1. Newton’s Laws of Motion

2. 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. First Law of Motion: “The Law of Inertia” 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. click for web page click for applet

3. 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 Second Law of Motion: “The Law of Acceleration” What does this law tell us? Objects that are not in equilibrium will accelerate. 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. “sigma” = sum F and a are vectors Net force (sum of all forces) on an object causes acceleration. click for applet

4. The Definition of Force 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. 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. “If you insist upon a precise definition of force, you will never get it!” - Richard Feynmann

5. Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Third Law of Motion: “The Law of Action-Reaction” 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 1 from object 2 force on object 2 from object 1

6. Newton’s Third Law Example What are the action and reaction forces in this example?

7. Newton’s Third Law Example What are the action and reaction forces in this example?

8. 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

9. Newton’s Third Law Example What are the action and reaction forces in this example?

10. Newton’s Third Law Example

11. Mass versus Weight Weight is defined as a force caused by gravity acting on a mass. Mass is defined as an amount or quantity of matter. Mass is universal; it doesn’t depend on location. Weight is local; it depends on gravity. Mass Weight massforce Metrickilogramnewton Britishslugpound CGSgramdyne

12. Spring Force vertical spring The force associated with a stretched spring, or any elastic material. Hooke’s Law The spring force varies linearly with the amount of displacement. scalar formvector form Spring Force, F s FsFs forcedisplacement Spring constant has unit of newtons/meter

13. Inertial and Gravitational Mass 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. Relates to how a mass responds to the force of gravity (also called a field force). 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. If you lift up a stalled car you are testing its gravitational mass. Inertial mass 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 mass gravitational mass

14. A contact force, often called a support force, that acts perpendicular to the surfaces in contact. Normal Force, F n A pulling force in strings, ropes, cables, etc. Tension, F T Normal Force, Tension, and Applied Force On a level surface, the normal force will balance the weight of an object, as long as no other forces act vertically. Normal means perpendicular. Tension force always pulls away from a mass (opposite of compression). FnFn PHYSICS F g = mg PHYSICS FTFT rope Applied Force, F a An applied force is any external force. PHYSICS FaFa

15. Friction Forces -coefficient of the surfaces in contact,  A contact force that always acts parallel to the surfaces in contact, and always opposes motion. Friction is dependent on: Static friction opposes the intended 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. FaFa PHYSICS FsFs -normal force, F n FaFa PHYSICS FkFk velocity Kinetic friction is less than static friction. Friction arises from molecular bonding between surfaces FsFs book pulled wheel driven book dragged

16. surfaces in contact ss kk leather-soled shoes on wood0.30.2 rubber-soled shoes on wood0.90.7 climbing boots on rock1.00.8 shoes on ice0.10.05 auto tires on dry concrete1.00.8 auto tires on wet concrete0.70.5 auto tires on icy concrete0.30.02 waxed skis on dry snow0.080.04 waxed skis on wet snow0.140.1 wood on wood0.40.2 glass on glass0.90.4 steel on steel - dry0.60.4 steel on steel - greased0.10.05 synovial joints in humans0.010.003 Coefficients of Friction

17. 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). When the sum of all forces is not equal to zero, the object accelerates in the direction of the resultant force. FaFa PHYSICS FsFs FaFa FkFk acceleration FnFn FgFg Unbalanced Forces FnFn FgFg click for web page