02_motion0 - Physics Basics, Part I Units Laws of Motion...

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Unformatted text preview: Physics Basics, Part I Units Laws of Motion UCSD: Physics 8; 2007 Units of Measurement • Physics provides a conceptual and mathematical description of the physical world – Concentrates on matter, energy, space and time • Physics uses measurements of physical quantities, such as – distance, mass, time • Measurements involve units • We’ll stick to MKS (SI) units in this course – MKS: meters; kilograms; seconds – As opposed to cgs: centimeter; gram; seconds – Or English system: feet, pounds, seconds • Distance in meters (m) – 1 meter is close to 40 inches • Mass in kilograms (kg) – 1 kg is about 2.2 pounds • Time in seconds (s) 2 UCSD: Physics 8; 2007 Secondary Units Units can be combined in a variety of ways to form complex units, many of which have their own names/symbols Physical quantity velocity acceleration formulation Complex unit dist/time m/s velocity/time m/s2 = m/s/s = m/s per s Name for complex unit — — force F=ma kg·m/s2 Newton (N) Work or energy W=F·d kg·m2/s2 Joule (J = N·m) energy/time kg·m2/s3 Watt (W = J/s) frequency cycles/second 1/s pressure force/area kg/m·s2 power Hertz (Hz) Pascals (Pa = N/m2) 3 UCSD: Physics 8; 2007 Electrical Units We’ll deal a lot with electrical phenomena in this course, with its own (but related) set of units: quantity formulation units charge I Coulombs current charge/time C/s Amps (A) voltage V = IR V Volts (V) resistance R = V/I volts/amp Ohms (Ω) P = VI = I2R = V2/R volt-amps Watts (W = J/s) voltage/distance V/m power electric field for short C — 4 UCSD: Physics 8; 2007 Mass: m • Mass is a physical quantity – It is related to the number of protons and neutrons in an object – It is related to how hard it is to get something to move, which we call inertia – It tells you how strongly gravity will pull • Mass is not weight – Mass is a constant that does not change as you move around – Weight measured how strongly gravity pulls you. • the same mass weighs different amounts on different planets 5 UCSD: Physics 8; 2007 Inertia Unless acted on by an outside force, an object in motion tends to remain in motion and an object at rest tends to remain at rest This applies to both moving and not moving because at rest or in motion are not physical quantities We can speak of motion relative to something • Motion relative to surface of Earth • Motion relative to Center of Earth • Motion relative to sun: Earth is orbiting the sun – Hence we can not say if something is moving or not. – For everyday examples, we implicitly mean motion relative to surface of Earth 6 UCSD: Physics 8; 2007 Motion is measured by speed Speed is given by (distance traveled) divided by time taken: Speed = distance/time The units are those of the components: Units of speed = (units of distance)/(units of time) = meters/second = m/s = ms-1 7 UCSD: Physics 8; 2007 Motion is also measured by Velocity • Velocity is a speed and direction of motion – 10 m/s toward the north – 50 m/s straight upward All three arrows have the same length, which represents same speed. Only top two have same velocity 8 UCSD: Physics 8; 2007 Newton’s first law of motion Unless acted on by an outside force, an object in motion moves at a constant velocity, along a straight line This does not necessarily feel correct to you. All your observations are done in the presence of friction, air resistance and gravity, all of which add outside forces. You have never seen something move with no outside force. 9 UCSD: Physics 8; 2007 Examples of Inertia and Newton’s 1st Law Standing in a bus or train when it suddenly changes speed, you loose balance. A traffic collision. Car comes to a stop suddenly when hits wall. You keep moving. http://www.physicsclassroom.com/mmedia/newtlaws/mb.html http://www.glenbrook.k12.il.us/GBSSCI/PHYS/mmedia/newtlaws/cci.htm (animations) While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb which abruptly halts the motion of the skateboard. Objects placed on dashboard of car will move when turn corner Clip drops vertically when supporting pen removed, or tennis ball on a plastic bottle. Egg drops vertically when supporting cylinder removed 10 UCSD: Physics 8; 2007 Examples of Inertia and Newton’s 1st Law Students participated in a relay race, carrying a plastic container of water around a race track. The water had a tendency to spill from the container during specific locations on the track. In general the water spilled when: – the container was at rest and you attempted to move it – the container was in motion and you attempted to stop it – the container was moving in one direction and you attempted to change its direction. • Running with a coffee cup – will spill whenever you change velocity. • the head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. 11 UCSD: Physics 8; 2007 Examples of Inertia and Newton’s 1st Law to dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and, thrust downward at high speeds and then abruptly halted. headrests are placed in cars to prevent whiplash injuries during rear-end collisions. 