forces Flashcards
when is work done
Work is donewhen an objectis movedover a distanceby a forceappliedin thedirectionof
its displacement
work done equation
work done (joules) = force (Newtons) * distance (m)
1 J = ?
1 Joule = 1 Newton metre
force and direction
Ifa forceacts in thedirection that an objectis moving,then theobjectwillgain energy
Iftheforceacts in theoppositedirection tothemovementthen theobjectwill loseenergy
how is nrg transferred to gpe nrg store of obj
mechancially
when friction is present wat happens
When friction is present, energy is transferredby heating
This raises thetemperature(energy is transferredtothethermal store) oftheobject
andits surroundings
Theworkdoneagainstthefrictionalforces causes this risein thetemperature
how does friction occur
Imperfections attheinterfacebetween theobject andthesurfacebumpintoandrubup
againsteach other
Notonlydoes this slow theobjectdown but alsocauses a transferofenergy tothe
thermal storeoftheobject andthesurroundings
air resisitance - what happens
Particles bumpintotheobject as itmoves through theair
As a result, energy is transferredby heatingduetotheworkdoneagainstthefrictional
forces
stationary objects - how many forces must be applied to change their shape?
For stationaryobjects, morethan oneforcehas tobeappliedtochangetheir shape
how can shape of obj change by
Their shapecan changeby:
Stretching(forces in oppositedirections away from theobject)
Bending(forces thatdistorttheobject)
Compressing(forces in oppositedirections towards theobject)
compression
An exampleofcompression is placinga mass on topofa springplacedon a flat surface
Thetwoforces are:
Theweightofthemass
Thereaction forcefrom thesurfacetothespring
the 2 forces are towards each other
stretching
An exampleof stretchingis placinga mass on thebottom ofa vertically hangingspring
Thetwoforces are:
Theweightofthemass
Thetension in thespring
forces are away from each othr
bending
An exampleofbendingis a divingboardbendingwhen a swimmer stands atthefarend
Thetwoforces are:
Theweightoftheswimmer
Thereaction forcefrom theblock tothedividingboard
forces act towards each other, but at different points on obj.
bending can alos be cuased by two forces at angle to each other
changes of shape
Achangeof shapeis calleda deformation andcan eitherbe:
Elastic
Inelastic
elastic deformaton
Whenobjects return totheiroriginal shapewhen thestretchingforceis removed
Examples ofmaterials that undergoelasticdeformation are:
Rubberbands
Fabrics
Steel springs
inelastic deformation
Whenobjects remain stretched and donot return completely totheiroriginal shape
even when thestretchingforceis removed
Examples ofmaterials that undergoinelasticdeformation are:
Plastic
Clay
Glass
hookes law
Therelationshipbetween theextension ofan elasticobject andtheappliedforceis defined
by Hooke’s Law
Hooke’s Law states that:
Theextensionof an elasticobject is directlyproportional totheforceapplied, up
tothelimit of proportionality
limit of proportionality
Thelimit of proportionality is whereifmoreforceis added,theobjectmay extendbutwill
not return toits original shapewhen theforceis removed(itwillbeinelasticallydeformed)
This varies accordingtothematerial
hookes lae eqaution
f = k * e
Where:
F =forcein newtons (N)
k=springconstantin newtons permetres (N/m)
e =extension in metres (m)
spring constant
how stif a spring is
higher the spring const, higher the stiffness
hookes law on graph
Hooke’s law is thelinearrelationshipbetween forceandextension
This is representedby a straight lineon a force-extension graph
materials not obeying this have non inear relationship - represenyed by curve on graph
Any materialbeyondits limitofproportionality will havea non-linearrelationshipbetween
forceandextension
spring constant from a graph
Iftheforceis on they axis andtheextension on thex axis,thespringconstantis the
gradient ofthestraight line(Hooke’s law) region ofthegraph
Iftheforceis on thex axis andtheextension on they axis,thespringconstantis 1 ÷gradient
ofthestraight line(Hooke’s law) region ofthegraph
elastic pot nrg
Theenergy stored in an elasticobject when work is doneon theobject
Providedthespringis notinelasticallydeformed(i.ehas notexceededits limitof
proportionality),theworkdoneon thespringandits elasticpotentialenergy storedare
equal
equation elsastic pot nrg
1/2 k e^2
Where:
E =elasticpotentialenergy in joules (J)
k=springconstantin newtons permetre(N/m)
e =extension in metres (m)
what is elastic pot nrg eq for
This equation is only for springs that havenotbeen stretchedbeyondtheirlimit of
proportionality
displacement
Displacement is a measureofhow far somethingis from its startingposition, alongwith its
direction
it is a vector
speed
scalar quantity
factors affecting speed
Age
Terrain
Fitness
Distance
for vehicles:
Shape
Design
Cost
Purpose
speed of sound
330 m/s
1500m/s in seawater
velocity
vector force
vlecoity- circular motion
when obj travels along circular path, velocity always changing
speed may be constant
direction always changing
distance time graph
straight line - constant speed
slope of straight line - magnitude of peed (steeper the line, faster the speed)
horizonalline - obj is stationary
curve - speed is changing
gradient of the line is the speed of the obj
instantaneous pseed
to calcualte speed at point in time use tangent
acceleration
rate of change of velocity
acc = delta velocity/time
acc = m/s^2
delta v = m/s
spd = s
acceleration - up and down
speed up - positive acc
speed don - neg acc
Veolcity time graph
straight line - const acc
slope - magnitude of acc
(steeper the slope, higher the acc)
flat line - const velocity
acc - gradient of the graph
area under velocity time graph - distance travelled
uniform (constant) acc
v^2= u^2 + 2as
Where:
s =distancetravelledin metres (m)
u =initial speedin metres per second(m/s)
v =final speedin metres per second(m/s)
a =acceleration in metres per secondsquared(m/s )
used for when time is not knon
freefall
In theabsenceofairresistance, allobjects fallwith thesameacceleration
This is calledtheaccelerationduetogravity:
acc due to graviy = 9.8m/s^2
how to work out weight from freefall
acc due to gravity * mass = weight
terminal velocity
When a skydiverjumps outofa plane, twoforces act:
Weight (duetogravity)
Airresistance(duetofriction)
as they fall, air resiistance increases
one air resistance = weight, no more resultant force.
