The

Forces

Are

With

Us

More than one force can act on an object at once.

The net force acting on an object is the totalof all the individual forces acting on the object.

Net force changes an object’s state of motion.

A force is a push or a pullthat one object exerts onanother object..

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The Fundamental Forces in Nature 

These forces act at a distance

1. Gravitational Force

2. Electromagnetic Force

3. Strong Force (binds protons and neutrons in the nucleus of the atom

4. Weak Force - force of nature that underlies some forms of radioactivity and certain interactions between subatomic particles.

4

Contact Forces

Contact Forces are forces in which interacting objects are in contact with each other.

Examples of contact forces are:

1. Friction Force – force that acts between materials that touch as they move past each other.

2. Air resistance Force – Acts upon objects as they travel through the air.

3. Applied Force – Force that is applied to an object by another object or by a person.

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Friction Force

F(frict)

The friction force is the force exerted by a surface as anobject moves across it, or makes an effort to moveacross it.

Most of the time, the friction force opposes the motion of an object.

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Tension Force

F(tens)

The tension force is the force which is transmitted througha string, rope, cable or wire when it is pulled tight by forcesacting from opposite ends.

The tension force is directed along the length of the wire andpulls equally on the objects on the opposite ends of the wire.

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Applied Force

F(app)

An applied force is a force which is applied to an objectby a person or another object.

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Air Resistance Force

F(air)

The air resistance force is a special type of frictional forceWhich acts upon objects as they travel through the air.

The force of air resistance is often observed to oppose theMotion of an object. It is most noticeable for objects whichTravel at high speeds or for objects with large surface areas.

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Normal Force

F(norm)

The normal force is the support force exerted upon anObject which is in contact with another stable object.

More on the normal force will come later.

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Gravity Force

F(grav)

The force of gravity acts over a distance.

The force of gravity if the force with which the earth, moonor other massively large objects attracts another object towarditself.

By definition, this is the weight of the object. The force of gravity on earth is always equal to the weight of the object asfound in the equation

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Anyone can learn about the forces!

• The force of gravity pulling you and the skateboard downward towardthe earth (gravitational force).

• The force of your pushing the skateboard with your foot (applied force).

• The force of the air pushing against you. (air resistance force). 

• The force of a wall pushing back on the skateboard when the skateboardhits the wall (applied force).

 

• The force exerted by the surfaces of the wheels and the floor as theymove across each other. (friction force).

Motion does not occur with balanced forces because they cancel each other out.

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When forces are balanced, net force is always 0.

Balanced ForcesWhen the forces acting on an object are in oppositedirections and equal in size, the forces are said to be balanced.

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The table pushes upwardon the book

Gravity pulls the bookdownward.

The forces acting on the book are balanced. They are equal in size but are acting in opposite directions.

The book is not moving.

Net force = 0.

Unbalanced forces (also called Net Forces) are unequalforces acting on an object. During the tug-of-war competition,one team is finally able to overcome and apply more pullingforce to the rope.  The rope starts to move in that teams favor.

Motion can only occur when an unbalanced force(net force) acts on an object.

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The downward force of gravity and the upward force of the tablesupporting the book are of equal magnitude, act in oppositedirections and thus balance each other. However, the force of friction acts leftwards, and there is norightward force to balance it. In this case, an unbalanced forceacts upon the book to change its state of motion and the bookslows.

Mass is not volume. Volume is a measure of the spacean object occupies.

The pillow takes up more space, therefore it has morevolume than the battery. However the batteryhas more mass. The battery is harder to move andhas more inertia. 19

Mass Is Not Weight.

Mass is the amount of matter in an object.

Weight is a measure of the gravitational force acting on an object..

Weight depends on an object’s location. Mass does not.

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Formula for calculating weight:

Weight = mass x gravity

Weight is a force. The unit for force is the Newton.

1 kilogram of mass is equal to 9.8 Newtons of force.

1 kg = 9.8 Newtons

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Weight can vary with location because gravity can bedifferent. Mass stays the same regardless of location.

The Earth has more mass than the Moon, so the gravitational force is greater on the Earth than it is on the Moon.

Aristotle gave three reasons for motion:

1. Natural motion, such as falling, came from the tendency of objects to go to their “natural” place, on the ground, and come to rest.

2. Natural motion was the type of motion exhibited by animals, which moved because they chose to.

3. Violent motion occurred when an object was acted on by some other object that made it move

Nicolaus Copernicus (1473-1543)

Formulated a theory for theMoving Earth.

Copernicus reasoned thatthe the best way tointerpret astronomicalobservations was to assumethat the Earth and otherplanets move around theSun.

Copernicus worked on hisideas in secret to avoidpersecution.

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Work primarily dealt with describing motion and kinematics.

Was the first to believe that a force was needed to keep an object moving only when friction was present.

He was aware of the concept of inertia.

He rejected Aristotle's ideas of violent and natural motions after studying falling or rolling objects and projectiles.

Galileo Galilei (1564 - 1642)

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A Master and an Apprentice

Sir Isaac Newton (1642 - 727)

Yoda??

Newton’s First Law is also called The Lawof Inertia.Inertia (mass) is the tendency of an object to resistany change in its motion.

Mass is a measure of the inertia of an object.

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Every object continues in a state of rest, orin a state of motion in a straight line atconstant line at constant speed, unless itis compelled to change by the state offorces acting on it.

The Law of Inertia deals with forces when they are balanced.

According to Newton, an object placed in motion in astraight line will continue in motion forever untilsome outside force acts on the object to changeits motion.

Law of Inertia

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A force is not needed to keep an object in motion.

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

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The law of inertia explains what happens in a car crash.

No seat belt!!!

If the car were to abruptly stop and the seat belts were not being worn, then the passengers in motion would continue in motion.

Assuming a negligible amount of friction between the passengers and the seats, the passengers would likely be propelled from the car and be hurled into the air.

Once they leave the car, the passengers becomes projectiles and continue in projectile-like motion

A force is not required to keep an object in motion; in actuality, it is a force which brings the object to rest.

In the absence of a changing force, objectswill continue in motion with the same speed and in the same direction – forever!

May The Force Be with You! The End!