Chapter 5 Work, Energy & Power Flashcards
Work Done
is when an object is moved over a distance by an external force applied in the direction of its displacement
when work is done
energy is transferred from one object to another -Work done can be thought of as the amount of energy transferred, hence its units are in Joules (J)
if a force acts in the direction that an object is moving
then the object will gain energy
If the force acts in the opposite direction to the movement
then the object will lose energy
The Principle of Conservation of Energy states that
Energy cannot be created or destroyed, it can only change from one form to another -This means the total amount of energy in a closed system remains constant, although how much of each form there is may change
Types of energy
-kinetic -gravitational potential -elastic -chemical -nuclear -internal
-kinetic energy
energy of a moving object
gravitational potential energy
the energy something gains when you lift it up, and which it loses when it falls
elastic energy
the energy of a stretched spring or elastic band(sometimes called strain energy)
chemical energy
energy contained in a chemical substance
nuclear energy
the energy contained within the nucleus of an atom
internal energy
the energy something has due to its temperature(or state). (Sometimes referred` to as thermal or heat energy)
Energy dissipation
-When energy is transferred from one form to another, not all the energy will end up in the desired form (or place) -Dissipation is used to describe ways in which energy is wasted -Any energy not transferred to useful energy stores is wasted because it is lost to the surroundings -These are commonly in the form of thermal (heat), light or sound energy -What counts as wasted energy depends on the system
The efficiency of a system
is the ratio of the useful energy output from the system to the total energy input —-If a system has high efficiency, this means most of the energy transferred is useful —-If a system has low efficiency, this means most of the energy transferred is wasted
efficiency = (energy)
useful energy output/ total energy input x100
efficiency = (power)
useful power output/ total power input x100
Defining Power
- The power of a machine is the rate at which it transfers energy
- Since work done is equal to the energy transferred, power can also be defined as the rate of doing work or the work done per unit time
- The SI unit for power is W-atts (W) where 1 W = 1 J s-1
Derivation Moving power
P=F x v
- This equation is only relevant where a constant force moves a body at constant velocity.
- Power is required in order to produce an acceleration
- The force must be applied in the same direction as the velocity
work done
W = F x d
power (work)
is the change of work power = w/t
at constant velocity
d = v x t therefore w = F x v t p= w/t = F x v x t / t
GPE = mgh
- Gravitational potential energy is energy stored in a mass due to its position in a gravitational field
- When a heavy object is lifted, work is done since the object is provided with an upward force against the downward force of gravity
- Therefore energy is transferred to the object GPE = M x G x H
Kinetic Energy
- Kinetic energy is energy an object has due to its motion (or velocity)
- A force can make an object accelerate; work is done by the force and energy is transferred to the object
- Using this concept of work done and an equation of motion, the extra work done due to an object’s speed can be derived KE = 1/2 x m x v^2
GPE v Height graphs
