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FORCES The Dynamics of Newton’s Laws Physics is Dynamic We now shift our focus from kinematics (a description of motion) to dynamics (a study of what causes motion). Newton’s Laws are at the heart of the study of dynamics. Dynamics is the study of motion and the forces that cause that motion. The 4 Forces of Nature Gravitational Electromagnetic Strong Nuclear Weak Nuclear Common Forces in our problem-solving Weight Normal force Tension Friction The 4 Forces of Nature GravitationalCaused by anything that has mass. Really noticeable when there is a large mass- like a star or planet. It is the weakest of the forces but never goes away. The 4 Forces of Nature Electromagnetic- Caused by electric charges or magnetic poles. It never goes away, but is much, much stronger than gravity. The 4 Forces of Nature Strong Nuclear- Only acts within a very short distance such as on the atomic level. This is the force that holds the nucleus together. It is the strongest of the forces. The 4 Forces of Nature Weak Nuclear- Responsible for radioactive decay. This force has recently been integrated within the electromagnetic forces as one and the same. It is called the electro-weak force. Isaac Newton Isaac Newton (1642-1727), mathematician and physicist, is one of the foremost scientific intellects of all time. He was born at Woolsthorpe, in England. Newton outlined his Laws of Motion in his book the Principia, which included how and why objects move, the fundamental force gravity, the motion of fluids, and planetary motion. He invented the mathematics of Calculus. Published Opticks which explained the phenomena of the color spectrum and light as a particle. Newton’s Laws of Motion 1st Law of Motion An object at rest remains at rest, and an object in motion remains in motion, unless acted upon by a net external force Inertia is directly proportional to an object’s mass Newton’s Laws of Motion 2nd Law of Motion An acceleration is imparted on a mass by a non-zero net force. The acceleration is proportional to the force and in the same direction as the force. The acceleration is inversely proportional to the mass. F = ma Newton’s Laws of Motion 3rd Law of Motion For every action force there is an equal and opposite reaction force The opposing forces are called action-reaction pairs. Forces are Vectors A force is a push or a pull. Sometimes there is contact between the objects, but sometimes there is non-contact, like gravity. Force is a vector. It has magnitude and it has direction. We represent a vector on paper with an arrow drawn in the direction of the force. Free-Body Diagrams A diagram that only shows the forces that are acting on one object. The forces are shown by drawing arrows on a representation of the object. Shove Pull Push Tug Problem Solving The 2nd Law Steps in all your dynamics problem-solving Draw a free-body diagram. Write a F = ma equation for each dimension, both x and y (horizontal and vertical.) Pay attention to the signs of the vectors. Solve the equation for the unknown. Do any kinematics as necessary. Units of Force The units of force are derived from the units of mass and acceleration. SI units of force: Newton (N) = 1 kg•m/s2 English System unit is the Pound. Weight Weight is the force of gravity that acts on an object when it is on (or near) a large planetary-like body. Weight = mass x acceleration due to gravity On Earth: weight = mass x 9.8 m/s2 Symbolically: Fg = mg Or w = mg Weight is always directed toward the center of the earth which is the bottom of the page/board. Weight vs. Mass Mass is a universal quantity because the mass of an object remains the same no matter where it is measured in the Universe. It is the measure of inertia that an object has. Weight is dependent upon the celestial body that the object is near. It increases when on a larger planet and decreases on a smaller one. It also can change on the earth. The farther you are from the center of our planet, the less the object’s weight. Normal Force The normal force FN is the force that a surface applies to an object that is in contact with it. The normal force is always perpendicular to the surface-no matter what the case. Surface pushing up on box FN mg Tension Tension is the force that is applied to an object by a rope, string, cable, wire, or chain. Tension is always directed along the rope away from the object. T mg Friction Friction is the force that opposes motion. There is friction present everywhere, however, many times we consider the case where there is NO friction. The best we can do is outer space. Types of friction: Static friction Kinetic friction Air resistance Rolling friction Water resistance Static Friction vs. Kinetic Friction Static Friction The force that opposes an object before it is moved. Static friction increases with the amount of applied force until the object begins it’s motion It’s maximum value is: Fs ≤ sFN Kinetic Friction The force that opposes an object’s motion while it is moving. Kinetic friction is equal to: Fk = kFN Friction is dependent only on , coefficient of friction, and the normal force. Coefficients of Friction The coefficient of friction (static or kinetic), is a unit-less number that depends on the types of surfaces that are in contact. Values are found on reference table Terminal Velocity Ff When the air resistance equals the weight of the parachute and jumper, the acceleration stops, and he reaches terminal velocity, or constant speed. mg The Elevator Problem Constant velocity Upward acceleration Downward acceleration Free-fall In Summary All forces are vectors The 4 basic forces are gravitational, electromagnetic, and the strong and weak nuclear Newton’s 1st law- Law of Inertia. Newton’s 2nd Law- F=ma Newton’s 3rd Law- Action-Reaction Weight is the force of gravity. Normal force is always to the surface. Friction is a force that opposes motion.