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Net force and Newton's first law
Newton's second law
Newton's third law
Frictional forces
Gravitation
Circular motion
Centripetal force
Static equilibrium and reference frames
Who are they???
Universal Laws of Classical Mechanics
P2 = ka3
Inertia
F = GMm/R2
F=ma
What is the difference between
Kinematics and Dynamics ?
Answer is Forces
Part 1
Net force and
Newton's first law
Review: Vector sum of Net force
• Net force: System of many vectors act to
one mass point O.
Example: F + F + F = FT
1
F1
O
F3(Fish)
2
3
(Bird)
F2
F1
F2(Tiger)
F3
O
FT
FT
Learning Check
• Can you explain all forces acting on a bus
when it is at rest ??
–Draw the arrows for them.
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 at rest.
* If v = C, it till move at that value.
@ Another way to say the same thing:
• No net force  call freely moving
object
• Velocity is constant or zero 
acceleration is zero
Learning check
• Suppose you are on an airplane travelling at constant
velocity with a speed of 500 miles per hour (roughly
200 m/s)
• If you throw a ball straight up, does it return to you?
•How long does it appear to you?
•How does the path of the ball look
to an observer on the ground?
Can you think of any experiment done inside the
airplane that would detect the motion of the airplane
at constant velocity?
Valuable Solution
1.25 m
To person on airplane
Time = 1 sec
1.25 m
200 m
To person on ground - Time =1 sec
What about pouring coffee?
(We exaggerate and assume the coffee is
poured 1.25 meters above the cup!)
To person on airplane
Time = 1/2 sec
1.25 m
1.25 m
To person on ground
Time =1/2 sec
100 m
Question 1
An airplane flying: Many forces act on the plane, including weight
(gravity), drag (air resistance), the thrust 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 force on the plane:
lift
1. is pointing upward
2. is pointing downward
3. is pointing forward
4. is pointing backward
5. is zero
correct
drag
thrust
weight
Velocity is constant implies there is no net force acting on it.
All components of the net force are zero.
Forces are balanced in all three dimensions.
Question 2
An object is held in place by friction on an inclined surface. The
angle of inclination is increased until the object starts moving.
If the surface is kept at this angle, the object
1. slows down
2. moves at uniform speed
3. speeds up
4. none of the above
When the object is at rest net force on it is zero.
When the object starts to move, there is change in velocity - i.e.,
there is acceleration or a net force due to gravity
The force remains constant when inclination is kept at that angle
leading to constant acceleration - continuous speed up.
Contact and Field Forces
MCQ Test
Forces that appear in contact
A. Friction
B. Electron acts to another electron
C. Gravity (you and the Earth)
D. Two mobiphones at distance of 5m.
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
Learning check
Look at all people in this ficture, who has smallest
inertia ?
b)
d)
a)
c)
Question 3
A passenger in a car and is not wearing your seat belt.
Without increasing or decreasing its speed, the car
makes a sharp left turn, and He find yourself colliding
with the right-hand side door. Which of the following is
correct of the situation?
1. Before and after the collision there is a
rightward force pushing him into the door
2. Starting at the time of collision, the door
exerts a leftward force on him
3. Both of the above
4. Neither of the above
Law of inertia: his body tends to move in a straight line
forward.
It collides with the door which being part of the car is
beginning to curve
leftward. When the contact happens he feel the door’s force on
him.
Mass vs. Weight
• Mass is an intrinsic property of an object.
– A rock has same mass whether it is on the moon or on
Earth
• Weight is the force exerted on an object by gravity.
– This is different depending upon the strength of the
gravitational force.
• On the surface of Earth, gravitational force is
constant so we can easily convert from mass to
weight.
1 pound = 1 slug - ft/s2
1 Newton = 1 kg - m/s2
1 kg x 9.8m/s2 = 9.8 Newtons
9.8 Newtons = 2.2 pounds
Principle of inertia
• Galileo’s Profound
Contributions to Physics
Include:
Principle: An object (Ball)
moving on horizontal surface
will continue to move in the
same direction at constant speed
unless it is disturbed.
(This becomes even more general
in the hands of Newton.)
Part 2
Newton’s
second law
Learning Check
• Can you explain all forces acting on a bus
when it is at rest ??
–Draw the arrows for them.
How Does a Car Move?
• Each arrow represents a force.
– Your car is accelerating forward, meaning
there is a net force in that direction.
• Identical concept for walking (or running).
Which has more momentum?
• A bullet, mass = 100 grams, speed =
1000m/s
100 grams = 0.1kg
p=mv = 0.1 kg x 1000m/s =
100 kg.m/s
A car, mass=1000kg, speed = 10m/s
p=mv = 1000 kg x 10m/s
= 10,000 kg.m/s
Newton’s Second Law
– This law tells us how motion changes
when a net force is applied.
acceleration = (net force)/mass

F



Fnet Ftot
in symbols : a 


M
M M
alternate way to write it:
Fnet  M a
Newton’s Second Law


Fnet  M a
– Units:
Prove:

 dp d (Mv )

dv
F

M
 Ma
dt
dt
dt
 [F] = [M] [a]
[F] = kg m/s2
1 Newton (N)  1 kg m/s2
– A scaler equation:
Fnet,x = Max
Fnet,y = May
Example: Newton’s 2. Law
F1
M
M=10 kg F1=200 N
Find a
a = Fnet/M = 200N/10kg = 20 m/s2
F1
M
F2
M=10 kg F1=200 N F2 = 100 N
Find a
a = Fnet/M = (200N-100N)/10kg = 10 m/s2
Part 3
Newton ‘s third law
Newton's Third Law...
• FA ,B = - FB ,A. is true for all types
of forces
Fw,m
Fm,w
Fm,f
Ff,m
Newton’s 2. and 3. Laws
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.FA = FS
correct Third Law!
2. FA > FS
3. FA < FS
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
correct
a=F/m
F same  lower mass gives larger a
Exercise: Action-Reaction
• Suppose a tennis ball (m= 0.1 kg) moving at a velocity v =
40 m/sec collides head-on with a truck (M = 500 kg) which
is moving with velocity V = 10 m/sec.
– During the collision, the tennis ball exerts a force on
the truck which is smaller than the force which the
truck exerts on the tennis ball.
TRUE or
FALSE ?
The tennis ball will suffer a larger acceleration during
the collision than will the truck. TRUE or FALSE ?
Suppose the tennis ball bounces away from the truck
after the collision. How fast is the truck moving after
the collision?
< 10 m/sec
= 10 m/sec
> 10 m/sec ?
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:
Fnet = ma
• Newton’s Third Law:
Fa,b = -Fb,a
Learning check
• ---------------LAWS OF MOTION----------------write the good word to ……..
• (I) ……….is constant absent external force
• (II) Force equals Mass x…….
• (III) Action equals…….
(I) Velocity
(II) Acceleration
(III) Reaction
Let try to familier with
Interactive Physics software
Short thesis
• Do the thesis about
1- Application of Crane truck
2- Combine harvester machine
3- Centrifuge machine
4- Automatic teller machine; ATM
5- Meat grinder machine
Please
Make friend with DVD experiments