Download File - Mr. Purdy`s Rocket Science

Safety First!
That’s what My
Daddy always
says!
EGS
Physics Unit
Describing and Measuring
Motion
• What is motion and are you really moving?
• Is your table moving?
• Is the building moving?
• Can you be sure if something is really
moving?
Describing and Measuring
Motion
What is all this talk
about motion? Are
we really moving in
space at 30 km per
second?
Describing and Measuring
Motion
• A reference point is a place or object
used for comparison to determine if
something is in motion.
• An object is in motion if it changes
position relative to the reference point.
Describing and Measuring
Motion
What is the
basic unit
used to
describe
motion?
Describing and Measuring
Motion
This one is easy,
METER
Constant Speed
• If you know the distance an object travels
in a given amount of time, you can use the
formula for speed to calculate the object's
constant speed.
• Speed = distance divided by time
•S=d/t
time distance
0
0
1
2
2
4
3
6
4
8
5
10
6
12
7
14
8
16
9
18
10
20
distance
calculating slope
25
20
15
10
5
0
time
distance
1
3
5
time
7
9
11
time distance
0
0
1
2
2
4
3
6
4
8
5
8
6
8
7
8
8
12
9
16
10
20
25
20
15
10
5
0
time
time
11
9
7
5
3
distance
1
distance
calculating slope
I think I
am
getting
this slope
stuff.
Velocity
• When you know both the speed and
direction of an object’s motion, you know
the velocity of the object.
• Speed in a given direction is called
velocity.
Velocity
• Speed
– 197 mph
– 88.07 m/s
– 317.05 km/hr
• Velocity
– 197 mph North
– 88.07 m/s to the left
– 317.05 km/hr
downward
We’re trying our best, but even
after two three four five
championships, We still can’t
teach Junior how to drive!
ACCELERATION
• Acceleration is the rate at which
velocity changes.
• Recall that velocity has both speed and
direction.
• So, acceleration refers to increasing
speed, decreasing speed and changing
direction.
CALCULATING
ACCELERATION
• Acceleration is equal to the final velocity
minus the initial velocity divided by the
time.
final velocity – initial velocity
• Acceleration= -------------------------------
time
CALCULATING
ACCELERATION
• A roller coaster car rapidly picks up speed
as it rolls down a slope. As it starts down
the slope, its speed is 4 m/s. But 3
seconds later, at the bottom of the slope,
its speed is 22 m/s. What is its average
acceleration?
CALCULATING
ACCELERATION
•
•
•
•
•
•
Final velocity = 22 m/s
Initial velocity = 4 m/s
Time = 3 seconds
22 m/s- 4 m/s
Acceleration = ----------------------- =
3 sec
CALCULATING
ACCELERATION
•
•
•
•
•
•
Final velocity = 22 m/s
Initial velocity = 4 m/s
Time = 3 seconds
22 m/s- 4 m/s
Acceleration = ----------------------- = 6 m/s2
3 sec
CALCULATING
ACCELERATION
• The same roller coaster car goes through
its final loop before the ride ends. Its
speed is 40 m/s and slows down to a
complete stop in 8 seconds. What is the
roller coaster’s acceleration
(deceleration)?
CALCULATING
ACCELERATION
• Final velocity = 0 m/s
• Initial velocity = 40 m/s
• Time = 8 seconds
•
0 m/s- 40 m/s
• Acceleration = ------------------- = -5.0 m/s2
•
8 sec
Newton’s First Law of Motion
• INERTIA –the tendency of an object to resist
change in its motion (even if it is at rest)
Newton’s First Law of Motion
• The amount of inertia an object has
depends on its mass.
• The greater the mass of an object, the
greater its inertia.
• Mass, then, can also be defined as a
measure of the inertia of an object.
Newton’s Second Law of Motion
• The net force on an object is equal to the
product of its acceleration and its mass.
•
•
•
•
Force = mass X acceleration
Units for mass is kg.
Units for acceleration is m/s/s or m/s2
Unit of force- Newton (N) or (kg X m/s2)
Newton’s Second Law of Motion
• What net force is needed to accelerate a
55 kg cart at 15 m/s2
•Force = mass X acceleration
•Mass = 55 kg
•Acceleration = 15m/s2
Newton’s Second Law of Motion
• Force = 55 kg X 15 m/s2
• Force = 825 (kg X m/s2)
• Force = 825 N
Newton’s Second Law
• If you push on a shopping cart, it will
accelerate.
• If you push 5 times harder, what happens
to the acceleration?
• Acceleration is 5 times greater.
Newton’s Second Law
• If you push the same,
but the cart is loaded
so it has five times as
much mass, what
happens to its
acceleration?
• Acceleration will be
less - 1/5 as much.
Where’s
my
Cart?
Newton’s Second Law
• If you push five times harder when it is
loaded with five times as much mass, what
happens to the acceleration?
