Download LINEAR KINETICS (Part 1)

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Examples of Inertia
1. Bag of groceries on car seat - slam on
brakes of car.
What happens?
Objects (groceries, passengers, car, etc.)
retain their state of motion unless a net
force acts of them.
Forces acting on the car?
...on the body?
2. Standing in the back of a pickup truck that
takes an unexpected turn. What
happens?
Forces acting on the body?
...on the truck?
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Let’s tie the concepts of inertia, mass, force, and
acceleration together...
NEWTON'S 2ND LAW OF MOTION:
A quantitative example...
What average force must a baseball catcher
apply to an 80 mph (35.8 m/s) pitch in order
to stop it?
What do we need to know?
What forces are involved?
Ball mass?
4.5 oz. weight = 0.13 kg mass
Impact time?
Ball acceleration?
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Consider...
Meaning of negative force?
Average vs. instantaneous force?
The effect of ∆t on F?
Some important interpretations of ΣF = ma:
*
Cause (ΣF) and effect (a) relationship
probably the most important concept in
this class!
*
a directly proportional to ΣF
(bigger F produces greater a...
i.e., more rapid change in velocity)
*
a is in the same direction as ΣF
*
To produce a given a, it takes a larger ΣF
for a larger object (i.e., higher mass
requires higher force for the same effect)
NEWTON'S 3RD LAW OF MOTION
Action-Reaction...describes how objects
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interact with one another:
If one body exerts a force on a second
body, the second exerts back on the first a
force of equal magnitude but opposite in
direction.
e.g., impact between a tennis ball and
racquet - an action-reaction pair?
racquet face
impact
ball
A confusing principle to many:
1. Where you find force, you will always find
two interacting bodies or objects.
According to action-reaction, there is
never just a single force but always a pair
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of forces.
2. If action and reaction forces are of equal
magnitude but opposite in direction, do
they cancel each other (i.e., result in ΣF =
0)?
___!! They _______ cancel each other.
Action and reaction forces affect the motion of
different bodies. They influence the
acceleration of different objects.
e.g., tennis ball and racquet - action force
applied to the ball by the racquet
accelerates ball; reaction force applied to
racquet by the ball accelerates the
racquet.
3. The label Aaction-reaction@ tends to suggest
that the action and reaction forces do NOT
occur simultaneously, but rather occur
sequentially... Correct?
e.g., Bouncing on a trampoline - an
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example of action-reaction?
Simultaneous or sequential action-reaction
forces?
_________! Action-reaction forces act
____________, not ______________!
Effect of action force (our push against the
ground)?
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our push against
the ground
earth
Negligible acceleration of earth - why?
Earth’s huge mass
Let’s see just how big earth’s mass is...
ΣF = mΑa
5 billion participants gather for World
Vertical Jump Games
Typical peak force in VJ
.4ΑBW . 2000 N
If all participants jump simultaneously:
ΣF = (5x109)Α(2000 N) = __________
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earth=s mass (me) = 6 x 1024 kg
ae = ΣF / me = (1 x 1013) / (6 x 1024 kg)
aearth = ____________ m/s2 ... ______!
NEWTON’S LAW OF GRAVITATION
A fundamental physical principle that
describes the concept of gravity...
Any two particles of matter (any bodies or
objects) attract one another with a force
directly proportional to the product of their
masses and inversely proportional to the
square of the distance separating them (i.e.,
distance between their centers):
G = gravitational constant =
6.7 x 10-11 NΑm2/kg2
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e.g., Like it or not, there is a force of attraction
between you and the person sitting next to
you.
However, this force is so small that you
don=t notice it.
When one of the objects is the earth (with
its huge mass), the force of attraction (i.e.,
gravity) is very significant.
( m1 m 2 )
Fg=G
2
r
e.g., students sitting 1.5 m apart
e.g., attraction between earth and student
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WEIGHT:
An object’s weight represents the force of
attraction between the earth and the object
Question: Is earth’s gravitational attraction
(i.e., force) the same for all objects on or near
the surface of the earth?
____...
this force is dependent on the mass of the
earth AND the mass of the object close to the
earth.
Fg=G
( m1 m2 )
2
r
Simplified relationship for the link between
weight and mass on earth:
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where ag = g (i.e., acceleration due to
gravity, 9.8 m/s2) is equivalent to:
PRESSURE
Force applied to an object is rarely applied at
a single point, usually distributed over an
area:
“force per unit of area”
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Consider:
Lying down vs. standing
Force?
Pressure?
What changes; what stays the same?
Other Examples...
Atmospheric pressure
Rock in shoe
Pressure - an important factor related to injury
protection
e.g., Impact situations:
Advantageous to spread force over as
large an area of contact as possible
protective equipment - helmets, boxing
gloves, protective padding
landing techniques: parachuting, high
jumping, pole vaulting
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Example problem: Which situation would
exert more pressure on your stomach (1) a
woman in high heels standing on one foot on
your stomach, or (2) an elephant standing on
your stomach?
What do you need to know?
MOMENTUM – “_______________”
Any object in motion has _____ and a
quantifiable ________.
units of measurement: ________
Momentum - especially useful in collisions...
Collision outcome directly related to
momentum of the colliding bodies just
before impact.
The greater the momentum of a body (not just
mass and not just velocity), the bigger the
effect it will have on other objects it
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collides with.
e.g.,
auto collision
two football players
tennis racquet and ball
head or foot and soccer ball
boxing glove and boxer's head
In impact/collision situations in sport, we often
want to manipulate the momentum of at least one
of the colliding objects to produce some desired
outcome of the collision:
e.g.,
baseball - home run vs. bunt
Link to kinetics...
One way to describe the effect of a net force
is by the ____________ of momentum
produced by the force (an alternative way of
expressing Newton's 2nd law):
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IMPULSE
product of average net force and the time over
which it is applied.
One interpretation of the equation...
F ∆t = mv f - m vi = ∆ mv
We can produce the same change in
momentum with many combinations of force
and time...
large force over a ______ time
small force over a ______ time
“Spreading an impact out over time”
e.g.,
catching an egg
landing from a vertical jump
(refer to “impact plots” from lab)
function of a bike helmet
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INERTIA/MOMENTUM vs. FORCE
A common misconception is that a moving
body possesses force.
You cannot possess ______; rather you can
only apply force to others. What you do
possess is _______ and (if you are moving)
__________.
The rate at which your momentum changes
equals the resultant force applied to you from
the outside.
In other words, since acceleration (_) is the
rate of change of velocity (_), (resultant) force
(__) is the rate of change of momentum (__).
If there is no _____________, then there is
no change in ____________
IMPACTS AND THE PRINCIPLE OF
CONSERVATION OF MOMENTUM
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When two bodies collide with each other, we
can treat the two bodies as a ________ and
examine their total momentum.
Newton’s First Law can be restated to predict
what will happen to the “system”:
In the absence of _______ forces acting
on a system, the total momentum of the
system remains constant (in both
________ and _________).
Example #1: A 100 kg running back carries
the ball forward with a speed of 9 m/s. He is
hit head on in the air at the goal line by a 130
kg defensive back running in the opposite
direction at 7 m/s. Who has more
momentum? Which direction will the
combined bodies move after impact?
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Example #2: Two ice skaters collide on the
ice. A man (mass 75 kg) moving forward at a
speed of 7 m/s runs into a stationary woman
(mass 55 kg). If they cling to each other to
keep from falling down, how fast and in which
direction do they move as a unit (assuming
“frictionless” ice)?