Work, energy & power Flashcards
What is Power?
What is its equation?
Power is rate of work done
Power (Js^-1) = Work done (J) /
Time (s)
How do you calculate the power developed by the driving force of a car (or any object which needs a constant force to maintain a constant speed), knowing velocity, force & distance?
P = W/t —> W = Fx
P = Fx / t
x/t = v
P = Fv
What is the equation for efficiency?
Efficiency = Useful energy Output / Total energy input x100
When an object falls from rest, a set distance, how can you calculate velocity (final) ?
Only valid if no resistive forces involved
As all energy is transferred from GPE to KE:
Mgh = 1/2 mv^2
GPE = KE
therefore 2Mgh / m = V^2
—> root 2gh = v
ONLY applies for objects starting at rest
Energy is dervived as the capicity to do work
What is the equation for work done?
“This is also the definition of work done by a force”
Work done = Force x Distance moved in the direction of force
W (Nm) = F (N) x(m)
When a force act at an angle to a objects direction of motion, how do you calculate the componant of force in the direction of the x - axis?
Hence, how do you calculate work done?
F Cos theta
W = F Cos theta X
What are two forces of energy related to mass?
1) Kenetic energy - Energy due to motion of an object with Mass
2) Gravitational Potential Energy - energy of an object due to its position in a gravitational field
What are the 7 types/forms of energy?
1) Chemical energy - Energy contained within chemical bonds (can be relased when atoms are rearranged)
2) Elastic potential Energy - Energy stored in a object as a result of a revisible change in shape
3)Electrical Potential Energy - energy of electrical changes due to position in an electric field
4) Nuclear energy - energy within the nucli of atoms - it can be released when the particles within the nucleus are rearranged
5) Radiant energy - energy associated with all EM waves stored within the oscilating electric & magnetic field
6) Sound energy - energy of mechanical waves due to the movement of atoms
7) Internal Energy - the sum of random potential and kenetic energies of atoms in a system. AKA heat or thermal.
What is stated by the conservation of energy?
The total energy of a closed system remains constant.
Work is done to convert energy from one form to another
How do you calculate Kenetic energy of an object from its mass and speed?
Ek (J) = 1/2 x m (kg) x v^2 (ms^-1)
GPE is the capacity of doing work as a result of an objects position in a gravitational field
How do you calculate change in gravitational potential energy?
Hint: it is expanded from W = Fx
Ep = mgh
Change in GPE (J) = mass (kg) x 9.81 (gravitaional field) x height (m)
W = Fx
W = (mg)(x)
Exam Technique:
When calculating energy lost when something hits the ground (work done against ground), what do you do?
DONT find difference in velocity, use KE equation.
1) Calculate energy before input
2) Calculate energy after input
3) Use the difference
Exam Technique:
How do you define entiities such as work done?
You use their equations
E.g Work done = force x distance
When stating what happens to work done on an object, what do you do?
You state what the energy is transferred to
E.g work done against friction generates thermal energy
When defining units like the watt, what do you do?
You state its units e.g Js^-1
Do not write things like rate of doing work
E.g 1 watt = 1 Js^-1
