Theos-Talk Archives (July 2005 Message tt00171)

Are there any scientists out there that can answer my queries in relation to Newton's laws, I am also pasting some links which may be of interest to Odin. As I am a very basic student of science I hope my explanations and interpretations are correct. I am happy to be corrected if I have misunderstood any concept.

Cheers

Cass

http://www.thefinaltheory.com/pages/5/index.htm

 

http://www.corollarytheorems.com/articles_1.htm

 

 

http://www.mkaku.org/articles/

 

http://www.indianscience.org/essays/ht_es_science.shtml


 

Prior to Newtonian science it was generally held that moving objects, wouldeventually stop moving; a force was necessary to keep an object moving. But if left to itself, a moving object would eventually come to rest and an object at rest would stay at rest.

NEWTON’S FIRST LAW OF MOTION

An object at rest tends to stay in rest. An object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force, that is objects tend to keep on doing what they’redoing.

It is the natural tendency for object to resist changes in their state of motion (velocity).

An object moving at a constant velocity (direction + speed) will continue to move at a constant velocity, if it is moving at a constant speed and in astraight line, unless a non-zero (net) (or total) force acts upon the object.

Non-zero net force can act upon an object by causing it to slow down when acted upon by an unbalanced force (eg friction) causing a deacelleration.

 

One predicts the behavior of stationary objects and the other predicts the behavior of moving objects

Newton’s first law of motion is sometimes referred to as the law of Inertia

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

 

AN OBJECT AT REST HAS A TENDENCY TO STAY AT REST

Force has two components, 

Force (A) the earth’s gravitational pull which exerts a downward force onan object or person 

Force (B) Normal force – exerts an upward force on an object or person

Eg. Ground pushes up on a person, gravity pushes down on a person. If these forces are of equal magnitude and in opposite directions they balance each other out. The forces are in equilibrium. If there is no unbalanced force acting upon the person, the person or object remains in a state of motion (equilibrium), it will not be accelerated.

A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction.

All forces (interactions) between objects can be placed into two broad categories:


contact forces, and 
forces resulting from action-at-a-distance 

Contact forces are types of forces in which the two interacting objects arephysically contacting each other. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance forces, spring force and applied forces. 

Action-at-a-distance forces are types of forces in which the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite a physical separation. Examples of action-at-a-distance forces include gravitational forces (e.g., the sun and planets exert a gravitational pull on each other despite their large spatial separation; even when your feet leave the earth and you are no longer in contact with the earth, there is a gravitational pull between you and the Earth), electric forces (e.g., the protons in the nucleus of an atom and the electrons outside the nucleus experience an electrical pull towards each other despite their small spatial separation), and magnetic forces (e.g., two magnets can exert a magnetic pull on each other even when separated by a distance of a few centimeters). 

Force is a quantity which is measured using the standard metric unit known as the Newton. 

For example, the influence of a 20-Newton upward force acting upon a book is cancelled by the influence of a 20-Newton downward force acting upon the book. In such instances, it is said that the two individual forces "balanceeach other"; there would be no acting upon the object.

A FORCE IS NOT NEEDED TO KEEP AN OBJECT IN MOTION –

This is confusing, either I have misinterpreted it, or what Newton is saying is that a force did not cause the book to move position. I cannot see howthis works, because the book moved position because 

a) the incline of the table,

b) the force coming from the person pushing the book, or 

c) the friction between the book and the table, which results in an equal force from the other direction to cease the movement of the book, considering that Force A and Force B are in equilibrium. In the absence of this balancing force (friction and counter friction) the book would continue in motion with the same speed and direction-forever.

Isaac Newton built on Galileo's thoughts about motion. Newton's first law of motion declares that a force is not needed to keep an object in motion. Slide a book across a table and watch it slide to a rest position. An objectin motion does not come to a rest position because of the absence of a force; rather it is the presence of a force - that force being the force of friction - which brings the object to a rest position. In the absence of a force of friction, the object would continue in motion with the same speed and direction - forever! A force is not required to keep a moving object in motion; in actuality, it is a force which brings the object to rest.

