Work and Energy

Question 1

when is work done

Answer 1

when the point of application of a force is moved in the direction of the force

Question 2

what is the equation for work done and what is it measured in

Answer 2

• work done = force x distance (moved by the force in the direction of the force)
• measured in joules

Question 3

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

Answer 3

• 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

Question 4

how would the work done then be calculated from this

Answer 4

• multiple the value you get for the horizontal component of the force by the distance moved
• as work done = Fs

Question 5

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

Answer 5

• 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

Question 6

what adjustment could be made to the work done equation if you were to practically use it and why

Answer 6

• 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

Question 7

what type of quantity is work done and why

Answer 7

• it is a scalar quantity
• because force and displacement are both vectors
• and vector times a vector is a scalar

Question 8

what is the definition of energy

Answer 8

the ability to do work

Question 9

what is therefore the relationship between work done and energy transferred

Answer 9

• 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

Question 10

why is the definition of energy not universal or prefect

Answer 10

• because not all energy transferred can do work

- like in heat engines

Question 11

what is potential energy

Answer 11

the ability of an object to do work by virtue of its position or state

Question 12

if a box is lifted off the ground by a height of h, how would the work done to lift the box be calculated

Answer 12

• 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

Question 13

in this case, what is the force exerted on the box equal to and why

Answer 13

• 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

Question 14

therefore what is the equation for the work done to lift a box

Answer 14

delta W = mg delta h

Question 15

what is this work done usually called

Answer 15

gravitational potential energy (GPE / Egrav)

Question 16

what is gravitational potential energy

Answer 16

the energy an object possesses by virtue of its position in a gravitational field

Question 17

when can this equation only be applied

Answer 17

• 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

Question 18

what is elastic potential energy

Answer 18

the ability of an object to do work by virtue of a change in its shape

Question 19

what is the equation for elastic potential energy and why

Answer 19

• delta EPE = Fav * delta x
• because the elastic object is stretched using an average force of Fav
• so that it extends by delta x

Question 20

what is kinetic energy

Answer 20

the ability to an object to do work by virtue of its motion

Question 21

what is the equation for kinetic energy and why

Answer 21

• 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

Question 22

what would the equation need to be if ‘u’ wasnt 0

Answer 22

KE = (mv^2 / 2) – (mu^2 / 2)

Question 23

how is energy transferred during the swing of a pendulum

Answer 23

• 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

Question 24

what is the motion of the pendulum described as

Answer 24

• a continuous variation of GPE and KE

- where GPE goes to KE goes to GPE

Question 25

would the pendulum swing forever in practice and why

Answer 25

• 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

Question 26

what is internal energy in a gas of fixed volume

Answer 26

the sum of the kinetic energy of all the molecules

Question 27

what does an increase in internal energy usually result in

Answer 27

an increase in temperature

Question 28

what are some other forms of energy that you should be familiar with

Answer 28

chemical energy
nuclear energy
electrostatic potential energy
radiant energy