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ลำดับตอนที่ #8 : [[R.S.]]Physics Test ::Chapter.3 & 6::6.10.09

The Summary of Physics for

Chapter 3 & 6 :: Newton’s First & Second Law of Motion

 

Chapter.3 :: Newton’s First Law of Motion – Inertia  [[Page.28]]

3-1 Aristotle on Motion [[Page.29]]

-          Aristotle, the foremost Greek scientist, studied motion and divided it into two types ::

1.       Natural motion = either straight up or straight down

ð It was ‘natural’ for heavy things to fall and for every light things to rise

ð circular motion was natural for the heavens (as well as planets, stars, Earth)

2.       Violent motion = imposed (กำหนด) motion

ð It was the result of forces that push or pull.

ð It had external cause and it was imparted to objects

-          Objects in their natural resting places couldn’t move by themselves; they had to be pushed or pulled.

-          Most thinkers at that time surely thought that an object moving against its nature must have some kind of force responsible for the motion. Also, they thought Earth didn’t move because it was in natural motion.

3-2 Copernicus and the Moving Earth [[Page.30]]

-          Copernicus reasoned that the simplest way to interpret astronomical observations was to assume that Earth and the other planet move around the sun.

-          Copernicus worked on his idea in secret to escape persecution (cuz people think it was extremely controversial); in his last days of life, he printed his work.

3-3 Galileo on motion [[Page.30]]

-          One of Galileo’s great contributions to physics was demolishing the notion that a force is necessary to keep an object moving.

-          A force is any push or pull.

-          Friction is the name given to the force that acts between materials that tough as they move past each other.

·         It is caused by the irregularities in the surfaces of objects that are touching.

-          Galileo argued that only when friction is present – as it usually is – is a force needed to keep an object moving.

·         If the ball rolls on an inclined plane with slope downward, it is rolling partly in the direction of the pull of Earth’s gravity. Therefore, its speed increases.

·         If the ball rolls on an inclined plane with slope upward, it is in a direction opposed by gravity. Therefore, its speed decreases.

·         If the ball rolls on a level surface (horizontally), it doesn’t roll with or against gravity and has almost constant velocity. So, the speed doesn’t change.

-          If the opposing incline is not inclined (perfectly horizontal), the ball would rolls forever if there is no friction.

 

-          Galileo stated that the tendency of a moving body to keep moving is natural and that every material object resists change to its state of motion.

·         The property of a body to resist changes to its state of motion is called inertia.

3-4 Newton’s Law of Inertia [[Page.33]]

-          Newton’s first law, usually called the law of inertia, is a restatement of Galileo’s idea that a fore is not needed to keep an object moving.

·         Newton’s first law states that every object continues in a state of rest, or of uniform speed in a straight line, unless acted on by a nonzero net force.

-          Example (as for objects at rest), dishes on a tabletop are in a state of rest and they tend to remain at rest although you quickly snap a tablecloth from beneath them.

·         This means that the brief & small forces of friction aren’t significant enough to appreciably move the dishes. 

·         Objects in at rest tend to remain at rest. Only a force will change the state.

-           Example (as for moving objects), sliding hockey puck along air table where friction is absent makes it slides with o apparent loss of speed.

·         In the absence of forces, a moving object tends to move in a straight line indefinitely. It moves by virtue of its own inertia.

-          Forces are needed to overcome any friction that may be present and to set objects in motion initially. Once the object is moving in a force-free environment, it will move in a straight line indefinitely.

 3-5 Mass – A Measure of Inertia [[Page.36]]

-         The amount of inertia an object has depends on its mass – which is roughly the amount of material present in the object.

·         The more mass an object has, the greater its inertia and the more force it takes to change its state of motion.

-         Mass and volume are entirely different concepts. Volume is a measure of space and is measured in units such as cubic centimeters, cubic meters, and liters. Mass is measured in the fundamental unit of kilograms.

·         If an object has a large mass, it may or may not have a large volume.

-         Mass and weight are not the same thing too.

·         Mass is a measure of the amount of material in an object and depends only on the number of and kind of atoms that compose it.

·         Weight is a measure of gravitational force acting on the object; it depends on an object’s location.

-         The object’s mass is the same in any locations. Therefore, the object’s inertia is solely a property of the stone not its location. However, the object’s weight varies in different locations (due to the pull of gravity)

·         Mass is the quantity of matter in an object; it is a measure of the inertia, or ‘laziness,’ that an object exhibits in response to an effort made to start it, stop it, or otherwise change its state of motion.

·         Weight is the force of gravity on an object. 

-         Mass and weight are proportional (but not equal) to each other in a given place.

(Objects with greater mass have greater weight; objects with little mass have little weights.)

-         In U.S., the traditional unit of weight is pound. In most parts of the world, the measure of matter is expressed in units of mass. The SI unit of mass is the kilograms (kg). 1 kg equals 2.2 pounds.   

-         The SI unit for force is the newton (N). One newton is equal to slightly less than a quarter pound. One kilogram weights 10 N in SI units.

3-6 The Moving Earth Again [[Page.38]]

-         Consider a bird sitting at rest on the branch looking at a worm on the ground below. The bird drops down vertically and able to catch the worm but people thought this would be impossible if Earth moves because the worm would have been swept away by the moving Earth for a distance of 30 kilometers. 

-         Actually, everything is moving at 30 km/s not only the worm.

-         The law of inertia states that objects in motion remain in motion if no unbalanced forces act on them.

