PHYS 1441 – Section 002 Lecture #8AnnouncementsSlide 3Reminder: Special Project for Extra CreditForceMore ForcesNewton’s First Law and Inertial FramesMassNewton’s Second Law of MotionUnit of the ForceFree Body DiagramExample 4.3Ex. Pushing a stalled carWhat is the acceleration the car receives?Vector Nature of the ForceEx. Stranded man on a raftSlide 17Now compute the acceleration components in x and y directions!!Example for Newton’s 2nd Law of MotionMonday, Oct. 4, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu1PHYS 1441 – Section 002Lecture #8Monday, Oct. 4, 2010Dr. Jaehoon Yu•What is the Force?•Newton’s Second Law•Free Body Diagram•Newton’s Third Law•Categories of forcesToday’s homework is homework #5, due 9pm, Tuesday, Oct. 12!!Announcements•Quiz #2 –Beginning of the class Wednesday, Oct. 6–Covers: Covers: CH. 3.5 – what we finish todayMonday, Oct. 4, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu2Monday, Oct. 4, 2010 4Reminder: Special Project for Extra Credit•Show that the trajectory of a projectile motion is a parabola!!–20 points–Due: Wednesday, Oct. 6–You MUST show full details of your OWN computations to obtain any credit•Beyond what was covered in page 8 of this lecture note!!PHYS 1441-002, Fall 2010 Dr. Jaehoon YuMonday, Oct. 4, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu5ForceWe’ve been learning kinematics; describing motion without understanding what the cause of the motion is. Now we are going to learn dynamics!!Can someone tell me what FORCE is?FORCE is what causes an object to move.FORCEs are what cause changes to the velocity of an object!! The above statement is not entirely correct. Why?Because when an object is moving with a constant velocity no force is exerted on the object!!!What does this statement mean?When there is force, there is change of velocity!!What happens if there are several forces being exerted on an object?Forces are vector quantities, so vector sum of all forces, the NET FORCE, determines the direction of the acceleration of the object.F1F2NET FORCE, F= F1+F2When the net force on an object is 0, it has constant velocity and is at its equilibrium!!What does force cause?It causes an acceleration.!!Monday, Oct. 4, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu6More ForcesThere are various classes of forcesContact Forces: Forces exerted by physical contact of objectsExamples of Contact Forces: Baseball hit by a bat, Car collisionsField Forces: Forces exerted without physical contact of objectsExamples of Field Forces: Gravitational Force, Electro-magnetic forceWhat are possible ways to measure strength of a force?A calibrated spring whose length changes linearly with the force exerted .Forces are vector quantities, so the addition of multiple forces must be done following the rules of vector additions.Monday, Oct. 4, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu7Newton’s First Law and Inertial FramesAristotle (384-322BC): A natural state of a body is rest. Thus force is required to move an object. To move faster, ones needs larger forces.Galileo’s statement is formulated by Newton into the 1st law of motion (Law of Inertia): In the absence of external forces, an object at rest remains at rest and an object in motion continues in motion with a constant velocity. A frame of reference that is moving at a constant velocity is called the Inertial FrameWhat does this statement tell us? •When no net force is exerted on an object, the acceleration of the object is 0. •Any isolated object, the object that do not interact with its surroundings, is either at rest or moving at a constant velocity.•Objects would like to keep its current state of motion, as long as there are no net force that interferes with the motion. This tendency is called the Inertia.Galileo’s statement on natural states of matter: Any velocity once imparted to a moving body will be rigidly maintained as long as the external causes of retardation are removed!!Is a frame of reference with an acceleration an Inertial Frame?NO!Monday, Oct. 4, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu8MassMass: A measure of the inertia of a body or the quantity of matter•Independent of the object’s surroundings: The same no matter where you go.•Independent of the method of measurement: The same no matter how you measure it.12mm�The same forces applied to two different masses result in different acceleration depending on the mass.The heavier the object, the bigger the inertia !!It is harder to make changes of motion of a heavier object than a lighter one.Note that the mass and the weight of an object are two different quantities!!Weight of an object is the magnitude of the gravitational force exerted on the object. Not an inherent property of an object!!! Weight will change if you measure on the Earth or on the moon but the mass won’t!!21aaUnit of mass?kgMonday, Oct. 4, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu9Newton’s Second Law of MotioniiF =�urThe acceleration of an object is directly proportional to the net force exerted on it and is inversely proportional to the object’s mass. How do we write the above statement in a mathematical expression?xiixmaF Since it’s a vector expression, each component must also satisfy:yiiymaF ziizmaF marNewton’s 2nd Law of Motion Fiuri∑a =r mFrom this we obtainMonday, Oct. 4, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu10Unit of the ForceFrom the vector expression in the previous page, what do you conclude the dimension and the unit of the force are?]][[ am]][[]][[][2LTMamForceThe dimension of force is The unit of force in SI is For ease of use, we define a new derived unit called, Newton (N) 1N �[ ]M2/ smkg 21 /kg m s14lbsiiF =�urmar( )kg2ms� �=� �� �2[ ]LT-Monday, Oct. 4, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu11A free-body-d iagram is a diagram that represents the object and the forces that act on it.Free Body DiagramMonday, Oct. 4, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu12Acceleration Example 4.3What constant net force is required to bring a 1500kg car to rest from a speed of 100km/h within a distance of 55m?This is a one dimensional motion. Which kinetic formula do we use to find acceleration?What are given?Thus, the force needed to stop the car isxF hkmvxi/100 ifxxixfxxavv 222What do we need to know to figure out the force?Acceleration!! Initial speed:sm
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