Transcript Dyanmics I slides

Today: Dynamics I
Mass, Inertia, Weight
Balanced Forces
Newton’s Laws
Free-body diagrams
Aristotle v. Newton
384 – 322BC
1643 – 1747AD
Aristotle’s Laws of
Motion
•
Nothing moves unless you push it.
mover]
[it is moved by a
» Some motion is natural for the sublunar elements,
rectilinear motion to or away from the earth's center
for the supralunar quintessence, circular motion
» All other motion is violent, and requires a mover
• [Anselm's nth proof of the existence of God]
• Because motion exists, there must be a self-moved mover,
i.e. a Prime Mover [later i.d. God]
•
•
There is natural, violent, and local motion;
rectilinear and circular motion
Speed is proportional to motive force, and inversely
proportional to resistance.
» v = k (F / R)
•
•
There cannot be a vacuum [therefore Natura is a
plenum]
The most Natural state: Rest [if terrestrial]
Newton’s First Law
Newton’s First Law
Objects in motion tend to
stay in motion, and objects
at rest tend to stay at
rest unless an outside
force acts upon them.
Inertia
Kitchen roll
Coin and cup
A hammerhead
Anvil and hammer
Airplanes
Weight?
Zero Net Force
(or, a delicious melon)
Newton’s Second Law
The acceleration of an object as
produced by a net force 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 object.
Newton’s Second Law in
symbols
F = m a
(unit: Newton / kg m s-2)
W = ?
Try:
• A car has a mass of 1000 kg. At one
point in its motion, when the combined
forces of air resistance and friction
(acting backwards) are 500 N, its
acceleration is 1.6 m s-2. What forward
driving force does it need?
Direction of motion
Air resistance
+ Friction
Driving force
Car (obviously)
Newton’s Third Law
When body A exert a force
on body B, body B exerts
a force on body A that is
equal in magnitude and
opposite in direction
Delicious-melon-on-table
Delicious-melon-on-tableon-planet
Think:
What action-reaction
pairs can you think of
happening right now?
Free-Body Diagram
Magnet-string-paperclip
Free-body diagrams
• RI boy’s head sleeping on a table
• RI boy suspended motionless from a
bar hanging from a ceiling by two
ropes
• RI boy free-falling from a nest in
a tree. Neglect air resistance.
• RI boy gliding from a tree to the
ground at constant velocity.
Consider air resistance.
Free-body diagrams
• A rightward force is applied to a RI
boy in order to move it across a desk
with a rightward acceleration.
• Same as above, but constant velocity.
• A RI boy rests a backpack on his
shoulder. The pack is suspended
motionless by one strap from one
shoulder.
• A skydiving RI boy is descending with
constant velocity. Consider air
resistance.
Free-body diagrams
• A force is applied to the right to
drag a RI boy across looselypacked snow with rightward
acceleration.
• A RI boy is flying upwards after
having been punted by a teacher.
Neglect air resistance.
• A car is coasting to the right and
slowing down (no RI boys
involved).
Conclusion
Mass, Inertia, Weight
Balanced Forces
Newton’s Laws
Free-body diagrams