FORCES CAUSE CHANGES IN MOTION

GALILEOLaw of Falling ObjectsGravity

NEWTONNewton’s LawsStructuresFriction

HISTORICAL SWEEP

What we call the scientific method had its real beginnings some four centuries ago in many fields of knowledge. The most impressive of the early triumphs came in physics and in the application of physics to astronomy for studying the apparent and real motions of the sun, the moon, the planets, and the stars.

Galileo made the first real contribution by discovering the natural laws which govern falling bodies and the swinging of the pendulum.

Kepler established the three laws which explain all the motions of the planets.

Newton explained Kepler’s Laws using just the law of gravitation, which applies invariably to all matter in the universe as small as a grain of sand or as large as the sun. He also codified his three Laws of Motion

This triumph of explaining a vast range of phenomena with a few simple principles, inspired workers in all fields of knowledge to trust scientific methods.

GALILEO and “THE LAW OF FALLING BODIES”

A fresco by Giuseppe Bezzuoli, shows the great scientist Galileo demonstrating the law of falling bodies. His experiments laid the foundations for scientific method and modern science.

GALILEO’S EXPERIMENT

GALILEO

It was Galileo who opened the door to an entirely new world of physics. At the age of 19 he timed with his pulse the swings of a great chandelier in the cathedral at Pisa and found that the swing always took the same time, even though the size of the excursion became smaller and smaller.

He then invented a simple pendulum for measuring time. This was a great improvement over the sand and water clocks then in use.

Galileo studied the motions of falling bodies and, in contradiction to Aristotle's claim, found that heavy bodies fall at exactly the same speeds as lighter ones when air friction is discounted.

He also studied accelerated motion by rolling balls down inclined planes. His experiments laid the foundation for modern mechanics. (See previous picture.)

Mechanics is the study of things that move and don’t move.

Law of Falling Objects

A hammer and a feather dropped at the same time on Earth will not accelerate at the same rate because of air resistance.

On the moon, which has no atmosphere, both objects will reach the ground at the same time.

GALILEO and the Law of Falling Bodies

The first scientific studies of gravity were performed Galileo at the end of the 16th century. Galileo measured the speed of falling objects by timing metal balls rolling down an inclined plane. He concluded that gravity imposes a constant acceleration on all objects.

That is, with each second of fall an object acquires a constant additional downward velocity. On Earth this acceleration of gravity is 32 feet (9.75 meters) per second per second. Thus, at the end of one second, a falling object is moving at a velocity of 32 feet per second and at the end of two seconds, 64 feet (19.5 meters) per second, and so on, before any adjustment for the resistance of the air it passes through.

Galileo found, contrary to the speculations of the Greek philosopher Aristotle centuries earlier, that all objects

are accelerated by gravity in the same way. A feather falls more slowly than a rock not because its acceleration

from gravity is less but because air resistance slows it more. The force of air resistance varies with the surface

area of an object, so that an object that spreads its weight over a greater area suffers more resistance and

thus drops more slowly. This is the principle used in the parachute

LAW OF FALLING OBJECTS

Law of FallingBodies In a vacuum, a

feather and an apple fall at the same speed.

In Aristotle’s view of motion, objects had a “desire” to be on the ground.

The bigger the object, the bigger the desire.

The Law of FallingObjects is Explainedby Isaac Newton asGravity

The force that causes objects to drop and water to run downhill is the same force that holds the Earth, the sun, and the stars together and keeps the moon and artificial satellites in their orbits.

Gravitation, the attraction of all matter for all other matter, is both the most familiar of the natural forces and the least understood.

GRAVITY in SPACE

Newton demonstrated mathematically that the law of gravitation he proposed predicts that the planets follow Kepler's three laws. Newton's vision of a world governed by simple, unalterable laws exerted a powerful influence for more than a century.

If the planets are attracted to the sun by gravity, why do they not fall in? Newton showed that if the velocity is high enough, a planet will always be accelerating toward the sun without ever leaving its orbit. This is because an object's motion is the result of both its previous direction of travel and speed--that is, its velocity--and the acceleration applied to it. Just as a rock whirling at the end of a string is continually pulled toward the hand holding the string as long as it is whirled fast enough, so objects in a gravitational field remain in their orbits if they are moving fast enough.

ISAAC NEWTON and NEWTON’s LAWS

Classical mechanics is governed by three basic principles, which were first formulated in the 17th and 18th centuries by Isaac Newton.

These principles are known as Newton's laws.

NEWTON’S FIRST LAW

Every body remains in a state of rest or in a state of uniform motion (constant speed in a straight line) unless it is compelled by impressed forces to change that state.

Under this law a moving body is at rest, as far as its own inertia is concerned, as long as its motion continues at the same speed and in the same direction.

Therefore, particles (or even worlds) of matter will keep flying through empty space forever, without being driven by any force, until something compels them to change their motion.

NEWTON’S SECOND LAW

Newton's second law describes the manner in which a force compels a change of motion, at a rate of change called acceleration. It can be stated as follows:

Change of motion is proportional to the applied force and takes place in the direction of the straight line in which that force is impressed.

It can be stated much more simply as a formula, using letters for force, mass, and acceleration:

• F = ma.

The wording of the law, however, makes clear how an impressed force acts. It simply makes a change in the body's motion its speed or direction toward the direction in which the force is acting.

NEWTON’S SECOND LAW

It is easy to make this ball move quickly after it makes contact with the racket. Because the ball has a relatively small mass and the hitter has a great force, the acceleration is great.

NEWTON’S THIRD LAW Newton's third law may be stated as

follows: Action and reaction are equal and opposite.

This law is often expressed as "for every action there is an equal and

opposite reaction." The law states a fact that can upset many

calculations unless it is taken into account.

It explains, for example, the saying that a man cannot literally lift himself by his own bootstraps. As he pulls up on his bootstraps, the bootstraps pull down on him.

Action and reaction are equal and opposite. A striking modern example of action and reaction is jet propulsion

NEWTON’S THIRD LAW

Forces always exist in pairs, action-reaction pairs.

NEWTON’s THIRD LAW and Jet Propulsion

Jet Propulsion is thrust imparting forward motion to an object, as a reaction to the rearward expulsion of a high-velocity liquid or gaseous stream.

A simple example of jet propulsion is the motion of an inflated balloon when the air is suddenly discharged. While the opening is held closed, the air pressure within the balloon is equal in all directions; when the stem is released, the internal pressure is less at the open end than at the opposite end, causing the balloon to dart forward. Not the pressure of the escaping air pushing against the outside atmosphere but the difference between high and low pressures inside the balloon propels it.

NO FORCE, NO CHANGE IN MOTION

If an object is motionless, the net force on it must be zero.

A book lying on a table is being pulled down by the earth’s gravitational attraction and is being pushed up by the molecular repulsion of the tabletop.

The net force is zero; the book is in equilibrium.

STRUCTURAL MECHANICS: When Things aren’t Supposed to Move

FRICTION

Microscopic bumps on surfaces cause friction. When two surfaces contact each other, tiny bumps on each of the surfaces tend to run into each other, preventing the surfaces from moving past each other smoothly. An effective lubricant forms a layer between two surfaces that prevents the bumps on the surfaces from contacting each other; as a result the surfaces move past each other easily.