Kinematics vs Dynamics Kinematics Dynamics Explains how objects move Explains why objects move A force a push or a pull Classes of Forces Contact forces involve physical contact between two objects Field forces act through empty space No physical contact is required Coulomb Force magnetic force gravitational force Aristotle 384 322 BCE The natural state of an object near the surface of the earth is to be at rest Incorrect A force push or pull is required to maintain the motion of an object Incorrect Galileo 1564 1642 The natural state of an object near the surface of the earth is to travel at a constant speed in a straight line or to be at rest A force changes the motion of an object a force is not required to maintain the motion of the object A force changes the motion of an object Changes the speed Speed up Slow down Changes the direction Changes the speed and direction This type of change in motion is called acceleration Inertia Galileo A force causes a change in motion One finding the same force applied to different objects causes different results Conclusion every object has some resistance to change in motion Inertia is the natural tendency of an object to remain at rest or to move at a constant speed in a straight line Mass and Inertia Mass is a measure of an object s inertia The two are essentially interchangeable An object with a large mass has a great amount of inertia a great tendency to remain at rest or moving at a constant speed in a straight line Mass A measure of how much material is present Measured in units of kilograms kg 1 kg 1 000 grams Mass vs Weight Mass is the quantity of matter in an object The mass of an object does not depend on location Measured in kilograms Weight is the force on the object due to gravity The weight of an object does depend on location Measured in Newtons N or pounds lb Mass and weight are directly proportional to one another On Earth 1 kg 10N 2 2 lb Calculating Weight from Mass weight mass X acceleration due to gravity or w mg Example What is the weight of a 4 5 kg cat on the Earth and on the moon Earth w mg 4 5 x 10 45 N Moon w mg 4 5 x 1 67 7 5 N What is the mass of the 4 5 kg cat on the Earth and on the moon 4 5 kg Newton s Laws of Motion Newton s First Law the law of inertia A restatement of Galileo s concept of inertia States that every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero force A change in motion requires a net force Example dishes remain in their initial state of res when a tablecloth is whipped from beneath them Inertia is the property of objects to resist changes in motion Newton s Second Law of Motion F ma The acceleration produced by a net force on an object is directly proportional to the net force in in the same direction as the net force and is inversely proportional to the mass of the object A net force causes a change in the motion based upon the mass of the object Less mass more acceleration inversely proportional More force more acceleration proportional F ma a acceleration m s 2 F net force F N m mass kg Net Force and Newton s 2nd Law Multiple forces may act on an object at the same time The net force is the sum of all the forces acting on the object You must treat the up down motion separate from the left right motion Equilibrium net force 0 Newton s First Law Example George pushes a box across a frictionless level surface with a force of 1 2 N The box has a mass of 4 kg What is the acceleration of the box F ma 1 2 4a a 0 3 m s 2 How fast is the box moving after 10 seconds v v i at v 0 0 3 10 v 3 m s Example 2 Lisa pushes a large crate of books across the floor with a force of 5 0 N A frictional force of 1 5 N opposes the motion If the crate has an acceleration of 0 5 m s 2 what must be the mass of the crate F ma 5 1 5 m 0 5 3 5 m 0 5 m 7 kg Types of Forces Support forces normal force Friction Tension Support forces Support force normal force is a force that is exerted when two objects are in contact with one another This force is directly perpendicular to the area of contact between the surfaces The harder that two surfaces push together the greater the normal force is Friction Friction occurs when two objects rub together Comes from surfaces being rough Is always directed opposite the motion Depends on the materials that are rubbing coefficient of friction Depends on how hard the surfaces push together support or normal force N f Static Friction No relative motion between the objects Force applied does not move due to friction Kinetic Friction Surfaces are moving with respect to one another Force applied moves at a speed but friction is still acting on the object In general Static Friction Kinetic Friction Harder to get an object in motion than to keep it in motion moving at a constant speed Tension Equilibrium Force that acts through a rope string or cord A system is said to be in equilibrium when the net force acting on the object is zero The object has no change in motion At rest Moving at a constant speed in a straight line F 0 The symbol stands for the sum or net and F stands for forces force has both magnitude and direction Spring Scale in Equilibrium A spring inside the scale stretches until the scale reads the appropriate weight The stretched spring is under a stretching force called tension Using the Equilibrium Rule You exert a 50N horizontal force on a heavy create that weighs 100N and the crate will not move Is there a friction acting on the crate If so how much and in what direction does the frictional force act on the crate Yes a frictional force of 50N in the opposite direction of the exerted force What is the coefficient of static friction s f 50N N 100N f N 50 100 s 0 5 no units You exert a 40N horizontal force on a heavy crate same crate as before causing it to move at a constant speed along the floor How much and in what direction does the frictional force act on the crate 40N in the opposite direction of the exerted force What is the coefficient of kinetic friction f N 40 100 0 4 Chapter 4 Newton s 3rd Law of Motion Air Resistance Newton s 3rd Law of Motion Whenever one object exerts …