1. Net force and Newton's first law2. Newton's second law3. Newton's third law4. Frictional forces5. Gravitation6. Circular motion7. Centripetal force8. Static equilibrium and reference frames

Who are they??? Universal Laws of Classical Mechanics

a = v2 /R

F = GMm/R2

Equal Areas in Equal Times

Orbit = Ellipse

P2 = ka3

MassF = m a

Force

Inertia

Action =

Reaction

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:

O

F2F1

FT

F3

FT

F1FTF2 F3

+ + =

OF2(Tiger)

F1 (Bird)

F3(Fish)

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

To person on airplaneTime = 1 sec

1.25 m

200 m

1.25 m

To person on ground - Time =1 sec

What about pouring coffee?

To person on airplaneTime = 1/2 sec

To person on groundTime =1/2 sec

(We exaggerate and assume the coffee is poured 1.25 meters above the cup!)

1.25 m

1.25 m

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 totaltotal 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

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.

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 aboveWhen 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 gravityThe force remains constant when inclination is kept at that angle

leading to constant acceleration - continuous speed up.

Question 2

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 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

Learning check

Look at all people in this ficture, who has smallest

inertia ?

a)

b)

c)

d)

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 above4. 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.

Question 3

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/s29.8 Newtons = 2.2 pounds

1 kg x 9.8m/s2 = 9.8 Newtons

• Galileo’s Profound Contributions to Physics Include:

Principle of inertia

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

A car, mass=1000kg, speed = 10m/s

100 grams = 0.1kg

p=mv = 0.1 kg x 1000m/s = 100 kg.m/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

M

F

M

F

M

F a :symbolsin totnet

net

alternate way to write it:

F M a

Newton’s Second Law

–Units:Prove: [F] = [M] [a][F] = kg m/s2

1 Newton (N) 1 kg m/s2

– A scaler equation:Fnet,x = Max

Fnet,y = May

a M Fnet

aMdt

vdM

dt

)vM(d

dt

pdF

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

Part 3

Newton ‘s third law

Newton's Third Law...

• FFA ,B = - FFB ,A. is true for all types of forces

FFw,m

FFf,m

FFm,w

FFm,f

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. LawsSuppose 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 2. FA > FS

3. FA < FS

correct a=F/mF same lower mass gives larger a

correct Third Law!

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 itis 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