1 Physics 101: Lecture 8 Newton's Laws Today’s lecture will be a review of Newton’s Laws andthe four types of forces discussed in Chapter 4.Concepts of Mass and ForceNewton’s Three LawsGravitational, Normal, Frictional, Tension Forces1
3 Newton’s First LawThe motion of an object does not change unless it is acted upon by a net force.If v=0, it remains 0If v is some value, it stays at that valueAnother way to say the same thing:No net force velocity is constantacceleration is zerono change of direction of motion
4 Mass or Inertia Inertia is the tendency of an object to remain at rest or in motion withconstant speed along a straight line.Mass (m) is the quantitative measure of inertia. Mass is the property of an object that measures how hard it is to change its motion.Units: [M] = kg
5 Newton’s Second LawThis law tells us how motion changes when a net force is applied.acceleration = (net force)/massStimulus is Force (cause)Resistance is mass (control rod)Response is acceleration (effect)
6 Newton’s Second Law Units: [F] = [M] [a] A vector equation: [F] = kg m/s21 Newton (N) 1 kg m/s2A vector equation:Fnet,x = MaxFnet,y = May
7 Newton’s 1. LawAn airplane is flying from Buffalo airport to O'Hare. Many forces act on the plane, including weight (gravity), drag (air resistance), the trust of the engine, and the lift of the wings. At some point during its trip the velocity of the plane is measured to be constant (which means its altitude is also constant). At this time, the total (or net) force on the plane: is pointing upward 2. is pointing downward is pointing forward is pointing backward 5. is zeroliftweightdragthrustcorrect
8 Newton’s 1. LawNewton's first law states that if no net force acts on an object, then the velocity of the object remains unchanged. Since at some point during the trip, the velocity is constant, then the total force on the plane must be zero, according to Newton's first law.liftweightdragthrustSF= ma = m0 = 0
9 Example: Newton’s 2. Law M=10 kg F1=200 N Find a F1 M a = Fnet/M = 200N/10kg = 20 m/s2MF1M=10 kg F1=200 N F2 = 100 NFind aF2a = Fnet/M = (200N-100N)/10kg = 10 m/s2
10 Newton’s Third LawFor every action, there is an equal and opposite reaction.Finger pushes on boxFfingerbox = force exerted on box by fingerFfingerboxBox pushes on fingerFboxfinger = force exerted on finger by boxFboxfingerThird Law:Fboxfinger = - Ffingerbox
11 Newton's Third Law... FA ,B = - FB ,A. is true for all types of forces Fw,mFm,wFf,mFm,f
12 Conceptual Question: Newton’s 3.Law Since Fm,b = -Fb,m why isn’t Fnet = 0, and a = 0 ?Fb,mFm,ba ??ice
13 Conceptual Question: Answer Consider only the box !Fnet, box = mbox abox = Fm,bWhat about forces on man?Fnet,man = mman aman = Fb,mFb,mFm,baboxSkateboard demo: get two students with obviously different masses.Show that acceleration is different although force is same.ice
14 Newton’s 2. and 3. Law Third Law! a=F/m Suppose you are an astronaut in outer space giving a brief push to a spacecraft whose mass is bigger than your own (see Figure 4.7 in textbook).1) Compare the magnitude of the force you exert on the spacecraft, FS, to the magnitude of the force exerted by the spacecraft on you, FA, while you are pushing: FA = FS FA > FS 3. FA < FSThird Law!correct2) Compare the magnitudes of the accelerationyou experience, aA, to the magnitude of the accelerationof the spacecraft, aS, while you are pushing:1. aA = aS2. aA > aS3. aA < aScorrecta=F/mF same lower mass gives larger a
15 Summary: Newton’s First Law: Newton’s Second Law: Fnet = ma The motion of an object does not change unless itis acted on by a net forceNewton’s Second Law:Fnet = maNewton’s Third Law:Fa,b = -Fb,a
16 Forces: 1. Gravity r12 m1 m2 F2,1 F1,2 F1,2 = force on m1 due to m2 = Direction: along line connecting the masses; attractiveG = universal gravitation constant = x N m2/kg2Example: two 1 kg masses separated by 1 mForce = 6.67 x N(very weak, but this holds the universe together!)
17 Fg W = mg Gravity and Weight m Re mass on surface Force on mass: MeRemass on surfaceof EarthmForce on mass:gFg W = mg
18 Forces: 2. Normal Force FN book at rest on table: What are forces on book?WWeight is downwardSystem is “in equilibrium” (acceleration = 0 net force = 0)Therefore, weight balanced by another forceFN = “normal force” = force exerted by surface on objectFN is always perpendicular to surface and outwardFor this example FN = W
19 Forces: 3. Kinetic Friction FNdirection of motionfkFWKinetic Friction (aka Sliding Friction): A force, fk, between two surfaces that opposes relative motion.Magnitude: fk = kFN k = coefficient of kinetic frictiona property of the two surfaces
20 Forces: 3. Static Friction FNfsFWStatic Friction: A force, fs, between two surfaces that prevents relative motion.fs ≤ fsmax= sFN force just before breakaway s = coefficient of static frictiona property of the two surfaces
21 Forces: 4. Tension T Tension: force exerted by a rope (or string) Magnitude: same everywhere in ropeNot changed by pulleysDirection: same as direction of rope.
22 Forces: 4. Tension example: box hangs from a rope attached to ceiling ySFy = mayT - W = mayT = W + mayTWIn this case ay = 0So T = W
23 Examples: InertiaSeat-belt mechanism (see textbook)A man dangles his watch from a thin chain as his plane takes off. He observes that the chain makes an angle of 30 degrees with respect to the vertical while the plane accelerates on the runway for takeoff, which takes 16 s.What is the speed of the aircraft at takeoff ?
24 Examples: TensionA lamp of mass 4 kg is stylishly hung from the ceilingby two wires making angles of 30 and 40 degrees. Findthe tension in the wires.
25 Examples: Consider two blocks of mass m1 and m2 respectively tied by a string (massless). Mass m1 sits on a horizontalfrictionless table, and mass m2 hangs over a pilley. Ifthe system is let go, compute the aceleration and thetension in the string.