forces notes 2010

FORCES

• Why does an object move?• Why does an object change directions?• Why does an object stop, or start to move?

WHAT IS A FORCE?

Force- push or pull.

Described by magnitude (strength) and direction

Measured in Newtons (N)

FORCES

When forces are exerted in the same direction the two forces are added.

6N+3N=9N

FORCES IN OPPOSITE DIRECTIONS

When forces are exerted in the opposite directions one force is given a + number the other is given a – number, and these numbers are added.

Balanced and Unbalanced Forces

UNBALANCED FORCE When one force is greater in magnitude than

the other it creates an unbalanced force. This is what causes objects to move, stop,

and change directions.

+ =

Force 1 + Force 2 = Net Force

UNBALANCED FORCE

1 direction

NALANCED FORCES CHANGE THE MOTION OF AN OBJECT

Unbalanced Forces

change the motion of an object

BALANCED FORCE

When both forces are equal in magnitude and in opposite directions it creates a balanced force.

The net force will then be equal to 0.

No movement occurs.

I bet this guy hopes the forces stay balanced!!

+ = 02 forces cancel each other

BALANCED FORCES

+ =

What about this force?

?

Describe the balanced and unbalanced forces in this given situation

THE MOVEMENT OF A CAR

What are the forces working on this car?

What other two forces are missing?

BIG IDEAS

•There are forces that exist in nature that create movement and allow objects to become stationary.

• Without forces, we would have mayhem.

Book sliding to the left

SPEED AND FORCES

Name of Force

Definition Examples

Gravity Gravity is a force that attracts bodies of matter toward each other. It is a force that is everywhere there is matterThe size of the force depends on two factors: mass and distance

-Force of attraction increases as mass increases-Force of attraction increases as distance decreases

Keeps the planets in orbit around the sun, moon in orbit around the earthHolds us to the earth's surface. Weight is a measure of the force of gravity on your massIs responsible for the tides.Causes acceleration of a falling objectResponsible for air and water pressure (weight of air and water!)

Surface Tension

Liquid molecules are attracted to each other (some more than others!)Attraction between molecules causes a “membrane” to form on the surface of liquids

Allows small insects to walk on waterMosquitoes “attach” eggs to surface of waterSoap, kerosene, can “break” the surface tensionBlowing bubbles – stretching surface tension

Magnetic An invisible force felt within the space around a magnet.  This space, called the magnetic field, can either attract (pull) or repel (push away) other magnets and some types of metal.All magnets have two poles. These poles are opposites and are called the north and south poles.  Opposite poles attract, while like poles, repel.

Compasses work because of Earth’s magnetic fieldIron and Nickel can be magnetized - Other metals cannotMagnets can be made with electricity.

Electrical

A force between two charged particlesOpposite charges attract, while like charges repel.The size of the force depends on two factors: size of charge and distance of objects

-Force of attraction/repulsion increases as the size of the charge increases-Force of attraction/repulsion increases as distance decreases

Static Electricity caused by touching or rubbing of two surfaces which causes buildup of charges

Walking across carpet – you gain electrons which are released when you touch a doorknob (attracted because it is more +)!

Turning Force

causes an object in uniform circular motion to move toward the center of the circular path.

a car making a turn the movement of a roller coaster

When you are riding in a car and the car accelerates, your body tends to move backward against the seat. Likewise, if the car stops suddenly, your body tends to move forward, in the direction of the dashboard

Buoyant Force (Upthrust)

Force equal to the weight of the fluid (liquid or gas!) that is displaced

by the object.Determined by an

object’s DENSITY (mass ÷ volume!)

An object will float ifDensity of Object <

Density of Fluid

Equal to the weight of the fluid (liquid or gas!) that is displaced by the objectExplains why ships don’t sink Why we only see about 10% of icebergs (“tip of the iceburg”)

Friction A force that acts in a direction opposite to the motion of a moving object.Friction will cause a moving object to slow down and finally stopDependent on two factors

-How hard surfaces are pushed together-The materials of which the surfaces are made

Sliding friction – when solid objects slide over one anotherRolling Friction – produced by wheels, ball bearings, etcFluid (liquid or gas!) Friction – when an object moves through a fluidLubricants reduce friction