PHYS 1443 – Section 001 Lecture #5AnnouncementsRelative Velocity and AccelerationForceMore ForceNewton’s First Law and Inertial FramesMassNewton’s Second Law of MotionExample 4.2Example for Newton’s 2nd Law of MotionGravitational Force and WeightNewton’s Third Law (Law of Action and Reaction)Example of Newton’s 3rd LawSlide 14Some Basic InformationFree Body Diagrams and Solving ProblemsSlide 17Example for Using Newton’s LawsTuesday, June 6, 2006 PHYS 1443-001, Summer 2006Dr. Jaehoon Yu1PHYS 1443 – Section 001Lecture #5Tuesday, June 6, 2006Dr. Jaehoon Yu•Newton’s Laws of Motion –Force–Newton’s Law of Inertia & Mass–Newton’s second law of motion–Gravitational Force and Weight–Newton’s third law of motion•Application of Newton’s Laws–Free-body diagrams–Application of Newton’s Laws–Motion without frictionToday’s homework is HW #3, due 7pm, Friday, June 9!!Tuesday, June 6, 2006 PHYS 1443-001, Summer 2006Dr. Jaehoon Yu2Announcements•Quiz this Thursday, June 8–At the beginning of the class–CH 1 – what we cover tomorrow•Mail distribution list–8 of you have been added to the list–Extra credit•5 points if done by Today, June 6•3 points if done by Thursday, June 8Tuesday, June 6, 2006 PHYS 1443-001, Summer 2006Dr. Jaehoon Yu3Relative Velocity and AccelerationThe velocity and acceleration in two different frames of references can be denoted, using the formula in the previous slide:OFrame Sr’O’Frame S’v0v0trr�=rGalilean transformation equation What does this tell you?The accelerations measured in two frames are the same when the frames move at a constant velocity with respect to each other!!!The earth’s gravitational acceleration is the same in a frame moving at a constant velocity wrt the earth.drdt�=rv�=rdvdt�=r0, when is constanta a v�=r r r0drvdt-rr0v v-r r0dv dvdt dt-r r0r v t-r rThe reference frame that moves at a constant velocity is called the Inertial Frame of Reference!!Tuesday, June 6, 2006 PHYS 1443-001, Summer 2006Dr. Jaehoon Yu4ForceWe’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 any change in 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.!!Tuesday, June 6, 2006 PHYS 1443-001, Summer 2006Dr. Jaehoon Yu5More ForceThere 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 Force?A calibrated spring whose length changes linearly with the force exerted .Forces are vector quantities, so addition of multiple forces must be done following the rules of vector additions.Tuesday, June 6, 2006 PHYS 1443-001, Summer 2006Dr. Jaehoon Yu6Newton’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 higher force.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 constant velocity is called an Inertial FrameWhat does this statement tell us? •When no force is exerted on an object, the acceleration of the object is 0. •Any isolated object, the object that do not interact with its surrounding, is either at rest or moving at a constant velocity.•Objects would like to keep its current state of motion, as long as there is no 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!!Tuesday, June 6, 2006 PHYS 1443-001, Summer 2006Dr. Jaehoon Yu7MassMass: A measure of the inertia of a body or quantity of matter•Independent of the object’s surroundings: The same no matter where you go.•Independent of 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 an object the bigger the inertia gets!!It is harder to make changes of motion of a heavier object than the lighter ones.Note that mass and 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.21aaTuesday, June 6, 2006 PHYS 1443-001, Summer 2006Dr. Jaehoon Yu8Newton’s Second Law of MotioniiF =�rThe 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 should also satisfy:From the above vector expression, what do you conclude the dimension and unit of 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) yiiymaF ziizmaF 1N �]][[2LTM2/ smkg 21 /kg m s14lbsmarNewton’s 2nd Law of MotionTuesday, June 6, 2006 PHYS 1443-001, Summer 2006Dr. Jaehoon Yu9Acceleration Example 4.2What 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
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