Work, Energy, Power

work done?

the product of the force and the distance moved in the direction of the force

the unit of work is the juole, a scalar

a joule?

the work done when a force of one newton moves its point of application one metre in the direction of the force

when is work positive?

If the distance is moved in the same direction as the force we say that work was done by the force

when is work negative?

if the distance moved is in the opposite direction to the force we say that work was done on the force

work is done against gravitational force when..

a body is lifted if the displacement is not in the direction of the force then find the magnitude of the force in the direction of motion

work done = Fcos* x distance moved

how do you find work done by expanding gas?

pressure=force/area

force exerted=p x a

work done = p x a x d

but a x d is the volume by which the gas has expanded. Therefore work done by expanding gas = pv

work done by contracting gas = pv

energy?

the ability to do work, scalar measured in juoles

law of conservation of energy?

energy is never created or destroyed, it is changed from one form to another

when work is done energy is changed from one form to another

work done = amount of energy transformed

potential energy?

the ability of an object to do work due to its position or shape

gravitational potential energy?

an objects ability to do work due to its position in a gravitational field

it is the energy an object has because of its height above the surface. It is work done against gravity in lifting it to that height

the amount of work the body can do if it moves back to the surface

when objects move further apart..

GPE increases

GPE depends on?

mass of the body

height above the surface

gravitational field strength

deriving the formula? GPE

consider a body with mass m

the weight of the body = mg

g = gravitational field strength

the work done to lift the body to height h, against gravitational force

force x distance

mg x h = mgh

the increase in GPE =..

mgh

if the GPE is 0 at the surface then at the height h above the surface it is mgh

the GPE does not depend on the path taken to reach the height but only height reached

kinetic energy?

the energy an object has because it is moving

depends on its mass and the speed at which it is moving

deriving KE formula?

consider a body of mass m at rest

a force, F is exerted on the body giving it an acceleration a, over a distance s

the work done on the body is F x s

but F = ma

so work done = mas

using the equation of motion

v^2 = u^2 + 2as

as = v^2 / 2

but work done = mas = mv^2/2

energy gained = 1/2 mv^2

if the body has an initial velocity of u..

then the change in kinetic energy is 1/2m(v^2 - u^2)

in a system when the only force acting is related to potential energy...

then KE + PE is constant

when a body is falling towards the Earth...

it is losing GPE and gaining KE

if the only force acting is the gravitational force then we have loss in GPE = loss in KE

when theres friction/air resistance..

the formula decrease in energy = increase in energy does not work

decrease in energy = increase in energy + work done

the work done is often against friction

if there is an external force acting then the general equation is work done by force = gain in kinetic energy + gain in potential energy + work done against friction

power?

the rate of doing work or rate of supplying energy

power = work/time

scalar measured in watts

watt?

one watt is the magnitude of the power when one juole of work is done in one second

power =

force x velocity

efficiency?

whenever energy changes take place or work is done some of the energy is changed into unusable or unwanted energy

efficiency is how much of the energy produced is available for use

efficiency =

useful work/energy/power out

divided by

total work/energy/power in