Physics 101: Lecture 8 Newton's Laws

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


Sir Isaac Newton, English Physicist, 1643-1727


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


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


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


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


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


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


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


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


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


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


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


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!


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


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!)


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


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


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


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


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


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


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 ?


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.


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.

Physics 101: Lecture 8 Newton's Laws

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