12 UCSD: Physics 8; 2007 Summary Newton’s 1st Law When no outside or external force In which direction does the golf ball travel? 13 UCSD: Physics 8; 2007 Force: F • • Pushes and pulls: tries to make things change velocity Examples: – gravity exerts a downward force on you – the floor exerts an upward force on a ball during its bounce – a car seat exerts a forward force on your body when you accelerate forward from a stop – the seat you’re sitting in now is exerting an upward force on you (can you feel it?) – you exert a sideways force on a couch that you push across a floor – a string exerts a centrally-directed (centripetal) force on a rock at the end of a string that you’re twirling over your head – the expanding gas in your car’s cylinder exerts a force against the piston • • Note the syntax: Agent exerts directed force on recipient Has both a size and direction, like velocity 14 UCSD: Physics 8; 2007 Velocity v and Acceleration a • Velocity is a speed and associated direction – 10 m/s toward the north – 50 m/s straight upward • Acceleration is any change in velocity – A change in speed OR direction OR both • Acceleration measured as rate of change of velocity – velocity is expressed in meters per second (m/s) – acceleration is meters per second per second – expressed as m/s2 (meters per second-squared) If at time t1=100 seconds v1 = 5 m/s, and at t2=102 seconds v2 = 5.6 m/s, then a = (v2-v1)/(t2-t1) = (5.6-5.0)/(102-100) = 0.3 m/s2 Question: What is the acceleration when.. 15 UCSD: Physics 8; 2007 Newton’s Second Law of Motion • Newton’s First law was for absence of outside force – Outside means not coming from within the body – Also called an external force • When there is an outside force we need Newton’s Second Law of Motion: F = ma Force = mass times acceleration. Question: What are the units of Force? 16 UCSD: Physics 8; 2007 Summary Newton’s 1st and 2nd Laws No outside force: constant velocity Velocity could be zero, or constant speed in a constant direction With outside force: velocity changes Changing velocity means acceleration is not zero Either the speed changes, or the direction of motion changes The second Law a=F/m is a summary of both laws No outside force means F=0 Hence a=0, Hence velocity =0 or velocity = constant With outside force, F is not zero Hence a is not zero Hence v is changing, either speed or direction or both changing 17 Jeff Koon’s Balloon Dog UCSD: Physics 8; 2007 The dog (1996 Venice) and rabbit (1986) seem to be shiny low mass, low inertia balloons or perhaps glass…. but they are solid metal. You could tell by finding the F to accelerate them 18 UCSD: Physics 8; 2007 The Force-Acceleration Connection • Whenever there is a net force, there will be an acceleration – A ball thrown into the air has the force of gravity operating on it, so its velocity continuously changes, resulting in a curved path – When you step on the gas, a forward force acts on your car, making it speed up – The force of gravity attracts the earth toward the sun. This has the effect of changing the direction of earth’s velocity, wrapping it into a circle around the sun – A car, slamming into the side of another car already moving forward, will exert a sideways force, changing the traveling car’s direction of motion – When a bat hits a ball, the large momentary force results in a large acceleration of the ball as long as contact is maintained 19 UCSD: Physics 8; 2007 All Forces Great and Small • The relation, F = ma, tells us more than the fact that force and acceleration go together – the relation is quantitative, and depends on mass • For the same applied force: – a small mass will have a greater acceleration – a large mass will have a smaller acceleration Force = mass×acceleration OR Force = mass×acceleration • If you want the same acceleration, a smaller mass requires a smaller force, etc. – this then relates mass and inertia in an intimate way: – how hard is it to get an object moving? Experiment with spring pulling mass Question: are there any forces acting on you now? 20 UCSD: Physics 8; 2007 Force is a Vector If a physical quantity has a direction, then it is a vector. If not, then it is a scalar. Speed is a scalar Velocity = speed and a direction is a vector Acceleration = change of velocity = vector because velocity is a vector. If an equation has a vector in it, then all terms (but not all factors) must also be vectors. From F=ma, F is a vector because a is a vector. We show vectors using arrows for the direction and the length for the magnitude (size) 21 UCSD: Physics 8; 2007 Vector Arrows We show vectors using arrows for the direction and the length for the magnitude (size) this is a small force to left, eg 2 Newtons this is a larger force to left, eg 5 N You can add vectors: = And they can subtract if directions differ: sum of top 2 = bottom Which of following is not a vector? 