Physics - Motion and Force by hcj

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```									Distance: describes how far an object has
moved, regardless of its direction
ex: 40km east + 25km west = 65 total km traveled

Displacement: describes both distance
and direction
ex: 40 km east + 25 km west = 15 km east
ex: 40 km east + 25 km east = 65 km east
n       How fast an object is traveling
regardless of direction
n       Unit: m or km
sec hour

Described in two ways:
1. Instantaneous speed: measured at a specific instant
n     initial speed and final speed are examples
n     this is what speedometers measure

2. Average speed: total distance traveled per unit of
time
n     if an object travels at a constant speed, then the
instantaneous and average speed will be equal
n     if the object is traveling at varying speeds, then the
average speed is calculated as total distance traveled over
total time
n   Describes both the speed of an object AND
its direction of motion

n   Units: speed + direction: 24 m north
sec

Measured in two ways:
1. Instantaneous velocity: velocity at a specific
instant
2. Average velocity: the total displacement per unit
of time
Velocity can change if:
1. the object’s speed is changed (increased or decreased)
2. the object’s direction can change
V = either velocity or speed
d = distance or displacement
v=d
t = time
t
Units count!!!!!!!!!!
Let’s practice!!!!
FLASHBACK
• What is the difference between distance
and displacement?

• What is the difference between speed
and velocity?

• Your average walking speed is 0.90
m/sec.  What is it in km/hour?
FLASHBACK
•  EOC WORKBOOK - pg. 48 [1,2,4,5]
• Why do you think some people confuse
speed with velocity?

• Does a speedometer show both speed and
velocity?

• You travel to and from RHHS, which is
located 5 mi to the west of your house.  What
is your distance & displacement?
PS 5.3 and PS 5.4
n   The rate of change of an object’s velocity
over the time it takes for that change to
occur

n   Equation:   a = (v f – v i )
t

n   Unit: m/s/s or m/s2 (most common)

However, the unit used for velocity (m/s) and
the unit used for time (s) do not necessarily
need to be the same:
km/hr per second
The change in acceleration may involve a
change in speed or direction

Can acceleration be negative???
Yes!!
When you are slowing down at a
stoplight, speed is decreasing,
so acceleration is opposite of
velocity... = negative
acceleration
1. constant acceleration: acceleration is zero
because the velocity does not change
ex: walking 2 meters every 15 seconds

2. positive acceleration: the object is
speeding up
ex: getting on the interstate in your car

3. negative acceleration: the object is
slowing down
ex: pulling into a parking lot to
A.   All objects accelerate as they fall because
the Earth exerts gravitational force on them
B.   All objects when released accelerate in the
direction of the force (downward)
C.   At initial release, the object has an initial
velocity of 0.0 m/sec
D.   As it falls, the object accelerates at a
constant rate of 9.8 m/s2
E.   This means the object will travel 9.8 m/sec
every second it is falling à as long as there
is no air resistance
F.   The value 9.8 m/sec2 is called the
acceleration of gravity (ag).
a = (v f – v i )
t
Positive Acceleration: (speeding up)

A jet airliner starts at rest and reached a
speed of 80 m/s in 20 seconds. What is
it’s acceleration?
a = (v f – v i )
t
Negative Acceleration: (slowing down)

A skateboarder is moving at a constant
speed of 3 m/s and comes to a stop in 2
seconds. What is his acceleration?
Assignments!

n Right Now:
pg 51 “Self Check” # 1-4
Marble Lab!!!
FLASHBACK
•  What needs to occur for an object to have
acceleration?
• Which of the following is a proper unit of
acceleration?
a. m/km2     b. km/h     c. m/s2     d. m2/s
3. When describing the rate of a race car going
around a track, should you use the term speed
or velocity, why?

4.  How long does it take for the sound of thunder
to travel 1485 m, if sound travels at 330 m/s?
FLASHBACK
•  If you ride your bike down a straight road for
500 m then ride back, your distance is ____ your
displacement.
a. greater than     b. less than c. equal to

2. Acceleration is rate of change of ____.
a. position    b. time    c. velocity    d. force

3. If you ride your bike up a hill, then ride down
the other side, your acceleration is ____.
a. all positive     b. first positive, then negative
c. all negative    d. first negative, then positive
Tennis Ball Activity
What changes when a tennis ball
bounces???
n   Draw and label for two bounces:
n Positive acceleration
n Negative acceleration
n Increasing and
decreasing speed
FLASHBACK
• How long will it take to reach Bi-Lo 15 km away
if traveling at 50 km/hr?

2. How long will it take a whale to travel 200 km
swimming 4 m/s?

3. What is the value of 9.8 m/sec2 called…

4. What is the formula for acceleration?
PS 5.7
n       “The velocity of an object will remain constant
unless a net force acts on it.”
n     This is called the Law of Inertia.

n       Inertia: is the tendency of an object to remain
at rest or in motion
n     Inertia is dependent upon an object’s mass.

n       Force: a push or pull that one object exerts on
another
n     It is measured in Newtons (N).

n     If one object has a greater force, then net force must
be calculated
How does this relate to Newton’s First Law?????
n   “When a net force acts on an object, it will
accelerate in the direction of the net force”

n   The larger the force, the greater the
acceleration

n   The larger the mass, the smaller the rate of
acceleration

n   Force = Mass (acceleration)

n   Friction and air resistance can affect net
force; they act as opposing forces.
III. Newton’s Third Law
A.   Newton’s Third Law : When one object
exerts a force on a second object, the
second object exerts a force on the first that
is equal in magnitude and opposite in
direction.
1. a swimmer pushes on the water, and the
water pushes back

2. a ball thrown on a wall, bounces off
Force, Mass, Weight, and
Gravity

PS 5.8 – 5.10
I. The Math Behind the Force
A.  Force is a derived unit; it involves
multiplying the mass of an object by its
acceleration:
F = mass * acceleration
B. For us, the mass is measured in Kilograms
(Kg) and the acceleration in m/sec2.
C. This means the final answer (Force) is
measured as Kg * m/sec2 or 1 Newton
D. 1 Newton is the amount of force required to
accelerate a 1 Kg object at a rate of 1
m/sec2
E. Look at these examples:

What is the force required to move a 2400Kg
car with an acceleration of 4 m/sec2?

If a force of 4.2 Newtons is applied to an
object with a mass of 75 Kg, what is the rate
of acceleration?
II. Newton’s Law of Gravitation
A.   States that there is a force of attraction
between all objects in the universe.
B.   The object with the larger mass will exert
the greater force. This is why objects weigh
more on the earth than on the moon. The
earth is larger so it has a greater force of
attraction (gravity).
C.   We do not notice the forces between most
objects on earth because the force of the
earth is so much greater than the force
between the two objects.
D. The amount of force the earth exerts on
an object is dependent upon the object’s
mass & distance. The greater the mass,
the greater the gravitational force.
E. Weight is a measure of an object’s
gravitational force and it can be found
using the formula:
Fw = mass (acceleration of gravity)
F. The acceleration of gravity is what?
9.8 m/sec2
G. Try these examples:

What is the weight of an object with a
mass of 10 Kg?

What is the mass of an object with a
weight of 25 N?

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