Transcript Chapter 7 Motion

Chapter 7
Motion
Applied Lab Physics
Mrs. Campbell
Ch. 7, Lesson 1
What are Motion and Speed?
• Motion - A change in position.
• Elapsed time - The length of time that passes from
one event to another.
• Speed - The rate at which the position of an object
changes.
• Distance - The length of the path between two
points.
Ch. 7, Lesson 1
What are Motion and Speed?, con’t
Example: Calculating Elapsed Time
• Your flight began at 8:00 PM and ended at 11:00
PM. How long did your flight take?
• Elapsed Time = Arrival Time – Departure Time
11:00 PM – 8:00 PM =
• 3 hours
Ch. 7, Lesson 1
What are Motion and Speed?, con’t
Practice Problem: Calculating Elapsed Time
• Your flight began at 7:00 PM and ended at 9:00
PM. How long did your flight take?
• Elapsed Time = Arrival Time – Departure Time
• Elapsed Time = 9:00 PM – 7:00 PM
• Elapsed Time = 2 hours
Ch. 7, Lesson 1 What are Motion and Speed?, con’t
Example: Calculating Average Speed
• An airplane travels 810 miles in 3 hours. What is the
average speed of the airplane?
• Average speed = Distance
Time
• Average speed = 810 miles
3 hours
• Average speed = 270 miles
1 hour
• The average speed of the airplane is 270 miles per
hour.
Ch. 7, Lesson 1 What are Motion and Speed?, con’t
Practice Problem: Calculating Average Speed
• An airplane travels 1230 miles in 5 hours. What
is the average speed of the airplane?
• Average speed = Distance
Time
• Average speed = 1230 miles
5 hours
• Average speed = 246 miles
1 hour
• The average speed of the airplane is 246 miles
per hour.
Ch. 7, Lesson 1 What are Motion and Speed?, con’t
Calculating Distance – knowing speed and time we
can calculate distance with the formula
• Distance = speed x time
• Example – If you can travel 50 miles per hour and
you travel for 6 hours, what is the distance
traveled?
• Distance = speed x time
• Distance = 50 miles
x 6 hours
1 hour
• Distance = 300 miles
Ch. 7, Lesson 1 What are Motion and Speed?, con’t
Practice Problem - Calculating Distance
• If a car travels 60 kilometers per hour and travels
for 7 hours, what is the distance traveled?
• Distance = 60 km x 7 hours
1 hour
• Distance = 420 km
Ch. 7, Lesson 1 What are Motion and Speed?, con’t
Calculating Time – knowing distance and speed we
can calculate the time it will take to travel that
distance.
• Time = distance
speed
Ch. 7, Lesson 1 What are Motion and Speed?, con’t
• Example – Calculating Time
You have a job marking lines on a sports field. The
distance to be marked is 80 meters. You mark at the
speed of 50 meters per minute. How much time will
it take to mark the line?
• Time = distance
speed
• Time = 80 meters
50 meters per minute
• Time = 1.6 minutes
Ch. 7, Lesson 1 What are Motion and Speed?, con’t
• Practice Problem – Calculating Time
A car went 70 miles at an average speed of 60 miles
per hour. How long did it take to travel the 70
miles?
• Time = distance
speed
• Time = 70 miles
60 miles per hour
• Time = 1.166666 = 1.17 hours
Ch 7, Lesson 2 Using a Graph to Describe Motion
Distance (km)
Time (hrs)
Graph showing constant speed
• Constant Speed –
Speed that does not
change.
• On a graph,
constant speed is
shown by a straight
line.
Ch 7, Lesson 2 Using a Graph to Describe Motion
• Velocity – the speed and direction in which an object
is moving.
– Example of Velocity – 200 m in 7 seconds, east.
Ch 7, Lesson 2 Using a Graph to Describe Motion
•Varying Speed –
Speed that is not
constant.
Distance (km)
•On a graph, varying
speed will be shown
by a non-straight line
Time (hrs)
Graph showing varying speed
Ch 7, Lesson 2 Using a Graph to Describe Motion
• Using a Graph to Find
Unknown Distances
• We can use a graph
for constant speed to
find an unknown
distance or time.
