economic english 9 ppt

CONTACT FORCE

Contact forces are forces that exist as a result of an inter￾action between objects that are physically in contact with

one another. They include frictional forces, tensional

forces, and normal forces.

The friction force opposes the motion of an object

across a surface. For example, if a glass moves across the

surface of the dinner table, there exists a friction force in

the direction opposite to the motion of the glass. Friction

is the result of attractive intermolecular forces between

the molecules of the surface of the glass and the surface

of the table. Friction depends on the nature of the two

surfaces. For example, there would be less friction

between the table and the glass if the table was moistened

or lubricated with water. The glass would glide across the

table more easily. Friction also depends on the degree to

which the glass and the table are pressed together. Air

resistance is a type of frictional force.

Tension is the force that is transmitted through a rope

or wire when it is pulled tight by forces acting at each

end. The tensional force is directed along the rope or

wire and pulls on the objects on either end of the wire.

The normal force is exerted on an object in contact

with another stable object. For example, the dinner table

exerts an upward force on a glass at rest on the surface of

the table.

NUCLEAR FORCE

Nuclear forces are very strong forces that hold the

nucleus of an atom together. If nuclei of different atoms

come close enough together, they can interact with one

another and reactions between the nuclei can occur.

Forms of Energy

Energy is defined as the ability to do work. We have

already stated that energy can’t be created or destroyed;

it can only change form. Forms of energy include poten￾tial energy and kinetic energy.

Potential energy is stored energy. Kinetic energy is the

energy associated with motion. Look at the following

illustration. As the pendulum swings, the energy is con￾verted from potential to kinetic, and back to potential.

When the hanging weight is at one of the high points, the

gravitational potential energy is at the maximum, and

kinetic energy is at the minimum. At the low point, the

kinetic energy is maximized, and gravitational potential

energy is minimized.

The change of potential energy into kinetic energy, and

kinetic energy into potential energy, in a pendulum

Examples of potential energy include nuclear energy

and chemical energy—energy is stored in the bonds that

hold atoms and molecules together. Heat, hydrodynamic

energy, and electromagnetic waves are examples of

kinetic energy—energy associated with the movement of

molecules, water, and electrons or photons (increments

of light).

Interactions of Energy

and Matter

Energy in all its forms can interact with matter. For

example, when heat energy interacts with molecules of

water, it makes them move faster and boil. Waves—

including sound and seismic waves, waves on water, and

light waves—have energy and can transfer that energy

when they interact with matter. Consider what happens

if you are standing by the ocean and a big wave rolls in.

Sometimes, the energy carried by the wave is large

enough to knock you down.

Waves

Energy is also carried by electromagnetic waves or light

waves. The energy of electromagnetic waves is related to

their wavelengths. Electromagnetic waves include radio

waves (the longest wavelength), microwaves, infrared

radiation (radiant heat), visible light, ultraviolet radia￾tion, X-rays, and gamma rays. The wavelength depends

on the amount of energy the wave is carrying. Shorter

wavelengths carry more energy.

When a wave hits a smooth surface, such as a mirror,

it is reflected. Sound waves are reflected as echoes. Mat￾ter can also refract or bend waves. This is what happens

when a ray of light traveling through air hits a water sur￾face. A part of the wave is reflected, and a part is refracted

into the water.

Maximum

Potential

Energy

Maximum

Potential

Energy

Maximum Kinetic

Energy

–PHYSICAL SCIENCE–

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