newton’s laws of motion history aristotle first to study motion galileo dropped big and small...
TRANSCRIPT
Newton’s Laws of Motion
HistoryAristotle
First to study motion
Galileo Dropped big and small rock
Fell at same rate If no interference – motion will go
on forever
History
Newton formalized ideas At 23 (1665) he overthrew
Aristotle’s 2000 year old ideas
LAWS OF MOTION
Newton’s First Law of Motion Law of Inertia
An object continues in its state of rest, or of uniform motion in a straight line, unless it is acted upon by an unbalanced force
“Object” – any body
“Continues” – keeps rest or moving
“Unbalanced force” – net force, not in equilibrium
Law of Inertia
Why Law of Inertia? Tendency of a body to resist
change of motion More matter = more inertia Ex. If chased by an elephant,
how should you run away?
Zig-zag!!
Mass and Inertia
Wood LeadCardboard
• Same volumes take up the same amount of space
• Which is harder to move?
- More inertia
- More mass (mass is a measure of inertia)
V V VI I IM M M
Mass and Weight
Does mass = weight? Mass – quantity of matter in a body Weight – measure of gravitational force which
acts on a body Depends on where the object is Ex. Earth, Moon, Space
g = 9.8 m/s2 g = 1.6 m/s2 g = ?
Mass and Weight
Gravity on the Moon http://www.youtube.com/watch?v=PE81zGhnb0w
Gravity around the Solar System http://www.aerospaceweb.org/question/astronomy
/q0227.shtml
Mass and Weight
How much would a 75 kg (165 lb) person weigh on the Moon? On Mars? gmoon= 1.62 m/s2
gmars= 3.77 m/s2
2.2 pound = 1 kilograms 1 pound = 4.45 Newtons
Moon: 121.5 N or 27.3 lbs Mars: 282.8 N or 63.5 lbs
Examples of Inertia
On bus (rest rest) Standing and bus moves forward You seem to fall backward Actually feet move forward with bus and body
remains at rest Bus stops?
Examples of Inertia
Dishes on Table (rest rest) Snap a cloth from beneath Friction between dishes and cloth not enough to
move dishes
Examples of Inertia
In stopping car (motion motion) Fly forward Keep moving while car stops
Newton’s Second Law of Motion Deals with acceleration Acceleration = a = v / t
Ex. Car died tow
5 N
50 N
Which provides the greater acceleration?
Which can get car from 0 mi/hr to 20 mi/hr faster?
GREATER FORCE
Newton’s Second Law of Motion
With same force, which can go from 0 mi/hr to 50 mi/hr faster?
Mass = 1000 kg
Mass = 5000 kg
SMALLER MASS
SMALLER INERTIA
Second Law
Force = mass x acceleration (F = ma)
The more force on an object, the more it accelerates. The more massive an object, the more it resists acceleration.
Second Law “The acceleration of a body is directly proportional
to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the body”
* “in the same direction as the net force” a in the same direction of body’s motion speed up a in opposite direction of body’s motion slow down a at right angles to direction of body’s motion deflect
circular Any other change in speed and direction
Units
mks cgs English
Length
Mass
Time
Force
Weight =
W = g =
Use ______________ as force units
A force is a __________ or a __________
Now also say: Force is anything that can ____________ a body.
History Revisited
Galileo gave no reason why bodies fall with an acceleration.
Newton did. Newton – a F pulls on a body and pulls it toward
the Earth. F = ma
Heavier body attracted with more F. Heavier body has more m and…
More I More resistance to change
Ex. If 2 x F and 2 x m same net acceleration
Newton’s Third Law of Motion Force: Causes acceleration A push or a pull Always involves two objects
interaction Ex. Hammer and Nail
Hammer exerts force on nail In process, nail slows down hammer Therefore, nail exerts force on
hammer Ex. Ice skater with rifle or with mitt
Newton’s Third Law of Motion To every action there is an equal and
opposite reaction One force is action force, other is reaction Neither can exist without the other
Action
Weightlifter lifts on barbell
Feet down on Earth
You sit on chair
Tires sit on road
You push on wall
Helical blades push down on particles
Rifle pushes bullet
Reaction
Barbell pulls on weightlifter
Earth pushes up on feet
Chair pushes up on you
Road pushes up on tires
Wall pushes back on you
Particles push up on blades
Bullet pushes back on rifle (KICK)
SAME FORCE: F/mb = a F/mr = a
Newton’s Third Law of Motion Sometimes not so obvious.
Reactions in free-fall Earth pulls on body = action Body pulls on Earth = reaction Therefore, can say boulder falls to Earth or Earth falls to
boulder F is equal but acceleration is very small
When step off curb – street comes up to meet you Bug against windshield – deceleration of car small,
deceleration of bug large
When objects are relatively the same mass, both forces are more easily noticed since both accelerations are about equal.