Science & You
BVIS HANOI (VV)
Phuong & Chi
- Energy is the ability to do work. When work is done, energy is transfered from one type to another. The total amount of energy remains constant ( energy can never be created or destroyed)
- Energy cannot be measured directly. You can only measure the work done (i.e the amount of energy transfer)
Energy, Work Done
Work done = Force x distance moved in the same direction as the force
W = Fs
Work donw and hence, energy had the SI unit Joule (J)
1 Joule is the work done when 1N is used to move an object through 1m
This is a derived unit :
J = Nm = kgms^-2m = kgm^2s^-2
θ
Work done = Fcosθ x s
If the force is in a different direction to the displacement, you need the component in the direction of the force
gravitational potential energy
Lift the object at a constant velocity so that only GPE is gained.
F = mg
Work done by F:
W = F x h = mg x h
=> The work done equals to the gain in GPE
GPE = mgh
kinetic energy
Apply a force to accelerate a mass to velocity (v). No friction and horizontally so only KE is gained
Work done by F:
W = F x s (1)
S U V A T
s 0ms^-1 v F/m
=> v^2 = u^2 +2as
=> v^2 = 2 x F/m x s => s = 1/2mv^2/F (2)
(1) (2) => W - 1/2mv^2
Work done = gain in KE so KE = 1/2mv^2
power
power developed at a constant speed
Power = work done/ time
= Force x distance/ time
= Force x velocity
P = F x v
definition & unit
Definition: the rate of doing work
Power (W) = work done (J)/ time (s)
Unit: Watt => 1 watt means 1 Joule energy is transfered in 1 second
1W = 1Js^-1 = 1kgm^2s^-3
Efficiency and sankey diagram
Efficiency = 70/100 =0.7
% efficiency = 70%
Sankey diagram shows the amount of energy transfered with arrows representing the proportion of each type of energy
Efficiency = useful energy out/ total energy in
Percentage Efficiency = useful energy out/ total energy in x 100