• It will have the same acceleration as it
had to begin with.
Newton’s Second Law
• Gravity is actually an acceleration equal to
9.8 m/s2
• So the force of gravity is the mass of an
object times its acceleration due to gravity
• F = mg
Newton’s Third Law of Motion
• Whenever one object exerts a force on
a second object, the second object
exerts an equal and opposite force on
the first.
• To every action there is always an
opposed equal reaction.
Newton’s Third Law of Motion
• Examples
– Walking across the floor
– Your car moving (burning rubber??)
– Swimming
– A person on ice
– Leaning on a wall
– Using a pen to write
Newton’s Third Law of Motion
• Concept check
• Which exerts more force, the Earth pulling
on the moon, or the moon pulling on the
Earth?
• The force is the same, only in opposite
directions.
• Which is farther, the distance from New
York to S.F. or S.F. to N.Y.? Hmm???
Newton’s Third Law of Motion
• Concept Check
• A high speed bus and an innocent bug
have a head on collision. The force of
the bus on the bug splatters the poor
bug all over the wind-shield. Is the
corresponding force of the bug on the
bus greater, less, or the same? Is the
resulting deceleration of the bus
greater, less, or the same as that of the
bug?
Momentum
• Momentum = mass X velocity
• Which has more momentum, a 3 kg
sledgehammer swung at 1.5 m/s or a 4 kg
sledgehammer swung at 0.9 m/s?
• 3 kg X 1.5 m/s = 4.5 kg *m/s
• 4 kg X 0.9 m/s = 3.6 kg * m/s
Momentum
• A golf ball travels 16 m/s, while a baseball
moves at 7 m/s. The mass of the golf ball
is 0.045 kg and the mass of the baseball is
0.14 kg. Which ball has the greater
momentum?
• 0.045 kg X 16 m/s = 0.72 kg *m/s
• .14 kg X 7 m/s = 0.98 kg * m/s
Conservation of Momentum
• Law of Conservation of Momentum states that
the total momentum of the objects that interact
does not change.
Work and Power
• Work- a transfer of energy that occurs
when a force makes an object move
• Work = Force (N) x Distance (m)
• Units for work are N∙m or joules (J)
Work and Power
• Power- the amount of work done in a
certain amount of time
• Calculating Power
– Power = Work (J) / Time (s)
– Units for power are J/s or watts (W)
• Horsepower (hp) is equal to 746 watts
Machines
• Any device that
makes doing work
easier
– Increase applied force
– Change direction of
applied force
– Increase the distance
over which a force is
applied
Machines
• Machines help move things that resist
being moved
• Input force- the force that you exert on the
machine
• Output force- the force exerted by the
machine on some object
Machines
• You know what input force and output
force are, so what do you think input
distance and output distance are?
• How could we calculate Input Work and
Output Work?
Energy
• Energy is the ability to cause change or do
work
• Many forms of energy can be classified
into two general types:
– Kinetic
– Potential
Energy
• Kinetic Energy (KE)
– Energy in the form of motion
– KE of an object is dependent on the object’s mass and
velocity (speed)
– KE = ½mv2
– Measured in joules (J)
Energy
• Potential Energy (PE)
– Energy stored in a stationary object
• Elastic PE- energy stored in things that stretch or compress
• Chemical PE- energy stored in chemical bonds
• Gravitational PE- energy stored by things that are above a
reference level, for example the Earth’s surface, the room floor,
a table top, etc…
Energy
• Gravitational Potential Energy (GPE)
– Depends on an objects mass, its acceleration
due to gravity, and its height above a
reference level
– GPE = mgh
– Units for GPE are also in joules (J)
– ‘g’ on Earth is 9.8 m/s2
– What is ‘mg’?
Energy
• Energy can change from one form to
another
• For example, radiant energy from the Sun
can be converted into chemical energy by
plants, which in turn which in turn were
buried millions of years ago and converted
into fossil fuels.
Energy
• Mechanical Energy (ME)
– ME = KE + PE
• Law of Conservation of Energy
– Energy may change from one form to another,
but the total amount of energy never changes
Energy
• Example
– A moving swing is constantly converting GPE into KE
and back.
– Eventually the swing stops because it loses energy
due to friction. As the swing loses an amount of
energy, the frame of the swing must gain that same
amount of energy.
– Friction converts the ME into thermal (heat) energy
Energy
• Einstein tells us that we must think of mass and
energy as being equivalent and that one can be
converted into the other
• This is especially important in nuclear physics.
During a nuclear reaction, as atoms are split or
fused to release energy, the total amount of mass
and energy is conserved.
Energy
• The food we eat has chemical potential energy
that can be converted into kinetic energy enabling
you to breathe, think, move, live, etc…
• Energy in food is measured in Calories. One
Calorie is equivalent to 4,180 J.