Presentation on theme: "Newton’s Second Law of Motion"— Presentation transcript:
1Newton’s Second Law of Motion Force and Acceleration
2Movement When we see something move we see StartSlowCurveStopAll these things represent a CHANGE in motion
3Formulas to remember Acceleration = change in velocity time interval What is the cause of acceleration?FORCE
4Force causes Acceleration Example – hockey puck in iceStill until force is placed on itStays moving in a straight path until another force causes it to accelerateChange directionSpeed upSlow downChange in velocity acceleration
5Net force causes acceleration Combination of force yields accelerationDouble the force – double the accelerationMathematically –Acceleration ~ net force“~” means “directly proportional to”
6Mass resists acceleration ExampleFull shopping cart vs. empty shopping cartThe greater the mass the more force it takes to accelerate the objectAcceleration is inversely proportional to massAcceleration ~massAs the denominatorincreased the wholequantity decreases
7Newton’s Second LawThe acceleration produced by the force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force and is inversely proportional to the mass of the objectMathematically:acceleration~ net forcemass
9Using consistent units a = Fma =acceleration (m/sec^2 )F = force (newtons)m = mass (kg)
10Acceleration – which way? Net force action on an object and its resulting acceleration are always in the same directionThe spool demo:Which way will it roll?Does it change from top to bottom?
11Problem SolvingOne Newton – the force needed to give a mass of one kilogram an acceleration of one meter per second per second.1 N = (1 kg) (1 m/sec/sec)1 N = 1 kg m/ sec^2If we know two quantities, we can solve for the third
12Problem 1How much force, or thrust, must a 30,000-kg jet plane develop to achieve an acceleration of 1.5 m/sec^2F = ma= (30,000 kg)(1.5 m/sec^2)= 45,000 kg m/sec^2= 45,000 N
13Problem 2What acceleration is produced by a force of 2000 N applied to a 1000-kg car?a = F/m= 2000 N/ 1000 kg= 2000 kg m/sec^2/1000 kg= 2 m/sec^2If the force is 4000 N, the acceleration doubles4000N/1000 kg = 4 m/sec^2
14QuestionsIf a car accelerates at 2 m/sec^2, what acceleration can it attain if it is towing another car of equal mass?Answer – the same force on twice the mass produces half the acceleration or1 m/sec^2
15QuestionsWhat kind of motion does a constant force produce on an object of fixed mass?A constant force produces motion at a constant acceleration, in accordance with Newton’s second law.
16Friction Is a force Must be in contact Direction opposite to motion Force is needed to overcome frictionCaused by irregular surface
17Extent of friction Depends on kinds of material How much surface are pressed togetherExamplesRubber on concreteSteel on steelGuard rails are now concrete instead of steel
18Friction in Fluids Fluids – liquids and gases Caused by object trying to move particles apart in order to pass thru itTry to run in water?Liquid friction can be quite highAir resistance – notice at high speedsBiking or skiing
19Friction and Force Free-body diagrams When friction is present, object may only move at a constant speed even if you apply force (instead of accelerating)The net force is zeroAir ResistanceFree-bodydiagramsPUSHWeightFRICTION
20QuestionsTwo forces act on a book resting on a table: its weight and the support force from the table. Does a force of friction act as well?No, not unless the book tends to slide. Friction only acts when there is motion
21QuestionSuppose a high-flying jet cruises with a constant velocity when the thrust from its engines is a constant 80,000 N. What is the acceleration of the jet? What is the force of air resistance acting on the jet?
22AnswerThe acceleration must be zero because the velocity is constant. Since the acceleration is zero, if follows a = F/m the net force is zero. This means the force of air resistance is 80,000 N and it acts in the direction opposite to the jet’s motion.
23Applying Force - Pressure No matter how you place a book on a table, the force is the sameTry varying the way it is placed on a scaleHowever – place a book on your palm or on top of a pencil which goes into your hand……PRESSURE – has to do with force and area
24Pressure The amount of force per unit area Pressure = force area of applicationP = FA P=pressure (pascals)F= force (newtons)A = area (meter^2)1 Pascal = 1 N/m^2
25Pressure comparisons Pressure on your foot Two feetOne footPointe (ballerina)Calculating the pressure on your foot with graph paper
26Bed of Nails QuestionsIn attempting to do a bed of nails, would it be wise to begin with a few nails and work upward to more nails?No, no, no! There would be less physics teachers with fewer nails. The resulting greater pressure would cause harm.
27Bed of Nails questionThe massiveness of the cement block plays an important role in this demo. Which provides more safety, a small block or a more massive block?The greater the mass, the smaller the acceleration of the block and the bed of nails towards the friend. Much of the force from the hammer goes into breaking the block. The block must be BIG and must BREAK!
28Free Fall explainedGalileo did his famous experiment off the leaning tower of Pisa.Dropped a 10 kg cannon ballDropped a 1 kg stone at same timeResult – accelerations are equalBut why?
29Newton’s law F = ma Therefore a = F/m If an item is large it has a large force and a large massIf an item is small, it has a small force and a small massEither way the RATIOS are the sameF/m = F/m
30Galileo's experiment a = F/m = 9.8 N/ 1 kg rock = 9.8 m/sec^2 a = F/m = 98 N/10 kg cannon ball= 9.8 m/sec^2Question – if you were on the moon an dropped a hammer and a feather at the same time, would they strike the surface of the moon at the same time?
32AnswerYes. Astronaut David Scott did this exact experiment on the moon. They both accelerated at 1/6 g.
33Falling and Air resistance Example – feather and coin in a tube.With air – coin falls rapidly, the feather flutters downWithout air – both reach the bottom at the same timeAir resistancefeathercoinweight
34Terminal Speed or Velocity Speed during freefall, when the air resistance on the object equals the weight of the falling object.Terminal speeds of various objectsFeather – 5 m/secCoin – 200 km/hrSkydiver – 150 – 200 km/hParachute – km/h
35QuestionIf a heavy person and a light person open their parachutes together at the same altitude and each wears the same size parachute, who will reach the ground first?The heavy person. Light person reaches terminal speed first and it will be slower than terminal speed for the heavy person.