Transcript The Force
“The Force” “An energy field created by all living things. It surrounds us, penetrates us, and binds the galaxy together.” The Force has two components: Light side Dark side The Real Force Something that causes an object’s motion to change (causes acceleration). A “push” or a “pull.” Common Examples of forces: Gravity fields, pushing on something, compressing a spring, a magnetic field, tension, friction, and the “normal” force. Units are Newtons (N) Newton’s Laws of Motion First Law of Motion: “The Law of Inertia” An object at rest remains at rest, and an object in motion continues in motion with constant velocity unless the object experiences a net external force. click for web page click for applet What does this law tell us? • Objects in equilibrium do not accelerate. Static equilibrium (rest) and equilibrium (constant velocity) are both the result of an object with zero net force. • Only a frame of reference (F.O.R) can distinguish between rest and constant velocity. An object at rest in one F.O.R can have constant velocity in another (F.O.R) • It defines the kind of frame of reference, called an inertial frame of reference, in which Newton’s Laws of Motion apply. Galileo’s Unique Idea Objects don’t need a force to keep moving! Every object naturally wants to maintain its state of motion or rest INERTIA! (resistance to change in motion) Refined by Newton in 1800’s: Basic Info: Inertia Inertia depends on: Mass Shape/Mass Distribution of object- rotational inertia Solid Cylinder (like a wheel of cheese,) Hoop (like a bicycle tire) Inertia does NOT depend on: Velocity/Speed of object It takes the same amount of force to speed a bus up as to slow it down! Second Law of Motion: “The Law of Acceleration” 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 v v F ma “sigma” = sum F and a are vectors What does this law tell us? • Objects that are not in equilibrium will accelerate. • Net force (sum of all forces) on an object causes acceleration. • Note the difference between a force and a net force. A good analogy is to compare deposits/withdrawals into a bank account with the account balance. click for applet The Definition of Force “If you insist upon a precise definition of force, you will never get it!” - Richard Feynmann Forces are not directly observable, but the effect of force is perceived . Newton’s Second Law defines force. • A newton is defined as the force required to accelerate one kilogram of mass at a rate of one meter per second squared. meter 1 newton 1 kilogram 1 second 2 • A newton is the metric equivalent of the pound. Both are units of force, not mass. • A newton converts to a little less than a quarter pound. 1 newton 0.225 pound 1 pound = 4.45 newton Inertial and Gravitational Mass Inertial mass Relates to how a mass responds to an external force (also called a contact force). If you push a stalled car into motion you are testing its inertial mass. Gravitational mass Relates to how a mass responds to the force of gravity (also called a field force). If you lift up a stalled car you are testing its gravitational mass. r F ma inertial mass Fg mg gravitational mass In the equation for weight, g is no longer considered the acceleration due to gravity, but rather the gravitational field strength, with units of newtons/kilogram. Inertial and gravitational masses have been tested and are believed to always be equal in amount. This is why all objects freefall at the same rate of acceleration. Mass versus Weight Mass Mass is classical defined as an amount or quantity of matter. The modern definition is the amount of inertia object possesses. Mass is universal; it doesn’t depend on location. Weight Weight is defined as a force caused by gravity acting on a mass. Weight is local; it depends on gravity. weight = mass gravity Fg mg mass force Metric kilogram newton British slug pound CGS gram dyne Spring Force Spring Force, Fs The force associated with a stretched spring, or any elastic material. Hooke’s Law The spring force varies linearly with the amount of displacement. Fs kx vector form scalar form Fs force from spring v x displacement k spring constant Spring constant has unit of newtons/meter vertical spring displacement v Fs kx Fs k force 1 slope Normal Force, Tension, and Applied Force Normal Force, Fn A contact force, often called a support force, that acts perpendicular to the surfaces in contact. Normal means perpendicular. On a level surface, the normal force will balance the weight of an object, as long as no other forces act vertically. Fn PHYSICS Fg = mg Tension, FT A pulling force in strings, PHYSICS ropes, cables, etc. Tension force always pulls away from a mass (opposite of compression). Applied Force, Fa An applied force is any external force. rope FT Fa PHYSICS Friction Forces Friction arises from molecular bonding between surfaces A contact force that always acts parallel to the surfaces in contact, and always opposes motion. Fs PHYSICS Friction is dependent on: book pulled - normal force, Fn Ff Fn - coefficient of the surfaces in contact, Fs Static friction opposes the intended wheel driven motion of two surfaces in contact but at rest relative to one another. Kinetic friction opposes motion of two surfaces in contact that are moving relative to one another. Kinetic friction is less than static friction. Fk velocity PHYSICS book dragged Fa Fa Coefficients of Friction surfaces in contact s k leather-soled shoes on wood 0.3 0.2 rubber-soled shoes on wood 0.9 0.7 climbing boots on rock 1.0 0.8 shoes on ice 0.1 0.05 auto tires on dry concrete 1.0 0.8 auto tires on wet concrete 0.7 0.5 auto tires on icy concrete 0.3 0.02 waxed skis on dry snow 0.08 0.04 waxed skis on wet snow 0.14 0.1 wood on wood 0.4 0.2 glass on glass 0.9 0.4 steel on steel - dry 0.6 0.4 steel on steel - greased 0.1 0.05 synovial joints in humans 0.01 0.003 Free Body Diagrams A free body diagram identifies all action forces on an object so that the resultant force can be determined. Balanced Forces When the sum of all forces is equal to zero the object does not accelerate (at rest or constant velocity). Fs Fn PHYSICS v F 0 Fg Unbalanced Forces When the sum of all forces is not equal to zero, the object accelerates in the direction of the resultant force. v v F ma Fa click for web page Fk Fn PHYSICS acceleration Fg Fa Basic Info: Force Diagrams Definition: A Diagram that shows all the forces acting on a body Does NOT include forces exerted by the body! Forces are drawn as vectors. Free Body Diagram Simple drawing of all forces working on an object or system Use a box or dot to represent the object or system All forces move away from the box. Remember: gravity will always affect an object and so Fg will always be in a F.B.D! Third Law of Motion: “The Law of Action-Reaction” Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. force on object 1 from object 2 v F1,2 F2,1 What does this law tell us? • There is no isolated force in the universe. Instead every force has a matching "counter-force”. Forces always come in pairs. • Action-reaction forces always act on different bodies. They do not combine to give a net force and cannot cancel each other. force on object 2 from object 1 Newton’s Third Law Example What are the action and reaction forces in this example? Newton’s Third Law Example What are the action and reaction forces in this example? Newton’s Third Law Example That Professor Goddard…does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react - to say that would be absurd. Of course, he only seems to lack the knowledge ladled out daily in high schools. The New York Times, January 13, 1920 Further investigation and experimentation have confirmed the findings of Isaac Newton in the 17th century, and it is now definitely established that a rocket can function in a vacuum as well as in an atmosphere. The Times regrets the error. The New York Times, July 17, 1969 Newton’s Third Law Example What are the action and reaction forces in this example? Newton’s Third Law Example