Junior cycle science physics forces and motion. By Theresa Lowry-Lehnen. Science Teacher.
28/05/13Junior CycleJunior CyclePhysicsPhysicsForces and MotionForces and MotionEdited and Reproduced byTheresa Lowry-LehnenRGN, BSc (Hon’s) Specialist Clinical Practitioner (Nursing), Dip Counselling, Dip Adv Psychotherapy, BSc(Hon’s) Clinical Science, PGCE (QTS) , H. Dip. Ed, MEd, Emotional Intelligence (Level 9) MHS Accredited
28/05/13MomentsMomentsA moment is a “turning force”, e.g. trying to open or close adoor or using a spanner. The size of the moment is given by:Moment (in Nm) = force (in N) x distance from pivot (in m)Calculate the following turning moments:100 Newtons5 metres200 Newtons2 metres
28/05/13Balancing momentsBalancing moments100 Newtons200 Newtons2 metres 2 metresTotal ANTI-CLOCKWISEturning moment = 200x2 =400NmTotal CLOCKWISE turningmoment = 100x2 = 200NmThe anti-clockwise moment is bigger so the seesaw willturn anti-clockwise
28/05/13Balanced or unbalanced?Balanced or unbalanced?
28/05/13StabilityStability1. Centre of mass is withinthe wheelbase – no problem!2. Centre of mass is directly abovethe edge of the wheelbase –car is onthe point of toppling3. Car falls over
28/05/13Centripetal forceCentripetal forceConsider a ball of Pleistocene attached to some string:The ball is kept in its path by thetension in the string – an example of aCENTRIPETAL FORCE. This force alsoproduces the change in velocity due tothe direction constantly changing.This force is INCREASED if you increase the mass of theobject, its speed or decrease the radius of the circle.Other examplesof centripetalforces:Orbits Electrons
28/05/13MomentumMomentumAny object that has both mass andvelocity has MOMENTUM. Momentum(symbol “p”) is simply given by the formula:Momentum = Mass x Velocity(in kgm/s) (in kg) (in m/s)PVMWhat is the momentum of the following?1) A 1kg football travelling at 10m/s2) A 1000kg Ford Capri travelling at 30m/s3) A 20g pen being thrown across the room at 5m/s4) A 70kg bungi-jumper falling at 40m/s
28/05/13Conservation of MomentumConservation of MomentumIn any collision or explosion momentum is conserved (provided that thereare no external forces have an effect). Example question:Two cars are racing around Teville Gate. Car A collides with the back ofcar B and the cars stick together. What speed do they move at after thecollision?Mass = 1000kg Mass = 800kgSpeed = 50m/s Speed = 20m/sMomentum before = momentum after……so 1000 x 50 + 800 x 20 = 1800 x V……V = 36.7m/sMass = 1800kg Speed = ??m/s
28/05/13More questions…More questions…1) A white snooker ball moving at 5m/s strikes a red ball and pots it.Both balls have the same mass. If the white ball continued in the samedirection at 2m/s what was the velocity of the red ball?2) A car of mass 1000kg heading up the M1 at 50m/s collides with astationary truck of mass 8000kg and sticks to it. What velocity doesthe wreckage move forward at?3) A defender running away from a goalkeeper at 5m/s is hit in the backof his head by the goal kick. The ball stops dead and the player’s speedincreases to 5.5m/s. If the ball had a mass of 500g and the player hada mass of 70kg how fast was the ball moving?4) A gun has a recoil speed of 2m/s when firing. If the gun has a mass of2kg and the bullet has a mass of 10g what speed does the bullet comeout at? (Hint: total momentum before is zero and the gun has anegative velocity afterwards)
28/05/13Two lorries are travelling in the same direction along a motorway.1) Calculate the momentum of Lorry A as it travels along the motorway.2) Calculate the momentum of Lorry B as it travels along the motorway.(3 marks)1) Lorry B collides with Lorry A and they stick together. Calculate thecommon speed of the lorries immediately after the collision.(3 marks)Lorry AMass = 20,000kgSpeed = 14m/sLorry BMass = 30,000kgSpeed = 20m/s
28/05/13Newton’s Laws of MotionNewton’s Laws of Motion1) If an unbalanced force acts on anobject that object will either accelerateor change direction:2) That force is given by F=ma3) When a force acts on an objectthere is an equal force acting inthe opposite direction (“Action andreaction are equal and opposite”)FAMThese are my three laws ofmotion (summarised):
28/05/13Newton’s 2Newton’s 2ndndLawLawInstead of F=ma Newton actually said that the force acting onan object is that object’s rate of change of momentum. Inother words…∆mvTFForce = Change in momentumTime(in N)(in kgm/s)(in s)For example, David Beckham takes a free kick by kicking a stationaryfootball with a force of 40N. If the ball has a mass of 0.5kg and hisfoot is in contact with the ball for 0.1s calculate:1) The change in momentum of the ball,2) The speed the ball moves away with
28/05/13Example questionsExample questions1) A golfer strikes a golf ball with a force of 80N. If the ballhas a mass of 200g and the club is in contact with it for0.2s calculate a) the change in momentum of the golf ball,b) its speed.2) A tennis player strikes a serve with a force of 30N. If theball has a mass of 250g and the racket is in contact with itfor 0.15s calculate the ball’s change in momentum and itsspeed.3) A footballer takes a goal kick by kicking a 0.4kg footballaway at 10m/s. If his foot was in contact with the ball for0.1 seconds calculate the force he applied to the ball.4) A golfer strikes a 200g golf ball away at 50m/s. If heapplied a force of 50N calculate how long his club was incontact with the ball for.
