物理学专业英语A.pptx

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,Here comes your footer,*,Physics Professional English,物理学专业英语,Physics Professional English,A1 Motion,mass,and forces,NEW WORDS Page.6,NEW WORDS Page.7,1,Here comes your footer,第1页,A1 Motion,mass,and forces,Scientists make measurements using SI units such as the metre,kilogram,second,and newton.These and their abbreviations are covered in detail in B1.However,you may find it easier to appreciate the links between different units after you have studied the whole of section A.,For simplicity,units will be excluded from some stages of the calculations in this book,as in this example:,total length=2+3=5m,Strictly speaking,this should be written:,total length=2m+3m=5m,Units of measurement,2,Here comes your footer,第2页,A1 Motion,mass,and forces,Displacement is distance moved in a particular direction.The SI unit of displacement is the,metre(m).,Quantities,such as displacement,which have both magnitude(size)and direction,are called,vectors.,A B,The arrow above represents the displacement of a particle which moves 12m from A to B.However,with horizontal or vertical motion,it is often more convenient to use a+or-to show the vector direction.For example:,Movement of 12m to the right:displacement=+12,Movement of 12m to the left:displacement=-12,Displacement,12m,3,Here comes your footer,第3页,A1 Motion,mass,and forces,Displacement is not necessarily the same as distance travelled.For example,when the ball below has returned to its starting point,its vertical displacement is zero.However,the distance travelled is 10m.,Displacement,5m,ball thrown up from here,ball returns to starting point,4,Here comes your footer,第4页,A1 Motion,mass,and forces,Average speed is calculated like this:,distance travelled,average speed=,time taken,The SI uint of speed is the metre/second,abbreviated as m s,-1,.For example,if an object travels 12m in 2s,its average speed is 6m s,-1,.,Average velocity is calculated like this:,displacement,average speed=,time taken,The SI unit of velocity is also the m s,-1,.But unlike speed,velocity is a vector.,Speed and velocity,5,Here comes your footer,第5页,A1 Motion,mass,and forces,The velocity above is for a particle moving to the right at 6m s,-1,.However,as with displacement,it is often more convenient to use,a+or-for the vector direction.,Average velocity is not necessarily the same as average speed.For example,if a ball is thrown upwards and travels a total distance of 10m before returning to its starting point 2s later,its average speed is,5 m s,-1,.But its average velocity is zero,because its displacement is zero.,Speed and velocity,6m,6,Here comes your footer,第6页,A1 Motion,mass,and forces,Average acceleration is calculated like this:,change in velocity,average acceleration=,time taken,The SI unit of acceleration is the m s,-2,(sometimes written m/s,2,).For example,if an object gain 6m s,-1,of velocity in 2s,its average acceleration is 3m s,-2,.,Acceleration is a vector.The acceleration vector above is for a particle with an acceleration of 3m s,-2,to the right.However,as with velocity,it is often more convenient to use,a+or-for the vector direction.,Acceleration,3m s,-2,7,Here comes your footer,第7页,A1 Motion,mass,and forces,If velocity,increases,by 3m s,-1,evey second,the acceleration is+3m s,-2,.If it,decreases,by 3m s,-1,evey second,the acceleration is-3m s,-2,.,Mathematically,an acceleration of-3m s,-2,to the right,is the same as an acceleration of+3m s,-2,to the left,.,Acceleration,8,Here comes your footer,第8页,A1 Motion,mass,and forces,On the velocity-time graph above,you can work out the acceleration over each section by finding the,gradient,of the line.The gradient is calculated like this:,gain along y-axis,gradient=,gain along x-axis,Acceleration,6,6,4,2,0,time in s,velocity in m s,-1,acceleration,=gradient,=6/2,=3ms,-2,acceleration,=gradient,=0/2,=0ms,-2,acceleration,=gradient,=-6/2,=-3ms,-2,9,Here comes your footer,第9页,Force is a vector.The SI is the,newton,(N).,If two or more forces act on something,their combined effect is called the,resultant force,.Two simple examples are shown below.In the right-hand example,the resultant force is zero because the forces are,balanced,.,A resultant force acting on a mass causes an acceleration.The force,mass,and acceleration are linded like this:,resultant force=mass acceleration,F=ma,For example,a 1N resultant force gives a 1 kg mass an acceleration of 1 ms,-2,.(The newton is defined in this way.),A1 Motion,mass,and forces,Force,10,Here comes your footer,第10页,A1 Motion,mass,and forces,The more mass something has,the more force is needed to produce any given acceleration.,When balanced forces act on something,its acceleration is zero.This means that it is,either,stationary,or,moving at a steady velocity(steady speed in a straight line).,Force,6N,6N,6N,6N,Resultant force=12N downwards,Resultant force=0,11,Here comes your footer,第11页,A1 Motion,mass,and forces,On Earth,everything feels the downward force of gravity.This gravitational force is called,weight,.As for other forces,its SI unit is the newton(N).,Near the Earths surface,the gravitational force on each kg is about 10N:the,gravitational field strength,is 10 N kg,-1,.This is represented by the symbol,g,.,Weight and,g,acceleration=weight/mass=10ms,-2,=g,Earth,mass,weight,1 kg,2 kg,m,10N,20N,mg,12,Here comes your footer,第12页,In the diagram above,all the masses are falling freely(gravity is the only force acting).From,F,=,ma,it follows that all the masses have the same downward acceleration,g,.This is the,acceleration of free fall,.,You can think of,g,:,Tither as a gravitational field strength of 10 N kg,-1,Or,as an acceleraton of free fall of 10 m s,-2,In more accurate calculation,the value of,g,is normally baken to be 9.81,rather than 10.,A1 Motion,mass,and forces,Force,13,Here comes your footer,第13页,The turning effect of force is called a,moment,:,moment of force=force perpendicular distance*,about a point from point,*measured from the line of action of the force.,The dumb-bell below balances at point O because the two moments about O are equal but opposite.,A1 Motion,mass,and forces,Moments and balance,6N,2m,4m,3N,anticlockwise moment about O=6 2=12N m,clockwise moment,about O=3 4=12N m,14,Here comes your footer,第14页,The dumb-bell is made up of smaller parts,each with its own weight.Together,these are equivalent to a single force,the total weight,acting through O.0 is the,centre of gravity,of the dumb-bell.,A1 Motion,mass,and forces,Moments and balance,6N,2m,4m,3N,anticlockwise moment about O=6 2=12N m,clockwise moment,about O=3 4=12N m,15,Here comes your footer,第15页,The density of an object is calculated like this:,The SI unit of density is the kilogram/cubic metre(kg m,-3,).,For example,kg of water occupies a volume of 2 m,3,.,50 the density of water is 1000 kg m,-3,.,Density values,in kg m,-3,alcohol 800 aluminium 2 700,iron 7900 lead 11 300,A1 Motion,mass,and forces,Density,16,Here comes your footer,第16页,Pressure is calculated like this:,The SI unit of pressure is the newton/square metre,also,called the,pascal,(Pa).For example,if a force of 12 N acts,over an area of 3 m,2,the pressure is 4 Pa.,Liquids and gases are called,fluids.,In a fluid:,Pressure acts in all directions.The force,produced isalways at right-angles to the,surface under pressure.,Pressure increases with depth.,A1 Motion,mass,and forces,Pressure,17,Here comes your footer,第17页,