-         Supposed if we flip a coin in a high-speed moving vehicle, we catch the vertically moving coin as we would if the vehicle were at rest. The coin keeps up with us; the vertical force of gravity affects only the vertical motion of the coin.

-         Our notions of motion today are very different from those of our distant ancestors.

 

Chapter.6 :: Newton’s Second Law of Motion – Force and Acceleration  [[Page.86]]

6-1 Force Causes Acceleration [[Page.87]]

-         Unbalanced forces acting on an object cause the object to accelerate.

-         Most often, the force we apply is not the only force acting on an object.

-         Acceleration depends on the net force (combination of forces acting on an object). To increase the acceleration of an object, the net force must be increased.

·         An object’s acceleration is directly proportional to the net force acting on it. 
Acceleration ~ net force ;; (the symbol ‘~’ stands for ‘is directly proportional to.’)

6-2 Mass Resists Acceleration [[Page.87]]

-         Acceleration depends on the mass being pushed.

-         For a constant force, an increase in mass will result in a decrease in the acceleration.

-         For a given force, the acceleration produced is inversely proportional to the mass.

acceleration ~ 1/mass

·         Inversely means the two values change in opposite directions. (as the denominator increases, the whole quantity decreases by the same factor)

6-3 Newton’s Second Law [[Page.88]]

-         Newton’s second law describes the relationship among an object’s mass, an object’s acceleration, and the net force on an object.

·         It states that the acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force and is inversely proportional to the mass of the object.

·         12acceleration= net forcemass or a= Fm '>By using newtons (N) for force, kilograms (kg) for mass, and the meters per second squared (m/s²) for acceleration, we get this equation ::

 

·         If net force is doubled = acceleration double; if mass is doubled = acceleration is half; if both net force & mass are doubled = acceleration unchanged. 

6-4 Friction [[Page.90]]

-         Friction is a force that acts on materials that are in contact with each other, and it always acts in a direction to oppose relative motion.

-         The force of friction between the surfaces depends on the kinds of materials in contact and how much the surfaces are pressed together.

-         Friction isn’t restricted to solids sliding or tending to slide over one another. Friction also occur in fluids (liquids and gases are fluids because they flow)

·         Fluid friction occurs as an object pushes aside the fluid it is moving through.

·         Air resistance is the friction acting on something moving through air.

-         When friction is present & the object moves with constant velocity even when an outside force is applied to it, then the friction force just balances the applied force

-         A diagram showing all the forces acting on an object is called free-body diagram. (the picture on the right is a simple free-body diagram)

6-5 Applying Force – Pressure [[Page.91]]

-         The differences in the way an object presses against the surface are due to differences in the area of contact.

-         For a constant force, an increase in the area of contact will result in a decrease in the pressure.

·         The amount of force per unit of area is called pressure.

12pressure= forcearea of application or P= FA'>

·         Pressure is measured in newtons per square meter, or pascals (Pa). One newton per square meter is equal to one pacal.

-          The smaller the area supporting a given force, the greater the pressure on that surface

-          For example, when the author applies a force to his friend, who is sandwiched between two beds of sharp nails, to break the cement block with the hammer. His friend is unharmed by the nails because much of the force is distributed over the nails that make contact with his body.

6-6 Free Fall Explained [[Page.93]]

-          Galileo showed that falling object accelerate equally, regardless of their masses.

·         This is strictly true if air resistance is negligible (if object is in free fall).

·         This is approximately true when air resistance is very small compared to the mass of the falling object.

·         = >For example, when Galileo dropped a 10-kg cannonball & a 1-kg stone from an elevated position at the same time; both fall together and strike the ground at the same time.

ð The 10 times greater weight of the cannonball to the stone doesn’t make the acceleration rate 10 times faster. The 10 times as much force acting on 10 times as much mass produces the same acceleration as the smaller force acting on the smaller mass

ð The ration of the weight (F) to mass (m) is the same acceleration at the same place on Earth.

-          All freely falling objects fall with the same acceleration because the net force on an object is only its weight, and the ration of weight to mass is the same for all objects.  

6-7 Falling and Air Resistance [[Page.95]]

-          When falling in air, two objects might not reach the ground at the same time. The force due to air resistance diminishes the net force acting on the falling objects.

-          The air resistance force depends on speed and is also proportional to the frontal area of the moving object

·         The air resistance force an object experiences depends on the object’s speed & area

Air resistance force ~ speed x frontal area

ð The expression shows that the air resistance force is directly proportional to the speed and frontal area of an object.

-         When the air resistance force on a falling object builds up to the point where it equals to the weight of the object, the net force on the object is zero and it stops accelerating. We say that the object has reached its terminal speed.

·         Terminal speed is the speed at which the acceleration of a falling object is zero because friction balances the weight.

·         Terminal velocity is terminal speed together with the direction of motion.

-         The heavy objects will attain a greater terminal speed than the lighter objects because the greater weight is more effective in ‘plowing through’ air. The larger surface areas of the object also increase the air resistance.

-         If we hold a light object and heavy object at arm’s length and release them together, both strike the floor at the same time. But if we drop them from the top of a building, the heavy object strikes the ground first due to the buildup of air resistance at higher speeds.

·         At low speeds, air resistance is often negligible, but at high speeds, it can make quite a difference. The effect of air resistance is more pronounced on the lighter object than on the heavier object, so the acceleration of the fall is less for the light ones.

-         The connection between acceleration, force, and mass, discovered by Newton in the 1600s, led to men landing on the moon in the 1900s.

 

Ps. Good luck in the two-chapter physics test =)

Sorry for my mistakes as well na 


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