Ch 7, Lesson 2 Using a Graph to Describe Motion
• Using a Graph to
Predict Distances
• We can use a graph
for constant speed to
predict a distance
that is not on the
graph.
Ch 7, Lesson 3 Acceleration
• Acceleration – the rate of change in velocity.
Acceleration = Change in velocity
Change in time
Change in velocity = final speed – original speed
Change in time = end time – start time
Ch 7, Lesson 3 Acceleration
• Acceleration – Example Problem
A car starts from a stopped position. After 5
seconds, it has a speed of 40 km/hr. What is
the car’s acceleration?
Acceleration = Change in velocity
Change in time
Acceleration = 40 km/hr
5 second
Acceleration = 8 km/hr per second
Ch 7, Lesson 3 Acceleration
• Deceleration – a form of acceleration
used for the rate of slowdown.
• Deceleration is indicated by a negative (-)
number or by using the word deceleration.
Ch 7, Lesson 3 Acceleration
• Example Problem – A car is traveling at 20 km/hr. The
driver suddenly puts on the brakes. The car comes to a
complete stop 4 seconds later. What is the rate of
acceleration?
Acceleration = Change in velocity
Change in time
Acceleration = 0 km/hr – 20 km/hr
4 sec
Acceleration = -20 km/hr
4 sec
Acceleration = -5 km/hr/sec
or the vehicle decelerated at 5 km/hr/sec
Ch 7, Lesson 3 Acceleration
• Practice Problem – Acceleration
A car starts from a stopped position. After 8 seconds,
the car has a speed of 40 km/hr. What is the car’s
acceleration?
• Formula Acceleration = Change in velocity
Change in time
• A = 40 km/hr – 0 km/hr
8 sec – 0 sec
• A = 40 km/hr
8 sec
• Acceleration = 5 km/hr/sec
Ch 7, Lesson 4
The Laws of Motion
• Sir Isaac Newton (1643 – 1727)
was an English scientist and
mathematician.
• He is famous for his discovery of
the law of gravity.
• He also discovered 3 laws of
motion that we call Newton's
Laws of Motion.
• These laws describe the motion of
all objects we experience in our
everyday lives.
Ch 7, Lesson 4
The Laws of Motion
The 1st Law of Motion
• An object at rest tends to stay at
rest and an object in motion tends
to stay in motion unless acted
upon by an unbalanced force.
Ch 7, Lesson 4 The Laws of Motion
What does this mean?
• Basically, an object will “keep doing what
it was doing” unless acted on by an
unbalanced force.
• If the object was sitting still, it will
remain stationary. If it was moving at a
constant velocity, it will keep moving.
• It takes force to change the motion of an
object.
Ch 7, Lesson 4 The Laws of Motion
• What is meant by unbalanced force?
• If the forces on an object are equal and opposite,
they are said to be balanced, and the object
experiences no change in motion.
• If the forces are not equal and opposite, then the
forces are unbalanced and the motion of the object
changes.
Ch 7, Lesson 4 The Laws of Motion
Some examples from real life…
• A soccer ball is sitting at rest. It takes an
unbalanced force of a kick to change its motion.
• Two teams are playing tug of war. They are
both exerting equal force on the rope in opposite
directions. This balanced force results in no
change of motion.
Ch 7, Lesson 4 The Laws of Motion
Newton’s First Law is also called the Law
of Inertia.
• Inertia: the tendency of an object to
resist changes in its state of motion.
• The First Law states that all objects
have inertia. The more mass an object
has, the more inertia it has (and the
harder it is to change its motion).
Ch 7, Lesson 4 The Laws of Motion
More Examples from Real Life
• A powerful locomotive begins to pull a long line of
boxcars that were sitting at rest. Since the boxcars are
so massive, they have a great deal of inertia and it
takes a large force to change their motion. Once they
are moving, it takes a large force to stop them.
