physics 101: lecture 8 newton's laws
DESCRIPTION
Physics 101: Lecture 8 Newton's Laws. Today’s lecture will be a review of Newton’s Laws and the four types of forces discussed in Chapter 4. Concepts of Mass and Force Newton’s Three Laws Gravitational, Normal, Frictional, Tension Forces. Sir Isaac Newton, English Physicist, 1643-1727. - PowerPoint PPT PresentationTRANSCRIPT
Physics 101: Lecture 8, Pg 1
Physics 101: Physics 101: Lecture 8Lecture 8Newton's LawsNewton's Laws
Today’s lecture will be a review of Newton’s Laws and the four types of forces discussed in Chapter 4.
Concepts of Mass and Force Newton’s Three Laws Gravitational, Normal, Frictional, Tension Forces
Physics 101: Lecture 8, Pg 2
Sir Isaac Newton, English Physicist, 1643-1727
Physics 101: Lecture 8, Pg 3
Newton’s First LawNewton’s First Law
The motion of an object does not change unless it is acted upon by a net force.
• If v=0, it remains 0• If v is some value, it stays at that value
Another way to say the same thing:• No net force
• velocity is constant • acceleration is zero • no change of direction of motion
Physics 101: Lecture 8, Pg 4
Mass or InertiaMass or Inertia
Inertia is the tendency of an objectto remain at rest or in motion with constant 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
Physics 101: Lecture 8, Pg 5
Newton’s Second LawNewton’s Second Law
This law tells us how motion changes when a net force is applied.
acceleration = (net force)/mass
M
F
M
F
M
F a :symbolsin totnet
a M F
:it write way toalternate
net
Physics 101: Lecture 8, Pg 6
Newton’s Second LawNewton’s Second Law
Units: [F] = [M] [a]
[F] = kg m/s2
1 Newton (N) 1 kg m/s2
A vector equation:Fnet,x = Max
Fnet,y = May
a M Fnet
Physics 101: Lecture 8, Pg 7
Newton’s 1. LawNewton’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 totaltotal (or net) force on the plane:
1. is pointing upward2. is pointing downward 3. is pointing forward 4. is pointing backward5. is zero
lift
weight
drag thrust
correct
Physics 101: Lecture 8, Pg 8
Newton’s 1. LawNewton’s 1. Law
Newton'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.
lift
weight
drag thrustF= ma = m0 = 0
Physics 101: Lecture 8, Pg 9
Example: Newton’s 2. Law
MF1
M=10 kg F1=200 NFind a
a = Fnet/M = 200N/10kg = 20 m/s2
MF1
M=10 kg F1=200 N F2 = 100 NFind a
F2
a = Fnet/M = (200N-100N)/10kg = 10 m/s2
Physics 101: Lecture 8, Pg 10
Newton’s Third LawNewton’s Third Law
For every action, there is an equal and opposite reaction.
• Finger pushes on box • Ffingerbox = force exerted on box by finger
Ffingerbox
Fboxfinger• Box pushes on finger
• Fboxfinger = force exerted on finger by box
• Third Law: Fboxfinger = - Ffingerbox
Physics 101: Lecture 8, Pg 11
Newton's Third Law...Newton's Third Law...
FFA ,B = - FFB ,A. is true for all types of forces
FFw,m FFm,w
FFf,m
FFm,f
Physics 101: Lecture 8, Pg 12
Conceptual Question: Newton’s 3.LawConceptual Question: Newton’s 3.Law
Since FFm,b = -FFb,m why isn’t FFnet = 0, and aa = 0 ?
a ??a ??FFb,m FFm,b
ice
Physics 101: Lecture 8, Pg 13
Conceptual Question: AnswerConceptual Question: Answer
Consider only the box only the box !FFnet,net, box = mbox aabox = FFm,b
aaboxbox
FFb,m FFm,b
ice
What about forces on man?FFnet,man = mman aaman = FFb,m
Physics 101: Lecture 8, Pg 14
2) Compare the magnitudes of the acceleration you experience, aA, to the magnitude of the acceleration
of the spacecraft, aS, while you are pushing:
1. aA = aS
2. aA > aS
3. aA < aS
Newton’s 2. and 3. LawNewton’s 2. and 3. LawSuppose 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:
1. FA = FS 2. FA > FS
3. FA < FS
correct
correct a=F/mF same lower mass gives larger a
Third Law!
Physics 101: Lecture 8, Pg 15
Summary:
• Newton’s First Law:The motion of an object does not change unless itis acted on by a net force
• Newton’s Second Law:Fnet = ma
• Newton’s Third Law:Fa,b = -Fb,a
Physics 101: Lecture 8, Pg 16
Forces: 1. GravityForces: 1. Gravity
r12
m1m2
F2,1 F1,2
F1,2 = force on m1 due to m2 =2
12
21
r
mmG = F2,1 = force on m2 due to m1
Direction: along line connecting the masses; attractive
G = universal gravitation constant = 6.673 x 10-11 N m2/kg2
Example: two 1 kg masses separated by 1 mForce = 6.67 x 10-11 N
(very weak, but this holds the universe together!)
Physics 101: Lecture 8, Pg 17
Gravity and WeightGravity and Weight
Force on mass:
mg gm mR
GMF
2e
eg
Me
Remass on surfaceof Earth
m
g
2
6e
24e
2e
e
m/s 9.81 g
m 10 x 6.38 R and kg 10 x 5.98 M using
R
GM g
Fg W = mg
Physics 101: Lecture 8, Pg 18
Forces: 2. Normal ForceForces: 2. Normal Force
book at rest on table:What are forces on book?
W
• Weight is downward• System is “in equilibrium” (acceleration = 0 net force = 0)• Therefore, weight balanced by another force
FN
• FN = “normal force” = force exerted by surface on object• FN is always perpendicular to surface and outward• For this example FN = W
Physics 101: Lecture 8, Pg 19
Forces: 3. Kinetic FrictionForces: 3. Kinetic Friction
• Kinetic Friction (aka Sliding Friction):A force, fk, between two surfaces that opposes relative motion.
• Magnitude: fk = kFN
k = coefficient of kinetic friction a property of the two surfaces
W
FN
Ffk
direction of motion
Physics 101: Lecture 8, Pg 20
Forces: 3. Static FrictionForces: 3. Static Friction
W
FN
Ffs
• Static Friction:A force, fs, between two surfaces that prevents relative motion.
• fs ≤ fsmax= sFN force just before breakaway
s = coefficient of static friction a property of the two surfaces
Physics 101: Lecture 8, Pg 21
Forces: 4. TensionForces: 4. Tension
• Tension: force exerted by a rope (or string)
• Magnitude: same everywhere in rope Not changed by pulleys
• Direction: same as direction of rope.
T
Physics 101: Lecture 8, Pg 22
Forces: 4. Tensionexample: box hangs from a
rope attached to ceiling
T
W
yFy = may
T - W = may
T = W + may
In this case ay = 0
So T = W
Physics 101: Lecture 8, Pg 23
Examples: InertiaExamples: Inertia
Seat-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 ?
Physics 101: Lecture 8, Pg 24
Examples: TensionExamples: Tension
A lamp of mass 4 kg is stylishly hung from the ceiling
by two wires making angles of 30 and 40 degrees. Find
the tension in the wires.
Physics 101: Lecture 8, Pg 25
Examples:Examples:
Consider two blocks of mass m1 and m2 respectively
tied by a string (massless). Mass m1 sits on a horizontal
frictionless table, and mass m2 hangs over a pilley. If
the system is let go, compute the aceleration and the
tension in the string.