 

 

Yet when you apply this to the Moon, it doesn’t make any sense as Newton says there is no counter force necessary to keep the book or let’s say the moon in orbit. Unless he is postulating a friction force upon the moon and the earth, separate from the gravity of the earth pulling down and the moon pushing up. 

? This implies that the moon’s natural tendency would continue in motion,at the same speed and in the same direction, and would therefore carry it away from our own planet and off into space in a straight line. So if the earth is pulling down and the moon is pulling up, Force A and Force B are inequilibrium, which begs the question what is causing the moon to orbit andnot to continue in a straight line?   

Since the unbalanced force is directed opposite to the state of motion why doesn’t the moon deaccelerate and eventually come to rest? So something beside gravity is keeping the moon in orbit, but Newton says, it is not a force. This energy expenditure is not balanced by a conversion of energy from any known power source. Yet the moon continues to spin in orbit by an unknown force which is not a force????  

The force of gravity pulling downward and the force of the floor pushing upwards on the object are of equal magnitude and opposite directions. These two forces balance each other. Yet there is no force present to balance the force of friction. As the object moves to the right, friction acts to the left to slow the object down. There is an unbalanced force; and as such, theobject changes its state of motion. The object is not at equilibrium and subsequently accelerates. Unbalanced forces cause accelerations. In this case, since the unbalanced force is directed opposite the object's motion, it will cause a deceleration (a slowing down of the object). 

He also says the greater an object’s mass the greater an objects inertia.That the strength increases with their masses and decreases with the distance between them squared. But the mass of the moon exceeds 1% of the earth’s mass, so why doesn’t the moon fly past the earth if nothing is forcefully constraining it? If it is merely the gravity that holds our planet together, what about the 4 billion years the earth has had to endure thistremendous crushing force, yet remained intact? 

OBJECTS MOVE AT A CONSTANT VELOCITY (DIRECTION + SPEED)

Further, objects, resist changes in their velocity (speed with a direction)through the principle of inertia. 

Inertia is the tendency of an object to resist acceleration (Force + objector mass) Newton's conception of inertia stood in direct opposition to morepopular conceptions about motion. An object at rest has zero velocity - and (in the absence of an unbalanced force) will remain with a zero velocity;it will not change its state of motion (i.e., velocity). 

In fact, it is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion isdescribed as inertia. 

Inertia is the resistance an object has to a change in its state of motion. 

At the time, Newton's concept of inertia was in direct opposition to the more popular conceptions about motion. The dominant thought prior to Newton'sday was that it was the natural tendency of objects to come to rest. Moving objects, or so it was believed, would eventually stop moving since a force was necessary to keep an object moving. If left to itself, a moving object would eventually come to rest and an object at rest would stay at rest; thus, the idea which dominated the thinking for nearly 2000 years prior to Newton was that it was the natural tendency of all objects to assume a rest position.

UNBALANCED FORCES (causes accelerations)

An object in motion with a velocity of 2 m/s, East will (in the absence of an unbalanced force) remain in motion with a velocity of 2 m/s, East; it will not change its state of motion (i.e., velocity). 

Newton says that a force is not needed to keep a moving object, say to the right.

The force of gravity pulling downward and the force of the object pushing upwards are of equal magnitude and opposite directions. For an object to move a force of friction is applied, yet there is no force present to balance the force of friction. There is an unbalanced force and as such the objectchanges its state of motion. As the object moves to the right, friction acts to the left to slow the object down.. There is an unbalanced force; andas such, the object changes its state of motion. The object is not at equilibrium and subsequently accelerates. Unbalanced forces cause accelerations. Since the unbalanced force is directed opposite the object's motion, it will cause a deceleration (a slowing down of the object). 

An object is said to be "acted upon by an unbalanced force" only when thereis an individual force which is not being balanced by a force of equal magnitude and in the opposite direction . 

To determine if the forces acting upon an object is balanced or unbalanced,an analysis must first be conducted to determine what forces are acting upon the object and in what direction. If two individual forces are of equal magnitude and opposite direction, then the forces are said to be balanced. An object is said to be "acted upon by an unbalanced force" only when thereis an individual force which is not being balanced by a force of equal magnitude and in the opposite direction . 

