Download Newton’s First Law of Motion - Lecture Slides - Basic Concepts of Physics and more Slides Physics in PDF only on Docsity!
Notes on Chapter 1: About Science• We will barely cover this in class, and it will
not^ be examined, but I encourage you to read it on your own. • Main points:^ Observable physical evidence
is at the basis of science. Scientific theories must be^ testable.^ Measurement
plays a crucial role (eg. read about measurements in 200’s BC of size of earth, moon, sun – and try some at home!)^ Mathematics
provides unambiguous, compact language for scienceTerminology: Hypothesis^ = educated guess^ Law^ =^ principle
= rule Theory = synthesis of body of info that encompasses well- tested and verifiable hypotheses about certain aspects of the natural world. Theoriesmay change in time!^ Beware of pseudoscience!
Lacks evidence and falsifiability test.
Chapter 2: Newton’s First Law of Motion -
Inertia
Before getting into this, note ideas on motion
prior^ to Newton^ (I won’t examine this)
-^ Aristotle^ (c. 320 BC), all motions are due to “nature” of theobject, or to “violent” influences (push or pull) .“Normal state” = at rest, except for celestial bodies.Heavier objects fall faster, striving harder to achieve their “proper place”.--^ Copernicus^ (c. 1500’s) doubted that everything revolvedaround earth. Formulated sun-centered system.--^ Galileo^ (c. 1600’s) agreed with Copernicus, and disagreedalso with Aristotle’s “natural state” idea, using observation and experiment.Dropped objects from Leaning Tower of Pisa and found they fell at the samerate (apart from small effect of air resistance). Inclined planes experiments.Concept of Inertia Read more in your book. --^ Newton^ (c. 1665) formulated Newton’s Laws of Motion…
Newton’s 1
st^ Law of Motion: Inertia
-^ Every object^ continues
in its state of rest, or of uniform motion in a straight line, unless acted on by a force.• Inertia^ = property of objects to resist changes in motionHeavier (more massive) objects tend to have more inertia
(more in next
chapter)^ – e.g. takes more work to shake flagpole back and forth than to shake afeather…
-^ Force^ = something that produces a change in motion, apush or a pull.•^ Source can be muscle effort, or gravitational, or electric, ormagnetic…Often we
denote force by^
F Newton, N^ = standard unit of force. Physicists’ equivalent of “pounds”, butnot the same numerically i.e. 1-lb = 4.448-N.Eg. 1-kg weighs 9.8-N and 2.2-lb. Net force^ = resultant force when several forces are actingEg. Tug of War – both teams pull on opposite ends. If they each pull withthe same magnitude of force, there is zero net force on the rope.^ Eg: Note that any force has a direction!
A familiar force is gravitational force = weight
Clicker Question
The staging shown weighs300 N and supports twopainters, one 250 N and theother 300 N.The reading on the left scale is400 N. what is the reading onthe right-hand scale?A) 300 NB) 400 NC) 450 ND)850 NE) None of the above
Answer
The staging shown weighs 300 N andsupports two painters, one 250 N andthe other 300 N.The reading on the left scale is 400 N.what is the reading on the right-handscale? The upward forces are (400 N + RH tension). By the equilibrium rule
^ F = 0,
A) 300 NB) 400 NC) 450 ND) 850 NE)^ None of the abovethis upward total must equal the downward forces are (250 N + 300 N + 300 N)= 850 N. Hence, RH tension must be 450 N.Note that although the two tensions must add to the total weight, the tension islarger in the rope nearer the heavier person.
Support Force (a.k.a. Normal Force)What forces are acting on the book lying on the table? Gravity (weight of book) acts downward. But since book is atrest, there must be an equal upward force.This upward force is called the
support^ force, or
normal^ force,
and equals the weight of the book.
^ F = 0, since at rest What creates the normal force? The atoms in the table behavelike tiny springs, so push back on anything (eg book) trying tocompress them.
Question
Say a 120-lb person steps on some bathroom scales.(i)^ How much is gravity pulling on her?
120-lb (=weight) (ii)^ What is the net force on her?
0 (since she’s at rest) (iii)^ Now suppose she stands on two bathroom scales, withweight evenly divided between them. What will eachscale read?
60 –lb each, since the sum of the scale readings must balance the weight.
Answer
Consider again the 120-lb person who steps on some bathroom scales.What is the net force on the bathroom scales?A) 0B) 120-lbC) 120 ND) None of the above
Because the scales are at rest .There are two forces on thescales: the downward weight ofthe person, exactly balanced bythe support force from the floor
. docsity.com
Equilibrium of Moving Things
-^ An object moving at constant speed in a straight line is also inequilibrium,
^ F = 0.
Question:^ Can any object on which only
one^ force is acting, be in equilibrium?
No!
Consider pushing a box across a floor.(1) What forces are acting on the box?Weight downward, support force upward, your push across, andfriction between the floor and the box opposing your push.(2) What can you say about the relative magnitudes of the forces if is movingwith unchanging speed across the floor ?Magnitude of weight = support force.Your push = friction, if speed unchanging.(If it is speeding up, then your push > friction.)
Clicker Question
When the pellet fired into thespiral tube emerges, whichpath will it follow? (Neglectgravity).
Answer When the pellet fired into the spiral tubeemerges, which path will it follow?(Neglect gravity). B: While in the tube, the pellet is forced to curve, but whenit gets outside, no force is exerted on the pellet and(law of inertia) it follows a straight-line path – hence, B.
When the ball at the end of the string swingsto its lowest point, the string is cut by a sharprazor.What path will the ball then follow?b)^ At the moment the string is cut, the ball is moving horizontally. Afterthe string is cut, there are no horizontal forces, so the ball continueshorizontally at constant speed. But there is the force of gravity whichcauses the ball to accelerate downward, so the ball gains speed inthe downward direction. The combination of constant horiz. speedand downward gain in speed produces the curved (parabolic) path..
Answer
