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1 Physical Science, 6e Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 2 Motion Motion can be defined as the act or process of changing position relative to some reference during a period of time Slide 2 2 Overview Description Position Velocity Acceleration Applications Horizontal motion on land Falling objects Compound (2-D) motion Explanation Forces Newtons laws Applications Momentum Circular motion Newtons law of gravitation Slide 3 3 Measuring motion Two fundamental components: Change in position Change in time Three important combinations of length and time: 1.Speed 2.Velocity 3.Acceleration Slide 4 4 Speed Change in position with respect to time Average speed - most common measurement Instantaneous speed - time interval approaches zero Slide 5 5 Calculate average speed between trip times of 1 h and 3 h v= 150km - 50km 2h = 50km h Example: average speed Slide 6 6 Velocity Describes speed (How fast is it going?) and direction (Where is it going?) Graphical representation of vectors: length = magnitude; arrowheads = direction Slide 7 7 Acceleration Rate at which motion changes over time Speed can change Direction can change Both speed and direction can change Slide 8 8 Forces - historical background Aristotle Heavier objects fall faster Objects moving horizontally require continuously applied force Relied on thinking alone Galileo and Newton All objects fall at the same rate No force required for uniform horizontal motion Reasoning based upon measurements Slide 9 9 Force A push or pull capable of changing an objects state of motion Overall effect determined by the (vector) sum of all forces - the net force on the object Slide 10 10 Horizontal motion on land Natural motion question: Is a continuous force needed to keep an object moving? No, in the absence of unbalanced retarding forces Inertia - measure of an objects tendency to resist changes in its motion (including rest) YES, with unbalanced retarding forces such as air resistance and friction forces Slide 11 11 Balanced and unbalanced forces Motion continues unchanged w/o unbalanced forces Retarding force decreases speed Slide 12 12 Balanced and unbalanced forces Boost increases speed Sideways force changes direction Slide 13 13 Falling objects Free fall - falling under influence of gravity without air resistance Distance proportional to time squared Slide 14 14 Falling objects Free fall - falling under influence of gravity without air resistance Speed increases linearly with time Slide 15 15 Falling objects Free fall - falling under influence of gravity without air resistance Trajectories exhibit up/down symmetries Acceleration same for all objects Slide 16 16 Compound motion Three types of motion: 1.Vertical motion 2.Horizontal motion 3.Combination of 1. and 2. Projectile motion An object thrown into the air Basic observations: 1.Gravity acts at all times 2.Acceleration (g) is independent of the objects motion Slide 17 17 Projectile motion Vertical projectile Slows going up Stops at top Accelerates downward Force of gravity acts downward throughout Horizontal projectiles Horizontal velocity remains the same (neglecting air resistance) Taken with vertical motion = curved path Slide 18 18 Fired horizontally versus dropped Vertical motions occur in parallel Arrow has an additional horizontal motion component They strike the ground at the same time! Slide 19 19 Example: passing a football Only force = gravity (down) Vertical velocity decreases, stops and then increases Horizontal motion is uniform Combination of two motions = parabola Slide 20 20 Three laws of motion First detailed by Newton (1564-1642 AD) Concurrently developed calculus and a law of gravitation Essential idea - forces Slide 21 21 Newtons 1 st law of motion The law of inertia Every object retains its state of rest or its state of uniform straight-line motion unless acted upon by an unbalanced force Slide 22 22 Newtons 1 st law of motion The law of inertia Inertia resists any changes in motion Slide 23 23 Newtons 2 nd law of motion Forces cause accelerations Units = Newtons (N) Proportionality constant = mass; units = kilograms (kg) More force, more acceleration (with constant mass) More mass, less acceleration (with constant force) Slide 24 24 Examples - Newtons 2 nd law More mass, less acceleration, (with constant force) again Focus on net force Net force zero here Air resistance + tire friction match applied force Result: no acceleration; constant velocity Slide 25 25 Examples - Newtons 2 nd law More mass, less acceleration, again Focus on net force Net force zero here Air resistance + tire friction match applied force Result: no acceleration; constant velocity Slide 26 26 Weight and mass Mass = quantitative measure of inertia; the amount of matter Weight = force of gravity acting on the mass Pounds and newtons measure of force Kilogram = measure of mass Slide 27 27 Newtons 3 rd law of motion Source of force - other objects 3rd law - relates forces between objects Whenever two objects interact, the force exerted on one object is equal in size and opposite in direction to the force exerted on the other object. Slide 28 28 Newtons 3 rd law of motion Source of force - other objects 3rd law - relates forces between objects Whenever two objects interact, the force exerted on one object is equal in size and opposite in direction to the force exerted on the other object. Slide 29 29 Momentum Important property closely related to Newtons 2nd law Includes effects of both motion (velocity) and inertia (mass) Slide 30 30 Conservation of momentum The total momentum of a group of interacting objects remains the same in the absence of external forces Applications: Collisions, analyzing action/reaction interactions Slide 31 31 Impulse A force acting on an object for some time t An impulse produces a change in momentum Applications: airbags, padding for elbows and knees, protective plastic barrels on highways Slide 32 32 Forces and circular motion Circular motion = accelerated motion (direction changing) Centripetal acceleration, a c present Centripetal force, F c must be acting Centrifugal force - apparent outward tug as direction changes Centripetal force ends: motion = straight line Slide 33 33 Newtons law of gravitation Attractive force between all masses Proportional to product of the masses Inversely proportional to separation distance squared Explains why g = 9.8 m/s 2 Provides centripetal force for orbital motion Slide 34 34 Newtons law of gravitation Attractive force between all masses Proportional to product of the masses Inversely proportional to separation distance squared Explains why g = 9.8 m/s 2 Provides centripetal force for orbital motion