28/05/13Energy loss in collisionsEnergy loss in collisionsWe’ve also said that in a collision momentum is conserved (unless anexternal force acts). The same cannot usually be said for kinetic energy…For example, consider the following collision. How much kinetic energy islost?Mass = 1000kg Mass = 800kgSpeed = 50m/s Speed = 20m/sKinetic energy = ½ x mass x velocity squaredin J in kg in m/sIn the “Forces” module we looked at how to calculate an object’s kineticenergy:Mass = 1000kg Mass = 800kgSpeed = 20m/s Speed = 30m/sBeforeAfter
28/05/13Energy loss in collisionsEnergy loss in collisionsConsider a head-on collision where the cars stick together. How muchkinetic energy is lost in this example? Where does all the energy go?In this example more kinetic energy was lost. We sayit was a “less elastic collision”. An “elastic collision” isone where the kinetic energy is conserved.Speed = 50m/s Speed = 30m/sSpeed = 10m/sBeforeAfter
28/05/13The Structure of the EarthThe Structure of the EarthA thin crust -10-100km thickA mantle – has theproperties of a solidbut it can also flowA core – made ofmolten nickel and iron.Outer part is liquidand inner part is solidThe average density of the Earth is much higher thanthe crust, so the inner core must be very dense
28/05/13Tectonic theoryTectonic theoryPeople once thought that the oceans and the continents were formed byshrinkage from when the Earth cooled down after being formed.Alfred Wegener proposed something different. Consider Africa and SouthAmerica:These continents looklike they “fit” together.They also have similarrock patterns and fossilrecords. These twopieces of evidence ledme to believe thatthere was once a singleland mass. This is myTECTONIC THEORY.
28/05/13The Answer:1) Scientists discovered 50 years later that the Earth generates massiveamounts of heat through radioactive decay in the core. This heatgenerated convection currents in the mantle causing the crust to move2) We also now know that the sea floor is spreading outwards from plateboundariesTectonic theoryTectonic theoryThe Evidence:1) Some continents look like they used to “fit” together2) Similar rock patterns and fossil recordsThe Problems:Wegener couldnt explainhow continental drifthappened so nobodybelieved himConclusion – scientists now believe Wegener’s Tectonic Theory
28/05/13Movements of the crustMovements of the crustWhen the lithosphere (“crust”) moves three things can happen:1) Plates move pasteach other, causingearthquakes2) Plates move away from each other – a“constructive plate margin”. The gap isfilled with magma which cools to formbasalt. This is called sea floor spreading.3) Platesmove towardseach other – a“destructiveplate margin”.The thinnerone slidesunderneath(“subduction”)and partiallymelts. Thiscausesvolcanoes andearthquakes.
28/05/13Evidence for sea floor spreadingEvidence for sea floor spreadingSince the Earth was formed the north and south poles haveperiodically “________ ____”. When tectonic plates moveapart and _____ fills the gap the iron particles in the magmaorientate themselves in line with the Earth’s ________ field.This means that the rock formed on the sea floor contains a“magnetic __________” of the changing field:These magnetic patterns can be used to prove that sea floorspreading does happen, and at a rate of about 2cm per _____.Words: impression, magma, swapped over, magnetic, year