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a physical quantity Physics is the study of the material world and the rules that govern its behavior. time • It is based on observation of nature. • It identifies discernible patterns in nature. • It allows one to make predictions. However, it does not explain nature. It only describes nature, and with limited accuracy. Questions to think about • What is the “position” of an object • What is “time” • What do we mean by “movement” Over here !!! center of mass A configuration of selected points associated with an object with respect to other objects in space is called the position of the object. Time is a quantity associated with changes in the universe. A repetitive process (a clock) is used to assign a value to each configuration of the universe (instant). The second has been defined (since 1967) as the duration of 9,192,631,770 oscillations of a particular radiation emitted from 133Cs atoms. Physics Laboratory, NIST Boulder, Colorado René Descartes and the fly z z θ r y René Descartes (1596–1650) philosopher, mathematician and physicist x ϕ x y Cartesian Coordinates of position: ( x, y, z ) Polar Coordinates of position: ( r, θ, ϕ) Position is a three dimensional vector quantity. Movement y If the position of an object changes with time, the objects moves. (The object is in motion). x z Comment 1 The motion of an object is relative. Objects always move with respect to other objects (reference frame). Comment 2 A universal stationary reference frame does not exist! Speed and Velocity 0s Speed (of an object) is defined as the distance traveled by the object (along a path) in an infinitesimal time interval divided by the time. y 2 cm 0.1 s 0.2 s 0.3 s 0.4 s 0.5 s 0.6 s x z Velocity is defined as the rate at which an object changes its position. It contains information about the speed of the object and the direction of its motion. speed = dis tan ce 2cm cm = = 20 time 0.1s s velocity = 20 cm parallel to x - axis s Velocity and Position Velocity of an object makes it possible to predict its future position. 500 m 100 m/s 750 m 150 m/s What will be the position of the plane in five seconds? Where is the plane going to be in the next five seconds after changing the velocity to 150 m/s while maintaining its direction? The Average Value of a (varying) physical quantity is such a constant value of that quantity that in a considered amount of time the effect of the average value is equal to the effect of the varying values. Example: In five seconds, a fly completes a circle with a circumference of 100 cm. What was the average speed of the fly? 1m = v av ⋅ 5s v av = 20 cm s What was its average velocity? 0m = v av ⋅ 5s v av = 0 m s Newton’s First Law of Motion It is possible to find such a reference frame, that an object free of external influences moves with constant velocity or remains at rest. Such a reference frame is called an inertial reference frame. In many practical situations, the reference frame of the earth can be considered to be inertial. The tendency of an object to maintain its state of motion (in an inertial reference frame) is called inertia. v earth pulls ice pushes Newton’s Apple Sir Isaac Newton (1642-1727) - English scientist; invented the mathematical basis of calculus; formulated the laws of motion; described gravitational interaction Sir Isaac Newton (1642 - 1727) inertia puzzle A rotary lawn mower spins its blade rapidly over the lawn and cuts the top of the grass off. Would the blade still cut the blades of grass if they weren’t attached to the ground? It would! In fact, when the clippings are not collected, the blade cuts (mulches) them into smaller pieces. interactions The velocity (in an inertial reference frame) of an object can change only by interaction with other objects. Several physical quantities (such as force, work, power, torque) are used to describe interactions. Changing Velocity When an object changes its velocity it accelerates. The object accelerates when its speed is increasing or decreasing. The objects accelerates when the direction of its velocity is changing. Acceleration The rate at which the velocity of an object varies is called the acceleration of the object. The acceleration of an object makes it possible to predict the object’s future velocity. 4 m/s Example. At 1 s: velocity = 4 m/s to the left 4 m/s acceleration = 10 m/s2 down 2 m/s change in velocity in 0.2 s = 10m/s2 ⋅ 0.2s down = 2m/s down At 1.2 s: the velocity of the ball is 4 m/s to the left and 2 m/s downwards 4 m/s Puzzle: What will be the velocity of the ball at 1.4s? 1.2 s 1.4 s 4 m/s 1s 10 m/s2 10 m/s2 May the force be with you! Force is a measure of interaction between objects. • Force can only be exerted on an object. • Only objects can exert a force. • Force is always associated with a type of interaction. Example: W - the earth exerts a gravitational force on the ball. N w N - the floor exerts a support force on the ball. Force and Acceleration small force USA USA Lucas Oil Lucas Oil large force If an unbalanced force is exerted on an object, the stronger the force the greater the object’s acceleration. Mass is an attribute of an object describing the object’s inertia. It is easy to accelerate a baseball. It has a small mass. It is difficult to accelerate a bowling ball. It has a large mass. Newton’s Second Law of Motion Fnet = m ⋅ a The resultant (net) force exerted on an object is equal to the product of that object’s mass and its acceleration a N1 N2 W W a Measure for Measure A measurement is a procedure in which a numerical value is assigned to a physical quantity. one three , two four In every measurement, the given physical quantity is being compared to a standard. SI Units Systéme Internationale d’Unités SI units are worldwide the most accepted system of units • based on powers of ten (decimal) • constructed on a few basic units (the meter, the kilogram, and the second) 1 kg