Fundamental Physics Lecture
Newton's Laws
by Prof. Ari H. Ramelan
Physics Department,
Sebelas Maret University (UNS) 24 September 2019
Today’s lecture will be a review of Newton’s Laws and the four types of forces discussed:
Concepts of Mass and Force
Newton’s Three Laws
Gravitational, Normal, Frictional, Tension Forces
Newton’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 Inertia
Inertia is the tendency of an object to 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 Law
This law tells us how motion changes when a net force is applied.
acceleration = (net force)/mass
M F M
F M
a F : symbols
in
net tot
a M F
: it write way to
alternate
net
Newton’s Second Law
Units:
[F] = [M] [a]
[F] = kg m/s
2
1 Newton (N) 1 kg m/s
2 A vector equation:
F
net,x= Ma
x F
net,y= Ma
ya M F net
Newton’s 1. Law
An airplane is flying from Buffalo airport to O'Hare (Chicago City).
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:
1. is pointing upward 2. is pointing downward 3. is pointing forward 4. is pointing backward
5. is zero lift
weight
drag thrust
correct
Newton’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 thrust
SF= ma = m0 = 0
Example: Newton’s 2. Law
F
1M M=10 kg F
1=200 N
Find a a = F
net/M = 200N/10kg = 20 m/s
2F
1M
M=10 kg F
1=200 N F
2= 100 N Find a
F
2a = F
net/M = (200N-100N)/10kg = 10 m/s
2Newton’s Third Law
For every action, there is an equal and opposite reaction.
• Finger pushes on box
• F
fingerbox= force exerted on box by finger F
fingerboxF
boxfinger• Box pushes on finger
• F
boxfinger= force exerted on finger by box
• Third Law:
F
boxfinger= - F
fingerboxNewton's Third Law...
F
A ,B= - F
B ,A. is true for all types of forces
F
w,mF
m,wF
f,mF
m,fConceptual Question: Newton’s 3.Law
Since F
m,b= -F
b,mwhy isn’t F
net= 0, and a = 0 ?
a ??
F
b,mF
m,bice
Conceptual Question: Answer
Consider only the box !
F
net, box= m
boxa
box= F
m,ba
boxF
b,mF
m,bice
What about forces on man?
F
net,man= m
mana
man= F
b,m2)
Compare the magnitudes of the acceleration
you experience, a
A, to the magnitude of the acceleration of the spacecraft, a
S, while you are pushing:
1. a
A= a
S2. a
A> a
S3. a
A< a
SNewton’s 2. and 3. Law
Suppose you are an astronaut in outer space giving a brief push to a spacecraft whose mass is bigger than your own.
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. F
A= F
S2. F
A> F
S3. F
A< F
S correctcorrect
a=F/m
F same lower mass gives larger a
Third Law!
Summary:
• Newton’s First Law:
The motion of an object does not change unless it is acted on by a net force
• Newton’s Second Law:
F
net= ma
• Newton’s Third Law:
F
a,b= -F
b,aForces: 1. Gravity
r
12m
1m
2F
2,1F
1,2F
1,2= force on m
1due to m
2=
2 12
2 1
r m m
G = F
2,1
= force on m
2due to m
1Direction: along line connecting the masses; attractive
G = universal gravitation constant = 6.673 x 10
-11N m
2/kg
2Example: two 1 kg masses separated by 1 m Force = 6.67 x 10
-11N
(very weak, but this holds the universe together!)
Gravity and Weight
Force on mass:
mg gm
R m F GM
2e
g e
M
eR
emass on surface
of Earth
m
g
2
6 e
24 e
2 e
e
m/s 9.81 g
m 10 x 6.38 R
and kg 10
x 5.98 M
using
R g GM
F g W = mg
Forces: 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 F
N• F
N= “normal force” = force exerted by surface on object
• F
Nis always perpendicular to surface and outward
• For this example F
N= W
Forces: 3. Kinetic Friction
• Kinetic Friction (aka Sliding Friction):
A force, f
k, between two surfaces that opposes relative motion.
• Magnitude: f
k=
kF
N
k= coefficient of kinetic friction a property of the two surfaces W
F
NF f
kdirection of motion
Forces: 3. Static Friction
W F
Nf
sF
• Static Friction:
A force, f
s, between two surfaces that prevents relative motion.
• f
s≤ f
smax=
sF
Nforce just before breakaway
s= coefficient of static friction
a property of the two surfaces
Forces: 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. Tension
example: box hangs from a rope attached to ceiling
T
W
y S F
y= ma
yT - W = ma
yT = W + ma
yIn this case a
y= 0
So T = W
Examples: 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:
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.
Thank you very much for your attention
