Work and Energy Flashcards
(28 cards)
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
