerate downward. In this section you will learn that these two forces affect nearly all _________________. II. Friction A. When you push a book across a table, the _____________ of the book rubs against the ____________ of the table. B. The force that one surface exerts on another when the two rub against each other is called _______________. III. The Nature of Friction A. ______________________acts in a direction _____________ to the object’s direction of motion. Without __________________, the object would continue to move at constant speed forever. Eventually friction will cause an object to come to a ________________. B. The strength of the force of friction depends on the ________________ of surfaces involved and how hard the surfaces _____________________________. C. _________________________ produce greater friction than smooth surfaces. The skiers in the photo get a fast ride because there is ______________ friction between their skis and the snow. D. The force of friction also ________________ if the surfaces push harder against each other. IV. Is Friction Useful? A. You are able to walk because ________________ acts between the soles of your shoes and the _______________. Without friction your shoes would only _____________ across the floor, and you would never move _______________. B. An automobile moves because of friction between its ______________ and the _________. V. Controlling Friction A. When solid surfaces slide over each other, the kind of friction that occurs is called __________________. B. When an object rolls over a surface, the kind of friction that occurs is ____________________. The force needed to overcome _______________ friction is much less than the force needed to overcome ________________ friction. ________________________ are one way of ____________________ friction between two surfaces. C. The friction that occurs when an object moves through a fluid is called _____________________________. The force needed to overcome _______________ friction is usually less than that needed to overcome ________________friction. VI. Gravity A. Newton realized that a force acts to pull objects straight down toward the center of Earth. ________________ is the force that pulls objects toward each other. B. B. When the only force acting on a falling object is gravity, the object is said to be in ________________. An object in free fall ________________ as it falls. In free fall the force of gravity is an __________________, and unbalanced forces cause an object to _______________. C. C. How much do objects accelerate as they fall? Near the surface of Earth, the acceleration due to the force of gravity is __________________. D. While it may seem hard to believe at first, all objects in free fall accelerate at the _________________________ regardless of mass. VII. Air Resistance A. Despite the fact that all objects are supposed to fall at the ______________________, you know that this is not always the case. 1. An oak leaf flutters __________________ to the ground, while an acorn ______________ straight down. 2. Objects falling through air experience a type of fluid friction called ____________________________. 3. Air resistance is an _________________. 4. The __________________ the surface area of an object, the ________________ the air resistance. That is why a leaf falls more slowly than an acorn. B. Air resistance increases with __________________. Eventually, the air resistance ______________ the force of gravity. Remember that when forces are balanced, there is no ___________________. VIII. Growth of Seedlings A. Have you ever noticed that most plants grow vertically—not sideways or downward? The reason is that plants respond to the __________________. This response is known as ______________. B. No matter how a young plant is positioned in the soil, its stem grows _____________ and its root grows ________________. C. Roots grow in the direction of the _________________, whereas stems grow in the ________________ direction. IX. Weight A. The force of gravity on a person or object at the surface of a planet is known as ______________. B. ______________ is a measure of the force of gravity on an object, and _____________is a measure of the amount of matter in that object. Since weight is a force, you can rewrite Newton’s second law of motion, Force = Mass × Acceleration, to find weight. Weight = Mass x Acceleration due to gravity ( W =mg) C. Weight is usually measured in ______________, mass in ______________, and acceleration due to gravity in ____________. So a 50-kilogram person weighs 50 kg × 9.8 m/s2 = 490 Newtons on Earth’s surface.