now they fall at const speed which is their terminal velocity
therefore smaller the weight of obj, smaller th terminal velocity
newton first law of motion
Newton’s first lawof motion states:
Objects willremain at rest, or movewith a constant velocity unless acted onby a
resultant force
This means iftheresultantforceactingon an objectis zero:
Theobjectwillremain stationary ifitwas stationarybefore
Theobjectwillcontinuetomoveatthesamevelocity ifitwas moving
newton second law of motion
Newton’s second lawof motion states:
Theaccelerationof anobject is proportional totheresultant forceactingon it and
inverselyproportional totheobject’s mass
what does netwtonn 2nd law of motion eplain
An objectwill accelerate(changeits velocity) in responsetoa resultant force
Thebiggerthis resultantforce,thelargertheacceleration
For a given force,thegreatertheobject’s mass,thesmallertheacceleration
experienced
fore and acc
force = mass * acc
force = netwton
mass = kg
acc = m/s^2
newton 3rd law of motion
Whenever twobodies interact, theforces they exert on eachother areequal and
opposite
what does newton third law explain
Allforces arisein pairs - ifobjectAexerts a forceon objectB,then objectBexerts an
equal andoppositeforceon objectA
Forcepairs areofthesametype- forexample, ifobjectAexerts a gravitational force
on objectB,then objectBexerts an equal andoppositegravitational forceon object
A
inertia def
Thetendency of anobject tocontinuein its stateof rest, orin uniformmotion
unless acted uponby an external force
inertia in more detail - when at rest and when in motion
In otherwords, inertia is an object’s resistancetoa changein motion
Ifan objectis at rest, itwilltendtoremain at rest
Ifan objectis movingat a constant velocity (constant speedin a straightline), itwill
continuetodoso
intertial mass
Inertial mass is thepropertyofan objectwhich describes how diffcult itis tochangeits velocity
is the ratio between force applied and acc it experiences
inertial mass eq
inertial mass = force / acc
inertial mass - kg
force - Netwons
acc - m/s^2
stopping distance def
Thetotal distancetravelled duringthetimeit takes for a car tostop in responseto
someemergency
stop dist eq
think dist + brake dist
all measured in metres
reaction time def
Ameasureof howmuch timepasses between seeingsomethingand reactingtoit
think dist def
Thedistancetravelled by a car fromwhen a driverrealises they need tobraketo
when they apply thebrakes
think dist eq
speed of car * driver reaction time
factors affecting think dist
car speed
tiredness
distractions
intoxication
factors affecting brake dist
car speed
vehicle condition - worn tires, poor brakes
road condition - wet/icy roads harder to decelerate
vehicle mass
braking and friction
when driver apply brakes, friction occur between brakes and wheels
meaning kin nrg of car decrs, thermal nrg brakes incrs
this means car decelerate
braking force and spd
greater spd of vehicle, greater braking foce needed to be applied
this means decelration will be large as well
due to newton 2nd law motion
danger large decleration
brakes overheating
loss of control of vehicle
estim decelrationg forces
brake force * brake distance = 1/2 * mass * velocity^2
calculting momentum
p = mv
p = momentum (kg m/s)
m = mass (kg)
v = velocity (m/s)
what does momentum do
keep an obj moving in same direction
makes it difficult to change direction of obj with large momentum
momentum dpend on direction of travel
therefore can be pos or neg
if obj travelling right has pos momentum, obj going left has neg momejtm
hen does momentum of obj change
if object accelerate or decelerate
if obj change direction
if obj mass changes
conservation of momentum
Theprincipleofconservation ofmomentum states that:
In a closed system, thetotal momentumbeforean event is equal tothetotal
momentumafter theevent
Thetotal momentumbeforea collision=Thetotal momentumafter a collision
what is a system
certian number of objects under consideration
can be 1 obj or multiple
is momentum scalar or vector
vector
is momentum conserved over time
always conserved over time
since momenum is vector, system of obj moving in opp directions have what
if objs moving in opp dir at same speed, have overall momentum of 0 as they cancel out
elastic collision
obj collid and move in opp dir
inelastic collision
obj collide and move in same dir
when elastic collision happen, the objs have
different velocity depending on its mass and initial momentum of system
when inealstic collision happen, objs have
combined mass and velocity
is momentum convervsed in collsion
always conserved in collsion
when analysing a collsion what do you do
consider motion before and after collsion and state the velcoity of each obj and direction each obj moves
state whether collision was elastic or inelastic and explain
describe any energy transfers if kin nrg not conserved
perfect elastic collsions
kin energy conserved - always equal
perfect inelastic collsion
two objs stick together after colliding
