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SCI206 Lecture Notes - Defrosting, Stainless Steel, Hydrophile

Course Code
Stefan Idziak

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Topic 1
Thursday, October 05, 2006
1:56 PM
Topic 1: Motion
From Lecture
Newton's Laws
What are the 3 ways to describe motion?
oPosition, velocity, and acceleration
How can we describe position?
oWe use a vector, because it can tell us direction and distance (from
some defined start point)
What are the 2 parts of velocity?
oHow fast we are going (speed)
oWhat direction we are going in (direction)
What is acceleration?
oIt is how fast (and in what manner) our velocity is changing
What is Newton's first law?
oAn object that is not subject to any net outside forces moves at
constant velocity
What is Newton's second law?
oThe force exerted on an object is equal to the product of the object's
mass times its acceleration
oThe acceleration is in the same direction as the force
What is Newton's third law?
oFor every force that one object exerts on a second object, there is an
equal but oppositely directed force that the second object exerts on
the first object
Gravity and Componential Motion
What is gravity? What determines the effect we will experience due to
oIt is a downward force produced by the earth which acts on us, pulling
us down
oThe gravitational effect we experience is always an acceleration of ~10
m/s2 -- REGARDLESS of our mass
This means that (neglecting air resistance) a feather falls just as
fast as a brick
Why doesn't gravity cause us to fall through the floor?
oBecause of the "normal force": an upward force produced by the
surface we are standing on that counteracts the gravitational force so
that we don't move
Describe the motion of a projectile launched at some angle with respect to the
oWe can break up its initial velocity into horizontal and velocity
Its horizontal velocity will never change, because there are no
forces present to change it (recall Newton's First Law)

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Its vertical velocity will be subject to gravity, so its upward
progress will slow, eventually become zero, then start to head
If you wanted to shoot a monkey in a tree, where should you aim? (Assume
that the monkey will drop and fall downwards as soon as the shot is fired).
oYou should aim straight at the monkey (not above it or below it),
because once the bullet is in the air and the monkey has dropped, both
objects are subject to the same force (only gravity), and so they will
meet at some point
Work and Energy
What is the difference between kinetic and potential energy?
oKinetic energy is the energy of movement: the amount of this one
possesses is related to one's velocity
oPotential energy is stored: the amount of this one possesses is related
to its ability to unleash energy
Discuss energy conversions.
oEnergy cannot be created or destroyed, but it can be converted from
one form into another
oThe types of conversions which occur and their efficiencies affect the
things we see in the real world
For example, some materials bounce better than others
because they do a better job of converting kinetic energy to
elastic potential (while others just convert kinetic into heat)
Discuss work, in particular the way it is related to potential energy.
oTo do work on something is to exert a force on it and cause it to move
some distance in the direction of that force
oWork is related to potential energy because we can give an object
potential energy by doing work on it
For example, lifting a ball from the ground to a table
Imagine a scenario where (with friction present) you are pushing a block
horizontally across the floor. Describe the different forces acting on the box.
oThere is gravity acting downwards, and the normal force acting
oThere is the human pushing force acting in the box's forward direction,
and friction acting opposite to that
The pushing force must exceed the frictional force in order for
the box to start moving from rest
Discuss the 2 kinds of friction. What kind of energy is produced by friction?
oStatic friction is friction that opposes the initiation of movement
oKinetic friction is friction that slows down movement which has already
oThe energy lost from friction (remember that we would expect a given
amount of energy produced given the work we put into moving
something…so when we have reduced energy due to friction, where
does that energy go?) becomes HEAT
Explain the "spiky" model of friction.
oOn a microscopic level, the two interacting surfaces are not smooth but
rather have many small bumps and crevices, almost like interlocking

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Thus moving the surfaces across each other requires energy to
overcome this interlocking mechanism
oWhen starting from rest, more of the "teeth" are "locked together" and
thus the frictional force experienced is greater
oWhen already moving, there are fewer teeth interlocked and thus the
force is smaller
oIf you press the two surfaces together (i.e. if the one on top is heavier),
again the "teeth" "interlock" more, and so friction increases (this is
why friction is related to the normal force)
Explain why a tablecloth can be pulled out from beneath a dinner set.
oIt is simply a function of the frictional force between the tablecloth and
the dinner plates: can we pull the tablecloth fast enough that it
overcomes the frictional force which is causing the plates to stick to
the cloth? If so, the cloth will come right out from beneath them...
Rotational Motion
Describe the motion of a tennis ball connected to a string being flung around
in the head in a CIRCULAR pattern.
oIts speed is always the same, but its velocity changes since its
direction is always changing
oIt has an acceleration that is always the same in magnitude but always
changing in direction (though it is always pointed towards the middle)
The force creating this acceleration (since f = ma) is known as
the "centripetal force", and it is embodied in the tautness of the
oIf at any time in the motion the ball was separated from the string, it
would fly off in a line tangent to the circle (this is why we feel like we're
falling out of the car when we go around curves)
Recall the experiment where a water-filled bucket was rotated vertically in a
circle. What determines whether the water falls out?
oThe relevant relationship here is that the centripetal force (discussed
earlier) is related to the velocity of the motion
And the centripetal force is made up of the tension which
swinging the bucket exerts on the arm PLUS the gravitational
force - so if the necessary centripetal force isn't even as much
as the gravitational force, then the remainder of the
gravitational force will be used to allow the bucket to spill
Why do figure skaters spin faster when they bring their arms close into their
oBecause something called angular momentum is conserved, which is a
function of rotational inertia (analogous to mass) and rotational speed
(analogous to velocity)
oAs long as torque (a force which affects rotational velocity) is not
added to or removed from the system, angular momentum must stay
constant - and so if we reduce our rotational inertia by bringing our
weight closer to the axis of rotation, the rotational velocity must
increase to counteract this
Why did Idziak's tricks with the strings and the hourglass work better when
the hourglass was spinning?
oBecause the hourglass wants to conserve angular momentum, and it
does this by continuing to spin on the same axis
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