Velocity is a vector quantity which refers to "the rate at which an object changes its position." Imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. While this might result in a frenzy of activity, it would also result in a zerovelocity. Because the person always returns to the original position, the motion would never result in a change in position. Since velocity is defined as the rate at which the position changes, this motion results in zero velocity. If a person in motion wishes to maximize his/her velocity, then that person must make every effort to maximize the amount that he/she is displaced from his/her original position. Every step must go into moving that person further from where he/she started. For certain, the person should never change directions and begin to return to where he/she started.

A state of motion is a vector quantity. A vector quantity is a quantity which has both magnitude and direction. To fully describe the force acting upon an object, you must describe both the magnitude (size) and the direction.Thus, 10 Newtons is not a full description of the force acting upon an object. In contrast, 10 Newtons, downwards is a complete description of the force acting upon an object; both the magnitude (10 Newtons) and the direction (downwards) are given.

Furthermore, because forces are vectors, the influence of an individual force upon an object is often canceled by the influence of another force. 

Other situations could be imagined in which two of the individual vector forces cancel each other ("balance"), yet a third individual force exists that is not balanced by another force. For example, imagine an object sliding across the rough surface of a table from left to right. The downward force of gravity and the upward force of the table supporting the book act in opposite directions and thus balance each other. However, the force of friction acts leftwards, and there is no rightward force to balance it. In this case, an unbalanced force acts upon the object to change its state of motion.

All objects resist changes in their state of motion. All objects have thistendency – they have inertia. Some objects have more of a tendency to resist changes than others. The tendency of an object to resist changes in its state of motion is dependent upon its mass. Inertia is a quantity which is solely dependent upon mass. The more mass an object has, the more inertiait has – the more tendency it has to resist changes in its state of motion. 

NEWTON’S 2ND LAW OF MOTION -ACCELERATION OF AN OBJECT IS DEPENDENT UPON TWO VARIABLES 

Newton's second law of motion pertains to the behavior of objects for whichall existing forces are not balanced. The second law states that the acceleration of an object is dependent upon two variables - the net force (The net force is the vector sum of all the forces which act upon an object. Thatis to say, the net force is the sum of all the forces, taking into accountthe fact that a force is a vector and two forces of equal magnitude and opposite direction will cancel each other out) acting upon the object and themass of the object and the acceleration of an object depends directly uponthe net force acting upon the object, and inversely upon the mass of the object. As the force of propulsion acting upon a rocket-chair increased, theacceleration of a rocket-chair increased. As the mass of a rocket-chair increased, the acceleration of a rocket-chair decreased.


 
Newton's second law of motion can be formally stated as follows:

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

In conclusion, Newton's second law provides the explanation for the behavior of objects upon which the forces do not balance. The law states that unbalanced forces cause objects to accelerate with an acceleration which is directly proportional to the net force and inversely proportional to the mass.

Free-fall is a special type of motion in which the only force acting upon an object is gravity. Objects which are said to be undergoing free-fall, arenot encountering a significant force of air resistance; they are falling under the sole influence of gravity. Under such conditions, all objects willfall with the same rate of acceleration, regardless of their mass. 

Applying Newton’s second law a 1000-kg baby elephant would experiences a greater force of gravity. This greater force of gravity would have a directeffect upon the elephant's acceleration; thus, based on force alone, it might be thought that the 10-kg rock would accelerate faster. But acceleration depends upon two factors: force and mass. The 10-kg elephant obviously has more mass (or inertia). This increased mass has an inverse effect upon the elephant's acceleration. And thus, the direct effect of greater force on the 10-kg elephant is offset by the inverse effect of the greater mass of the 10-kg elephant; and so each object accelerates at the same rate - approximately 10 m/s/s. The ratio of force to mass (Fnet/m) is the same for the elephant and the mouse under situations involving free fall; this ratio (Fnet/m) is equivalent to the acceleration of the object.

