Download forces, motion, gravity lecture

Lecture: Forces, Motion,
Gravity and Friction
http://www.youtube.com/watch?v=YyJSlcIb
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What is the metric unit for mass?
What is the English unit for Mass (watch it, it’s
slimy)?
What is the English unit for weight?
What is the metric unit for weight?
How does weight relate to a force?
Variable:
English:
Metric:
Mass
slug
Gram (g)
Weight
pound
Newton (N)
(Newton is a force)
(F= Mass x Accel)
(1N= 1kg x 1m/s2)
the Newton is a measure of two things:
1. weight
2. force
THEY ARE THE SAME THING: (YOUR NAMES?)
But, this also means weight is a different measure than mass
1. Force: can only occur as two types:
A. pull or
B. push
C. all forces transfer energy to an object,
whether it moves or not: wall?
D. motion can only be caused by an applied force
(w/ direction and magnitude),
thus force is a vector
E. the amount of force needed to move an object
depends on:
1) the mass of the object (prius vs hummer?) and
2) the amount of friction applied to the object
resisting motion(ie: smooth vs. rough floor?)
F. The metric unit for force is the Newton (N):
the amount of force to cause 1kilogram to
accelerate at 1 meter per second squared:
-the same force a mouse applies to a table
top due the gravity acting on its mass;
-also = cube of butter in your hand
G. force formula: 1N = 1kg x 1m (note: mass in kg)
s2
(measured with a Newton spring scale: ?)
H. forces always come in pairs:
applied force (draw box w/ pairs?) and
the object’s resistant force
I. Mass is a measure of the amount of matter (atoms:
# amu, see periodic table)
J. Weight (not mass!!)
a measure (in Newtons of force) of the mass of the
object and
the amount of acceleration acting on the object:
1. so, what two variables affect weight (force)?
weight (force in N) = mass(kg) x acceleration
F = M x A (Newton’s 2nd law of motion)
2. so, how much does 1kg of mass weigh on
Earth?
a. weight = 1kg x 9.8 m/s2
so it is 9.8N, because the acceleration
of gravity on Earth is, for all objects,
9.8m/s2 (spring scale) and
weight is always in units of Newtons
b. if we were on the moon, would we weigh
six times less or six times more?
c. are people on the moon really weightless?
K. Show your work: (k, ?k, F, cu, ca, sf):
A 50.0 kg mouse runs w/ an acceleration of
2.30 m/s2, what force is it running at?
A 1.2 g humming bird is caught in a birding net
w/ an applied force of 0.07 N. What was the
bird’s acceleration at the time of capture?
L. Forces video #2:
http://www.youtube.com/watch?v=MztWyY9z1j
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2. There are four universal forces:
A. gravity force
1. always a pull force
2. the size of this force depends on two aspects:
a. the mass of the object:
thus everything with mass creates its own
gravity and
the more massive the object, the
greater its gravitational field:
moon vs Earth vs Sun:
this is what keeps planets in orbit,
otherwise they fly off into space
you vs your neighbor?
b. the distance from another
object:
the greater the distance = less effect of its
gravitational force
The sun is a more massive object, but
what object in our skies effects our tides
and why?
c. so: increase mass =
increase g force
(direct relationship)
increase distance =
decrease g force
(indirect relationship)
B. electromagnetic force
1. when electricity flows, it creates a magnetic field.
Thus, there are two types of forces created which are
related:
a. electric
-between opposite charged particles: balloon
-holds atoms together:
protons/electrons
-stronger than gravity:
why the ceiling does not fall down
b. magnetic force field
-can attract or repel (push or pull): magnets?
-magnetic levitation trains?
C. strong nuclear force
1. protons and neutrons are made of quarks
those quarks are each individually held together
by a strong force
2. occurs only over very short distances, w/ non
contact
3. break the strong force:
produce a nuclear explosion with electricity release
D. weak nuclear force
1. holds particles (p & n) within the nucleus
together w/non-contact: weak force
2. weak force decay causes neutrons to change
(“flavor”) into protons
3. loss of weak force causes the nucleus to
decay, releasing radioactivity (beta decay)
3. Friction
A. a force opposing motion
B. when surfaces move past each other they
create this force, of which there are three types:
1. sliding friction: greatest frictional
force
a. objects slide past one another:
feet sliding, tires screeching to a stop
b. weight of object effects its amount:
greater = more friction
rubbing hands together?
c. surface types effect its amount:
rough vs smooth?
2. rolling friction
a. object rolling over another
-bowling ball
-tires rolling on street
-ball bearings
3. fluid friction: least frictional force
a. fluid is any object which flows
b. ie: gases (wind), liquids (water)
c. lubricants change sliding friction to
fluid friction: reduces friction
C. friction is not always bad:
need it to walk, drive: start/stop
D. terminal velocity: due to the friction acting on the
object such that it is still moving but
is slowed by the friction so that it is not accelerating
= 0 m/s2 thus reached it maximum (terminal) velocity
4. Forces and equilibrium
things begin moving only if the forces acting on them
are unequal.
A. balanced forces are when all the forces acting on
an object equal zero; thus,
there is no speeding up or down in that direction:
it is a constant motion (including stopped)
for the airplane, which forces are balanced?
is it moving in that direction?
which forces are unbalanced?
is it moving in that direction?
B. when the net force is zero the object is in
equilibrium:
the object is either:
not moving:
or moving at a constant speed
if the net force is not zero (unbalanced), the object
is accelerating:
positive = going faster
negative = going slower
if the net force is not zero (unbalanced), the object
is accelerating:
positive = going faster
negative = going slower
C. Normal force:
an object resting on a surface is applying a
downward force on the surface it is resting upon.
Because the object does not sink into the surface
there is a force pushing it upward.
This upward force is the Normal force.
It occurs opposite to the direction pushing into the
surface.
D. free-body diagram
a single object only
with all forces acting are drawn on the object
up forces are positive
down forces are negative