Physics1101W
Fri,Feb.24,2017
Howtomakeamusicvideoinzerog
Theplan
• Today-Monday:Ch.5
• Nextweek:Quiz2!
• Readingassignment:
• Current:finishCh.5
• Monday:nonewreadingassignment
• Labreports– rewritedueMonday.
Today’stopics
• Conceptreview:
• SolvingproblemsusingNewton’slaws
• Newconcepts:
• Weightvsapparentweight
• Moreonfriction
Forceproblems
Mainstrategy:useNewton’s2nd law:
Σ𝐹# = 𝑚𝑎#
Σ𝐹' = 𝑚𝑎'
à Question:isthereacceleration?Iftheacceleration
iszero(equilibrium)thentherighthand sideiszero:
Σ𝐹# = 0
Σ𝐹' = 0
Whichequationscanweuse?
• Agirderisliftedatconstantspeedbyacrane.
a) Σ𝐹' = 𝑚𝑎'
b) Σ𝐹' = 0
• Agirderisloweredbyacrane.Itisslowingdown.
a) Σ𝐹' = 𝑚𝑎'
b) Σ𝐹' = 0
• You’restrainingtoholda200lb barbelloveryourhead.
a) Σ𝐹' = 𝑚𝑎'
b) Σ𝐹' = 0
Whichequationscanweuse?
• Ajetplanehasreacheditscruisingspeedandaltitude.
a) Σ𝐹# = 𝑚𝑎# andΣ𝐹' = 𝑚𝑎'
b) Σ𝐹# = 0 andΣ𝐹' = 0
• ArockisfallingintotheGrandCanyon.
a) Σ𝐹' = 𝑚𝑎'
b) Σ𝐹' = 0
• Aboxinthebackofatruckdoesn’tslideasthetruckstops.
a) Σ𝐹# = 𝑚𝑎#
b) Σ𝐹# = 0
• Aboxinthebackofatruckdoesn’tslideasthetruckstops.
a) Σ𝐹' = 𝑚𝑎'
b) Σ𝐹' = 0
Reviewofmassvsweight
• 𝐹⃗ = 𝑚𝑎⃗ è weight=mg(orw=mg)
• gisdeterminedbyyourplanet!
• Makesurenottogiveamassasananswerfora
weightorviceversa.
• Sohowtoachieve“weightlessness”?
• Option(a):Gotointergalacticspacefarfromanystaror
planet!
• Option(b):JointhebandOkGo:
www.cnn.com/2016/02/11/entertainment/ok-goweightless-video-feat/
Howdidtheyachieve“weightlessness”?
• Theirmassesdidn’tchange.
• Theiractualweight(howhardtheEarthpullson
them)didn’tchange.
• Buttheyfilmedinsideaplanethatisfallingjustas
quicklyastheyare.
à Nonormalforce.
• Result:theychangedtheirapparentweight.
• Magnitudeofsupportingcontactforces.
• Canbemore,less,orthesameasactual weight.
• Apparentweightiswhatascalewouldreadifyouare
standingonit.
Mass and Weight
–w = may = m(–g)
w = mg
(Fg = mg)
Slide 5-19
Apparent Weight
Apparentweightisthemagnitudeofthe
supportingcontactforces.
Slide 5-20
Example
A 50 kg student gets in a 1000 kg elevator at rest. As the
elevator begins to move, she has an apparent weight of 600 N
for the first 3 s. How far has the elevator moved, and in which
direction, at the end of 3 s?
Slide 5-21