Physics - Work, Energy and Power Flashcards
Explain the principle of conservation of energy
Energy cannot be created or destroyed in a closed system, only converted from one form to another.
Define work done in terms of energy
Work done is the transfer of energy
Define work in terms of force
Work done is force multiplied by the distance moved in the direction of the force
Write work in terms of force as an equation
W=F×d
Explain how the vector nature of the force and distance are important when calculating work done
Force and distance moved must be parallel
Explain what causes an object to have GPE
GPE is the energy stored in an object due to its position within a gravitational field
Write the equation for GPE
GPE=mgh
Derive the equation for GPE by considering the work done needed to lift a mass, m, a height h
W=F×d=mgh
Why is it difficult to define absolute GPE?
You need to define zero GPE, but it is not easy to choose where this is (it is infinitely far away)
Draw the gravitational field lines around the Earth
(Draw the Earth, and arrows pointing inwards towards the Earth in a circle around the earth)
Why can we consider the field lines parallel near the surface of the earth
We only consider a small section of the Earth’s surface, which is nearly flat, and only small changes in height, so the separation of the field lines is roughly constant
Draw equipotentials for GPE around the earth
(Draw the same diagram as the gravitational field lines but draw circles around the arrows. They should be close in close to the Earth, and get more spaced out the further out you go.) Note: equipotentials are lines of equal field strength
How are field lines and equipotentials related?
They are always perpendicular and the closer the field lines are the closer the smaller the separation of the equipotentials
Explain what causes an object to have KE
A moving object has kinetic energy
Write down the equation for KE
KE=1/2 mv^2
Kinetic energy = 1/2 x mass x velocity squared
How can the KE of an object be increased or decreased?
When work is done on/by the object that causes its motion to change
Describe how the energy of a ball changes when it is thrown vertically in the air
KE is transferred to GPE until there is no more KE - the ball stops. Then GPE is transferred into KE due to the gravitational force
How is the change in KE related to the change in GPE assuming all other energy losses can be ignored? (Ball thrown in the air) Explain
They are equal due to conservation of energy
Write an equation to demonstrate the change in KE related to the change in GPE (Ball thrown in the air)
ΔKE=ΔGPE
Write down an equation linking the final velocity of an object to the initial velocity of an object when a force is applied over a certain distance
v^2=u^2+2as
Final velocity (m/s) = initial velocity (m/s) + 2 x acceleration (m/s^2) x displacement (m)
In a speed/time graph, what does the area represent?
The distance
Define power
The rate of transfer of energy: The rate at which work is done
Write the definition of power as an equation
P=ΔW/Δt
Power = work done/time taken
What is the unit of power and how is it related to the Joule?
Watt - 1 watt = 1J/s
Derive an equation for power in terms of force and velocity
P = W/t = (F×d)/t = F×v
Power = Force x velocity (because distance divided by time is the same as velocity)
How is power related to work and how is it different?
Power is the rate at which work is done. Work does not depend on the time it takes.
List the units and unit symbols for: mass, length, velocity, acceleration, force, time, gravitational field strength
Mass (kilogram, kg), length (metre, m), velocity (metre/second, m/s), acceleration (metre/second2, m/s2), force (Newton, N), time (second, s), gravitational field strength (newton/kilogram, N/kg)
List the units and unit symbols for: moments and momentum
Moments (newton metre, Nm), momentum (kilogram metre/second, kg m/s)