22 UCSD: Physics 8; 2007 Net Force • I’ve got forces acting on me right now, but I’m not accelerating anywhere – very perceptive, and this is where the concept of net force comes in. Forces have direction, and can cancel. Total Force Force #2 Force #1 Force #1 Total Force = 0 Force #2 23 UCSD: Physics 8; 2007 Examples of Zero Net Force • Sitting in your seat, gravity is pulling down on you, but the seat reacts by pushing up on you. The forces cancel, so there is no net acceleration • Pushing against a huge crate the force of friction from the floor opposes this push, resulting in no net force and thus no acceleration 24 UCSD: Physics 8; 2007 Newton’s Third Law • For every force, there is an equal and opposite force – every “action” has a “back-reaction” – these are precisely equal and precisely opposite Force on box by floor (normal force) box floor Force on box by gravity • You can’t push without being pushed back just as hard – in tug-of-war, each side experiences the same force (opposite direction) – when you push on a brick wall, it pushes back on you! Experiment with two spring balances. 25 UCSD: Physics 8; 2007 Force Pairs Illustrated Force on box by person Force on floor by box Force on person by box Force on box by floor Force on person Force on floor by person by floor Not shown are the forces of gravity and the associated floor forces 26 UCSD: Physics 8; 2007 Examples of Newton’s 3rd Law A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards. In turn, the road reacts by pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface. Bug strikes a windscreen.. Bullet leaves gun.. 27 UCSD: Physics 8; 2007 Wait: We cheated two slides back… • When we drew the box and floor, with the “normal” force from the floor canceling the force of gravity, these weren’t strictly force pairs – but these are the two canceling forces on the box that result in zero acceleration of the box • The real pairs have to involve the earth: box floor • Force Pairs: • earth-box (grav) • box-floor (contact) • earth-satellite (grav) satellite earth 28 UCSD: Physics 8; 2007 Don’t all forces then cancel? • How does anything ever move (accelerate) if every force has an opposing pair? • The important thing is the net force on the object of interest Force on box by person Net Force on box Force on box by floor 29 UCSD: Physics 8; 2007 Gravity Described by Newton’s Universal Law of gravity: Fgrav = GMm/r2 – – – – where M is mass of earth, m is mass of you (or object of interest), and r is distance (separation) between object and earth’s center G is a constant: 6.67×10-11 in MKS units 30 UCSD: Physics 8; 2007 Force of Gravity is a Vector Connects the centers of mass. From center of the lead weight in Leonardo’s inclinometer to the center of the Earth. 31 UCSD: Physics 8; 2007 Gravity on earth’s surface Since F = ma, we can say ma = GMm/r2 and we can cancel the m, the mass of the object a = GM/r2 = g is the acceleration at the surface of Earth caused by gravity of Earth, or the acceleration of gravity. For earth GM = 3.986×1014 m3/s2 and r = radius of earth = 6,378 km, g = 9.8 m/s2 the acceleration of gravity (on Earth’s surface) • Falling objects accelerate at 9.8 m/s2 near the surface of the earth – • • downward velocity increases by about 10 m/s with each passing second Notice that a and v are negative if we choose up as positive. Notice that the mass of the object is not needed, as long as much smaller than Earth’s mass. Time (s) a (m/s2) v (km/s) 0 -9.8 0 1 -9.8 -9.8 2 -9.8 -19.6 3 -9.8 -29.4 32 UCSD: Physics 8; 2007 Gravity and Weight • To support a 1 kg book against the pull of gravity, one must exert F = ma = (1kg)·(10m/s2) = 10 Newtons • This is the object’s weight w= mg – Support for the book is just the “normal” force required to keep the book from accelerating – in other words: to make the net force on the book zero • We now see why weight is not constant – If move further from center of Earth, r increases, g decreases, weight decreases 33 UCSD: Physics 8; 2007 Pressure • Pressure is force per unit area: P = F/Area – measured in N/m2, or Pascals (Pa) – the pressure of the atmosphere at sea level is about • 105 = 100,000 Pa or • 14.6 pounds per square inch (psi), pounds are a unit of force • Pressure is same everywhere in a liquid at rest – F1 = A1P1 is much less than F2 = A2P2, where P1=P2 – Hydraulics work this way: move F1 a large distance to move F2 (heavier object) small distance 34 UCSD: Physics 8; 2007 Density Density is mass per volume = mass/volume – – – – – measured in kg/m3 water is 1000 kg/m3 (same as 1 g/cm3 in cgs units) air is about 1.3 kg/m3 rock is 3300 kg/m3 gold is about 19,300 kg/m3 • Density depends on both the elements in a material and on how densely those atoms are compressed. – When heat a metal the atoms vibrate faster and on average move farther apart form their neighbors, taking up more volume, so the density become smaller. The ABS must have higher density to have higher mass and weight, since same volume as PP 35 UCSD: Physics 8; 2007 Assignments • Establish WebCT connectivity & register transmitter • Read Chapter 1 of book – You can skip sections on velocity, position of falling balls, as well as section on projectile motion (pp. 15–21) • • • Tu Jan16 (week 2) Transmitters will start counting Tu Jan16 by 11:55pm First Q/O due via WebCT Th Jan18 First HW will be due 11:55pm WebCT 36 UCSD: Physics 8; 2007 Next • Next topic: – energy in its myriad forms – a simple model for molecules/lattices – electrons, charge, current, electric field 37 ...
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