Work, Energy, Power Flashcards
Define Power
The rate of energy transferred or the rate of work done
Use formulae for KE and GPE to find an expression for the speed of an object free falling distance h from rest:
Gain in KE=Loss in GPE
Ek = Ep
1/2mv^2 = mgh
1/2v^2 = gh
v^2 = 2gh
v= sqroot 2gh
Efficiency formula in terms of energy:
(Useful energy output/total energy input) x 100
Efficiency formula in terms of power:
(Useful power output/ total power input) x 100
Give the equation linking power, force and velocity:
P = Fv
P = power
F = driving force
v = velocity
Show that P = Fv
P = W/t and W = Fx
so P = Fx/t
since v = x/t, then P = Fv
State the principle of conservation of energy:
Energy cannot be created or destroyed, only transferred between different stores and objects. The total energy in a closed system remains constant.
Give the quantities and their units for the equation W = Fxcostheta
W = Work done (J)
F = Force (N)
x = Displacement (m)
Theta = Angle between force and displacement
Define the watt
The rate of energy transfer equal to one joule of energy transferred in one second
(1W = 1Js^-1)
Define the joule
One joule is the work done when a force of one newton moves an object through a distance of one metre (1J = 1Nm)
Why are energy transfers in the real world never 100% efficient?
Work done against resistive forces (such as friction and drag) means that some energy is always dissipated to the thermal energy store on an object and its surroundings.
Give the formula for gravitational potential energy near the Earth’s surface:
GPE = mass x gravitational field strength x change in height
Ep = mgh
Define work done:
Work done = force x distance moved in the direction of the force
How much work is done by a force F moving an object a displacement x when F and x are
a) parallel b) perpendicular
a) Fx b) 0
