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# phys1111ch4notes - CHAPTER 4 - FORCES AND NEWTON'S LAWS OF...

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CHAPTER 4 - FORCES AND NEWTON'S LAWS OF MOTION A force is defined as a push or pull. Any physical quantity that causes a change in speed or direction. Contact forces require the objects involved to be touching. An example would be a bat striking a ball. Action at a distance forces exist regardless of contact. Gravity and magnetic forces are examples. Forces are vector quantities with direction and magnitude. Example - your weight has a magnitude(number of pounds you weigh) and its direction is always down. Mass is a measure of the amount of matter of which an object is composed. Determined by the number of protons, electrons, and neutrons within the object. Newton's First Law of motion is called the Law of Inertia. It states: AN OBJECT MAINTAINS A CONSTANT STATE OF MOTION(VELOCITY) UNLESS IT IS ACTED UPON BY A NET FORCE. This means an object at rest stays at rest and an object moving continues to do so unless some force acts on it to cause a change. Remember that net force means the vector sum of all the forces acting on an object. Inertia is generally considered to be the measure of an object's tendency to resist changes of motion when acted upon by a net force. It is directly related to the mass of the object.

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The SI unit of mass is the Kilogram. Remember weight is a force and is expressed in the unit of force, the Newton. Inertial reference frame - Newton's Laws are valid. Non-inertial Reference Frame - Newton's Laws are not valid. In an inertial reference frame, the origin must be moving at a constant velocity.(No acceleration) If the origin is being accelerated, like in a car that suddenly stops, things begin to move with no apparent force causing them to accelerate. The earth's surface is an approximation of an inertial reference frame for small distances(< 1 Km). Newton's Second Law of Motion describes the relationship among force, mass, and acceleration. It states: THE ACCELERATION OF AN OBJECT DEPENDS DIRECTLY ON THE NET EXTERNAL FORCE APPLIED AND INVERSELY ON THE MASS OF THE OBJECT. It says if you increase the net force and keep the mass constant, the acceleration is larger. It also says if you increase the mass and keep the force constant, the acceleration decreases. The equation that results from this is: F = MA If mass is in kilograms and acceleration is in meters per second squared, the resulting force unit is called a Newton(named for Sir Isaac Newton). A newton is defined as the force required to accelerate a 1 Kg mass 1 meter per second per second.
When solving problems involving forces acting on an object, drawing a freebody force diagram is helpful. Example

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## This note was uploaded on 05/01/2008 for the course PHY 1408 taught by Professor Russell during the Spring '07 term at Baylor.

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phys1111ch4notes - CHAPTER 4 - FORCES AND NEWTON'S LAWS OF...

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