1. Net force and Newton's first law 2. Newton's second law 3. Newton's third law 4. Frictional forces 5. Gravitation 6. Circular motion 7. Centripetal force 8. Static equilibrium and reference frames

Who are they??? Universal Laws of Classical Mechanics a = v 2 /R F = GMm/R 2 Equal Areas in Equal Times Orbit = Ellipse P 2 = ka 3 Mass F = 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 F2F2 F1F1 FTFT F3F3 FTFT F1F1 FTFT F2F2 F3F3 ++= O F 2(Tiger) F 1 (Bird) F 3(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 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 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 airplane Time = 1 sec 1.25 m 200 m 1.25 m To person on ground - Time =1 sec

What about pouring coffee? To person on airplane Time = 1/2 sec To person on ground Time =1/2 sec (We exaggerate and assume the coffee is poured 1.25 meters above the cup!) 1.25 m 100 m

Question 1 total 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: 1. is pointing upward 2. is pointing downward 3. is pointing forward 4. is pointing backward 5. 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 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. 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 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 ? 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 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. 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/s 2 1 Newton = 1 kg - m/s Newtons = 2.2 pounds 1 kg x 9.8m/s 2 = 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

Newton’s Second Law – Units: è Prove: è [F] = [M] [a] è [F] = kg m/s 2 è 1 Newton (N)  1 kg m/s 2 – A scaler equation: è F net,x = Ma x è F net,y = Ma y

Example: Newton’s 2. Law M F1F1 M=10 kg F 1 =200 N Find a a = F net /M = 200N/10kg = 20 m/s 2 M F1F1 M=10 kg F 1 =200 N F 2 = 100 N Find a F2F2 a = F net /M = (200N-100N)/10kg = 10 m/s 2

Part 3 Newton ‘s third law

Newton's Third Law... FF F A,B = - F B,A. is true for all types of forces F F w,m F F f,m F F m,w F F m,f

2) 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 S 2. a A > a S 3. a A < a S 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, F S, to the magnitude of the force exerted by the spacecraft on you, F A, while you are pushing: 1.F A = F S 2. F A > F S 3. F A < F S correct a=F/m F 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 ?

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,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

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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

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