Suppose that an elephant and a feather are dropped off a very tall buildingfrom the same height at the same time. We will assume the realistic situation that both feather and elephant encounter air resistance. Which object -the elephant or the feather - will hit the ground first? Most people are not surprised by the fact that the elephant strikes the ground before the feather. But why does the elephant fall faster? 

Answering these questions demands that we combine our understanding of Newton's first and second law with the concept of terminal velocity. According to Newton's laws, an object will accelerate if the forces acting upon it are unbalanced; and further, the amount of acceleration is directly proportional to the amount of net force (unbalanced force) acting upon it. Falling objects initially accelerate (gain speed) because there is no force big enough to balance the downward force of gravity. Yet as an object gains speed, it encounters an increasing amount of upward air resistance force. In fact,objects will continue to accelerate (gain speed) until the air resistance force increases to a large enough value to balance the downward force of gravity. Since the elephant has more mass, weighs more and experiences a greater downward force of gravity, it will have to accelerate (gain speed) for a longer period of time before their is sufficient upward air resistance tobalance the large
downward force of gravity.

Once the upward force of air resistance upon an object is large enough to balance the downward force of gravity, the object is said to have reached a terminal velocity. The terminal velocity is the final velocity of the object; the object will continue to fall to the ground with this terminal velocity. In the case of the elephant and the feather, the elephant has a much greater terminal velocity than the feather. As mentioned above, the elephant would have to accelerate for a longer period of time since it requires a greater speed to accumulate sufficient upward air resistance force to balancethe downward force of gravity. In fact, the elephant never does reach a terminal velocity. There is still an acceleration on the elephant the moment before striking the ground. 

According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces and are the subject of Newton's third law of motion. Formally stated, 

Newton's third law is:

"FOR EVERY ACTION THERE IS AN EQUAL AND OPPOSITE REACTION."

The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.

A variety of action-reaction force pairs are evident in nature. Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards. The size of the force on the air equals the size of the forceon the bird; the direction of the force on the air (downwards) is oppositethe direction of the force on the bird (upwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reactionforce pairs make it possible for birds to fly.

Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards. In turn, the road reacts by pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface.

Suppose that there are two seemingly identical bricks at rest on a table. However, one brick consists of mortar and the other brick consists of Styrofoam. Without lifting the bricks, how could you tell which brick was the Styrofoam brick? You could give the bricks an identical push in an effort to change their state of motion. The brick which offers less resistance is the brick with less inertia – and therefore the brick with less mass (i.e., the Styrofoam brick). 

 

 


Velocity is a vector quantity. As such, velocity is "direction-aware." Whenevaluating the velocity of an object, you must keep track of its direction. It would not be enough to say that an object has a velocity of 55 mi/hr. You must include direction information in order to fully describe the velocity of the object. For instance, you must describe an object's velocity as being 55 mi/hr, east. This is one of the essential differences between speed and velocity. Speed is a scalar and does not keep track of direction; velocity is a vector and is direction-aware. 

The task of describing the direction of the velocity vector is easy! The direction of the velocity vector is the same as the direction in which an object is moving. It does not matter whether the object is speeding up or slowing down, if the object is moving rightwards, then its velocity is described as being rightwards. If an object is moving downwards, then its velocity is described as being downwards. Thus an airplane moving towards the west with a speed of 300 mi/hr has a velocity of 300 mi/hr, west. Note that speedhas no direction (it is a scalar) and that velocity is simply the speed with a direction.

Average Speed and Average Velocity

As an object moves, it often undergoes changes in speed. For example, during an average trip to school, there are many changes in speed. Rather than the speedometer maintaining a steady reading, the needle constantly moves upand down to reflect the stopping and starting and the accelerating and decelerating. At one instant, the car may be moving at 50 mi/hr and at anotherinstant, it may be stopped (i.e., 0 mi/hr). Yet during the course of the trip to school the person might average a speed of 25 mi/hr. 

The average speed during the course of a motion is often computed using thefollowing equation: 

 

 

 

 

 


 

 

 

 
 
 

 

 
 
 
 
 

 
 

		
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