Transcript Power Machines Pressure

Mechanics
Physics project
Name: Tamara Hall
Form: 5B
Power
• Power is the work done or energy converted
per unit time.
• P= E/t (where P-power, E- energy, t-time)
• The unit for power is the Watt (W). Large
amounts of power are measured in kilowatts
(kW) or megawatts (MW).
• 1W=1Js-1 i.e. 1 watt of power is dissipated if 1 joule of work
is done or 1 joule of energy is transformed in 1 second.
Friction
• Friction is the force that opposes the relative
motion of surfaces in contact and of bodies
moving through fluids.
• Friction increases with the roughness of the
surfaces in contact.
• Friction can be reduced by smoother
surfaces, grease and other lubricants, rollers,
wheels and ball bearings.
• Whenever work is done against friction, heat
is produced
Diagram of a machine
Machines
• A machine is any device which enables a force
to be applied at one point to overcome
another.
• The applied force is called the effort, E.
• The force moved or overcome is known as the
load, L.
• There are many purposes for machines such
as: i. multiplying forces
ii. transferring energy
Electrical energyan electrical motormechanical energy
iii. Transferring energy from one place to
another
iv. Increasing speed
v. changing the direction of a force
vi. Multiplying distance
• Examples of machines include levers, pully
systems, bicycles and the forearm of
humans.
Efficiency of Machines
• efficiency of a machine= energy input / energy output
= work output/ work input
= power output/ power input
• Mechanical advantage is the ratio of the load moved
to the effort applied
• Velocity ratio is the ratio of the distance moved by
the effort to the distance moved by the load.
•
% efficiency=mechanical advantage/velocity ratio x 100
• one factor which affect efficiency is friction.
• Energy may be used more efficiently and
economically in the Caribbean in forms such
as:
• Solar energy using solar water heaters. These
produce abundant hot water and the cost is
not too high. But they are not yet popular.
Photovoltaic solar systems are getting close
to the cost of generating electricity from
fossil sources because of the intense sunlight
and high cost of diesel.
• Geothermal energy- Islands in the lesser Antilles are
known to have potential for geothermal energy. Eg.
Guadeloupe has a 4MW plant from shallow high temperature
wells since 1984.
Geothermal energy is capable of grid connection as
relatively large amounts of energy can be generated
from a single plant.
• Natural gas- natural gas as a source of energy is
important in Trinidad and Tobago currently eg.
Petrotrin. However, with the large oil companies
entering into electricity generation it is possible that
gas could replace oil as an alternative energy source
in the Caribbean.
Hydrostatics
Pressure
• pressure is the force acting perpendicular to
unit surface area.
• The SI unit is Nm-2 and is called the Pascal
• Pressure= force/area
Pressure in fluids
• pressure in fluids depend on depth below the
surface and the density of the fluid.
• Pressure at the same depth acts equally in all
directions.
• Pressure is uniformly transmitted through a liquid.
One example of this application is the use of the
hydraulic jack. A hydraulic jack is two cylinders
connected. Among these two cylinders one is small
and one is large. If a force is applied to the small
cylinder it would result in a given pressure and this
pressure would be the same in the larger cylinder.
Since the larger cylinder has a greater area the force
emitted by it would be greater. The pressure remains
the same but there is a greater force.
The Manometer
A manometer is a device used to measure excess gas pressure. A
simple design is to seal a length of glass tubing and then bend
the glass tube into a U shape. It is then filled with mercury so
all trapped air is removed from the sealed end of the tube. It is
then positioned with the curved region at the bottom and the
mercury settles at the bottom.
After this occurs, a vacuum is produced in the sealed tube and
the open tube is connected to the system whose pressure is
being measured. In the sealed tube there is no gas to exert a
force on the mercury. The net result is that the column of
mercury in the left (sealed) tube is higher than that in the right
(unsealed) tube. The difference in the heights of the columns of
mercury is a measure of the pressure of gas in the system.
A manometer
Archimedes Principle
• Archimedes principle states that when a body is fully
or partially submerged in a fluid the upthrust equals
the weight of the fluid displaced.
• An object will float if the upthrust is greater than
the weight of the object. Eg. A cork floating in water
• An object will be partially submerged if the weight of
the object equals the upthrust. Eg. Wood in water
• An object will sink if the upthrust is less than the
weight of the object. Eg. When water is pumped into the tanks
of a submarine the weight becomes greater than the upthrust and it
sinks