Work and Energy Flashcards
when is work done
when the point of application of a force is moved in the direction of the force
what is the equation for work done and what is it measured in
- work done = force x distance (moved by the force in the direction of the force)
- measured in joules
if someone was pulling a pram at an angle upwards with a force of F, how would you calculate the the force that is actually pulling the pram in the direction of the force
- you would use trigonometry to work out the horizontal component of the force using the given angle
- that is the force that is actually pulling the pram horizontally
- as some force would be pulling it upwards which isnt needed in a horizontal plane
how would the work done then be calculated from this
- multiple the value you get for the horizontal component of the force by the distance moved
- as work done = Fs
if F was someone pulling the pram at an angle slightly upwards with theta being the angle between that and the horizontal component, and the pram was pulled X distance, what equation could you derive to calculate the work done on the pram
- as F slightly upwards would be the hypotenuse and the horizontal would be the adjacent that theta is in between, you would use cos (CAH)
- therefore, cos theta = A / F, so F cos theta = the horizontal component of the force
- and if distance X is travelled, the work done on pram = (F cos theta) x X
what adjustment could be made to the work done equation if you were to practically use it and why
- delta work done = F(av, meaning average) x delta s
- because the forces acting on moving objects isnt always constant
- so it is better it express the work done in terms of the average force
what type of quantity is work done and why
- it is a scalar quantity
- because force and displacement are both vectors
- and vector times a vector is a scalar
what is the definition of energy
the ability to do work
what is therefore the relationship between work done and energy transferred
- energy is transferred to an object when work is done on it
- in a closed system the sum of these will always be the same
why is the definition of energy not universal or prefect
- because not all energy transferred can do work
- like in heat engines
what is potential energy
the ability of an object to do work by virtue of its position or state
if a box is lifted off the ground by a height of h, how would the work done to lift the box be calculated
- you would need to work out the force exerted on the box in order to lift it that height
- for the equation work done = Fs to work
in this case, what is the force exerted on the box equal to and why
- its weight
- because force essentially = weight given that acceleration and gravity are the same due to earth
- F= ma and W = mg where a=g
therefore what is the equation for the work done to lift a box
delta W = mg delta h
what is this work done usually called
gravitational potential energy (GPE / Egrav)
what is gravitational potential energy
the energy an object possesses by virtue of its position in a gravitational field
when can this equation only be applied
- when the variations in height are still close to the earths surface
- because g would be assumed to be constant
- however it h was really large and the object was far away from the surface, g would noticeably decrease
- meaning it wouldnt be a constant in the equation any more
what is elastic potential energy
the ability of an object to do work by virtue of a change in its shape
what is the equation for elastic potential energy and why
- delta EPE = Fav * delta x
- because the elastic object is stretched using an average force of Fav
- so that it extends by delta x
what is kinetic energy
the ability to an object to do work by virtue of its motion
what is the equation for kinetic energy and why
- gain in kinetic energy = work done
- KE = Fs
- F = ma so Fs = ma s = m as
- using the equation v^2 = u + 2as where u = 0
- v^2 = 2as
- so as = v^2 / 2
- meaning m as = mv^2 / 2
- gain in KE = mv^2 / 2
what would the equation need to be if ‘u’ wasnt 0
KE = (mv^2 / 2) – (mu^2 / 2)
how is energy transferred during the swing of a pendulum
- when the ball is displaced to one extreme (fully left or right) it has the maximum amount of GPE it can have
- this is because the height between the lowest point it can get to and where it is is at its highest
- at the same time, its kinetic energy at this point is at its minimum as it isnt moving at these extremes
- however, GPE gets transferred to KE as it swings back down to the midpoint until KE reaches its maximum value where h = 0
- when the midpoint is passed the ball swings back up to the other extreme where it gains GPE and loses KE as delta h increases
what is the motion of the pendulum described as
- a continuous variation of GPE and KE
- where GPE goes to KE goes to GPE
would the pendulum swing forever in practice and why
- no, it would begin swinging at a decreases amplitude
- because some energy is being lost as work is being done against air resistance
- as well as against friction at the support
what is internal energy in a gas of fixed volume
the sum of the kinetic energy of all the molecules
what does an increase in internal energy usually result in
an increase in temperature
what are some other forms of energy that you should be familiar with
chemical energy
nuclear energy
electrostatic potential energy
radiant energy
