物理学专业英语A.ppt

A1 Motion,mass,and forcesNEW WORDS Page.6NEW WORDS Page.71Here comes your footerA1 Motion,mass,and forcesScientists 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=5mStrictly speaking,this should be written:total length=2m+3m=5mUnits of measurement2Here comes your footerA1 Motion,mass,and forcesDisplacement 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 BThe 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=+12Movement of 12m to the left:displacement=-12Displacement12m3Here comes your footerA1 Motion,mass,and forcesDisplacement 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.Displacement5mball thrown up from hereball returns to starting point4Here comes your footerA1 Motion,mass,and forcesAverage speed is calculated like this:distance travelledaverage speed=time takenThe 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:displacementaverage speed=time takenThe SI unit of velocity is also the m s-1.But unlike speed,velocity is a vector.Speed and velocity5Here comes your footerA1 Motion,mass,and forcesThe 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 velocity6m6Here comes your footerA1 Motion,mass,and forcesAverage acceleration is calculated like this:change in velocityaverage acceleration=time takenThe SI unit of acceleration is the m s-2(sometimes written m/s2).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.Acceleration3m s-27Here comes your footerA1 Motion,mass,and forcesIf 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.Acceleration8Here comes your footerA1 Motion,mass,and forcesOn 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-axisAcceleration66420time in svelocity in m s-1acceleration=gradient=6/2=3ms-2acceleration=gradient=0/2=0ms-2acceleration=gradient=-6/2=-3ms-29Here comes your footerForce 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=maFor 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 forcesForce10Here comes your footerA1 Motion,mass,and forcesThe 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).Force6N6N6N6NResultant force=12N downwardsResultant force=011Here comes your footerA1 Motion,mass,and forcesOn 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 gacceleration=weight/mass=10ms-2=gEarthmassweight1 kg2 kgm10N20Nmg12Here comes your footerIn 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-1Or as an acceleraton of free fall of 10 m s-2In more accurate calculation,the value of g is normally baken to be 9.81,rather than 10.A1 Motion,mass,and forcesForce13Here comes your footerThe 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 forcesMoments and balance6N2m4m3Nanticlockwise moment about O=6 2=12N mclockwise moment about O=3 4=12N m14Here comes your footerThe 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 forcesMoments and balance6N2m4m3Nanticlockwise moment about O=6 2=12N mclockwise moment about O=3 4=12N m15Here comes your footerThe density of an object is calculated like this:The SI unit of density is the kilogram/cubic metre(kg m-3).For example,2000 kg of water occupies a volume of 2 m3.50 the density of water is 1000 kg m-3.Density values,in kg m-3 alcohol 800 aluminium 2 700 iron 7900 lead 11 300A1 Motion,mass,and forcesDensity16Here comes your footerPressure is calculated like this:The SI unit of pressure is the newton/square metre,alsocalled the pascal(Pa).For example,if a force of 12 N actsover an area of 3 m2,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 forcesPressure17Here comes your footer