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Transcript

Deciphering Newton’s First Law “Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.” —Sir Isaac Newton Principia Mathematica (1687) Write: In your own words, what does this mean? Unless an unbalanced force acts on an object, An object at rest stays at rest An object in motion stays in motion Isaac Newton, 1642-1727 Galileo Galilei: “I found it first!” I call it “inertia.” Inertia is an object’s resistance to changing its motion. I wrote: “A body moving on a level surface will continue in the same direction at constant speed unless disturbed.” Sure, Newton stated it more thoroughly, but they don’t call me “the father of modern physics” for nothing! Galileo Galilei, 1564-1642 Newton’s 1st Law and You Wear seat belts! Mass—A Measure of Inertia • 2.2 pounds near Earth’s surface. Mass—A Measure of Inertia • Mass is the quantity of matter in an object. • Weight is the force of gravity on an object. A force is a push, pull or twist Examples of contact forces Applying a force can change an object’s velocity. accelerate Applying a force can cause an object to _________. Types of Forces Contact forces: interactions between objects that touch applied force Non-contact forces: attract or repel, even from a distance magnetic force spring force electric force drag force gravitational force frictional force Forces around us • What happens when forces are in equilibrium? Common Forces Force Gravity Symbol Fg Function Force of attraction between all objects Normal FN Force perpendicular to the surfaces of objects in contact Tension FT Force exerted by materials, such as ropes, fibres, springs, and cables, that can be stretched Friction Ff Force that resists motion or attempted motion between objects in contact Kinetic Friction FK Force that acts against an object’s motion Static Friction FS Force that tends to prevent a stationary object from starting to move Air Resistance Fair Frictional force that opposes an object’s motion through air Applied Force Fapp Any contact force not previous included Unbalanced Forces 3 Newtons 7 Newtons drag (air resistance) What two forces are acting upon the umbrella? weight (gravitational force) System Diagrams and FBD • A system diagram is a sketch of all of the objects involved in a situation. • A Free Body Diagram (FBD) is a drawing of a single object that shows only the forces (as vectors) acting on that object. – Always indicate the +x and +y directions System Diagrams vs. FBD Check Your Understanding • You toss a ball vertically upward. Draw an FBD of the ball just before it leaves your hand. When net force is zero… Newton’s Second Law of Motion • An object accelerates in the direction of the net force. • The acceleration is directly proportional to the net force and inversely proportional to the object’s mass. 2nd Law Newton’s Second Law of Motion • F = ma a = F m • Acceleration increase as F increases • Acceleration decreases as m increases 2nd Law • When mass is in kilograms and acceleration is in m/s/s, the unit of force is in Newtons (N). • One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second. 2nd Law (F = m x a) • How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second? • Write the formula • F=mxa • Fill in given numbers and units • F = 1400 kg x 2 meters per second/second • Solve for the unknown • 2800 kg-meters/second/second or 2800 N Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces. • We know that objects with different masses accelerate to the ground at the same rate. • However, because of the 2nd Law we know that they don’t hit the ground with the same force. F = ma F = ma 98 N = 10 kg x 9.8 m/s/s 9.8 N = 1 kg x 9.8 m/s/s Check Your Understanding • 1. What acceleration will result when a 12 N net force applied to a 3 kg object? A 6 kg object? • 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass. • 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? • 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec? Check Your Understanding • 1. What acceleration will result when a 12 N net force applied to a 3 kg object? 12 N = 3 kg x 4 m/s/s • 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass. 16 N = 3.2 kg x 5 m/s/s • 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? 66 kg-m/sec/sec or 66 N • 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec? 9800 kg-m/sec/sec or 9800 N Check Your Understanding • The mass of a hot-air balloon, including the passengers, is 9.0 x 102 kg. The density of the air inside the balloon is adjusted by adjusting the heat output of the burner to give a buoyant force on the balloon of 9.9 x 103 N [up]. Determine the vertical acceleration of the balloon. Check Your Understanding Solution • Fnet = Fapp – Fg = 9900 N – 8820 N = 1080 N • a = Fnet/m = 1080 N / 900 kg = 1.2 m/s2 [up]