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Transcript
Chapter 6 Notes-Forces Great Scientists and their thoughts on Motion Aristotle-old theory Copernicus Galileo-his theories are still used today Force - A push or a pull on an object The object or group of objects that get pushed or pulled is called the system. The world around it that exerts the force is called the environment. There are 2 groups of forces. 1. Contact Force-touching Examples: 2. Long range force-exerted without contact Examples: Types of Forces 1. Friction Force-contact force that acts to oppose a sliding motion between surfaces 2. Normal Force-Contact force exerted by a surface onto an object 3. Spring Force-Restoring Force, the push or pull a spring exerts on an object 4. Tension Force-the pull exerted by a string, rope or cable when attached to a body and pulled tight 5. Thrust Force-A general term for the forces that move objects such as rockets, planes, cars and people 6. Weight Force-long range force due to gravitational attraction between 2 objects (generally the earth and a 2nd object) Characteristics of Forces Net forces will change the state of motion of an object Can be exerted through long distances Occur in Pairs In each pair, each force works opposite of each other Mass vs. Weight The force called WEIGHT is a gravitational force. Weight is not mass!!! Mass-A measure of the amount of material in an object (it is not a force) The mass of an object will be the same in any location. Mass is measured in kilograms. Weight-A measure of the gravitational force acting on an object The weight of an object depends on its location and the amount of gravitational pull at that location. Weight is measure in Newtons. W=Fg A Newton is 1 kg*m/s2 F=mg So, the force of an object measured in Newtons is equal to the mass of an object times the amount of gravity acting on that mass. Examples 1. The mass of an object is 23.2 kg and the amount of gravity acting on the object is 9.8 m/s2. What is the Force of Weight of the object? 2. The force of weight of an object is -220 Newtons and the amount of gravity pulling on the object is -9.8 m/s2. What is the mass of the object? 3. Determine Your Ideal Mass/Weight You already know how much you “weigh” in pounds Lets try to figure out your ideal weight on earth in kilograms and Newtons, then on the moon. 1 kg = 2.2 lbs There is 1/6 less gravity on the moon than on earth. Vectors Because forces have magnitude and direction, and can be combined with other forces, we use VECTORS to draw forces. Net Force-the combination of all the forces acting on an object. Adding Subtracting Equilibrium Objects move when a net force is applied to it. It is the Net Force that changes an objects state of motion. Adding Vectors Subtracting Vectors Remember, Forces are measured in Newtons because Isaac Newton came up with the idea about forces. He wrote 3 statements concerning forces-they are called Newton’s Laws of Motion. Newton’s First Law “An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an outside force” Objects tend to keep doing what they are doing. Forces that have an influence on this law are friction and intertia. The Net Force changes the velocity of an object. So, something will keep doing what it is doing unless some Net Force acts on it. Newton’s Second Law “The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object” The combination of forces acting on an object is the net force. Acceleration depends on the net force. To increase the acceleration of an object you must increase the net force acting on it. So, the acceleration is directly proportional to the net force. Acceleration depends on the mass being pushed or pulled. They are inversely proportional to each other.