• On your way to school, a bug flies into your
windshield. Since the bug is so small, it has very little
inertia and exerts a very small force on your car (so
small that you don’t even feel it).
Ch 7, Lesson 4 The Laws of Motion
• If objects in motion tend to stay in motion, why
don’t moving objects keep moving forever?
• Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon it.
• A book sliding across a table slows down and
stops because of the force of friction.
• If you throw a ball upwards it will eventually
slow down and fall because of the force of
gravity.
Ch 7, Lesson 4 The Laws of Motion
In outer space, away from gravity and any
sources of friction, a rocket ship launched
with a certain speed and direction would
keep going in that same direction and at
that same speed forever.
Newton’s Second Law
Force equals mass times acceleration.
F = ma
• Acceleration: a measurement of how
quickly an object is changing speed.
What does F = ma mean?
• Force is directly proportional to mass and
acceleration. Imagine a ball of a certain
mass moving at a certain acceleration.
This ball has a certain force.
• Now imagine we make the ball twice as big
(double the mass) but keep the
acceleration constant. F = ma says that
this new ball has twice the force of the old
ball.
What does F = ma mean?, con’t
• Now imagine the original ball
moving at twice the original
acceleration. F = ma says that
the ball will again have twice
the force of the ball at the
original acceleration.
More about F = ma
You double the mass, you double the force. If you
double the acceleration, you double the force.
What if you double the mass and the acceleration?
(2m)(2a) = 4F
Doubling the mass and the acceleration
quadruples the force.
So . . . what if you decrease the mass by half? How
much force would the object have now?
What does F = ma say?
F = ma basically means that the force of
an object comes from its mass and its
acceleration.
• Something very massive (high mass) that’s
changing speed very slowly (low
acceleration), like a glacier, can still have
great force.
What does F = ma say? con’t
• Something very small (low
mass) that’s changing speed
very quickly (high acceleration),
like a bullet, can still have a
great force. Something very
small changing speed very
slowly will have a very weak
force.
Newton’s Third Law
For every action there is an equal and
opposite reaction.
What does this mean?
• For every force acting on an object, there is an
equal force acting in the opposite direction.
• Right now, gravity is pulling you down in your
seat, but Newton’s Third Law says your seat is
pushing up against you with equal force. This
is why you are not moving.
• There is a balanced force acting on you–
gravity pulling down, your seat pushing up.
Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push
against a wall? You slide in the
opposite direction (away from the wall),
because you pushed on the wall but the
wall pushed back on you with equal and
opposite force.
Why does it hurt so much when you
stub your toe? When your toe exerts a
force on a rock, the rock exerts an
equal force back on your toe. The
harder you hit your toe against it, the
more force the rock exerts back on
your toe (and the more your toe hurts).
Review
• Newton’s First Law: Objects in motion
tend to stay in motion and objects at rest
tend to stay at rest unless acted upon by
an unbalanced force.
• Newton’s Second Law: Force equals
mass times acceleration (F = ma).
• Newton’s Third Law: For every action
there is an equal and opposite reaction.
Ch 7, Lesson 5 Gravity
What is a Force?
• FORCE = Any push or pull which causes
something to move or change its speed or
direction.
Ch 7, Lesson 5 Gravity
Gravity: An attraction force between any two
objects that have mass.
Newton’s Universal Law of Gravitation: Every
object in the universe exerts a gravitational
attraction to all other objects in the universe.
The amount of gravitational force depends upon the
mass of the objects and the distance between the
objects.
Ch 7, Lesson 5 Gravity
The greater the mass, the greater the force.
The greater the distance, the less the force.
Acceleration due to gravity =
9.8 m/s/s or 9.8 m/s2
Ch 7, Lesson 5 Gravity
Weight is a measure of the gravitational force
between two objects.
The greater the mass the greater the force
(weight).
Measured in units called Newtons (N).
Ch 7, Lesson 5 Gravity
Weightlessness – free from the
effects of gravity.
Ch 7, Lesson 5 Gravity
Gravity and Acceleration
 Gravity causes all objects to have the
same acceleration as they fall.
 Another force, air resistance also acts on
falling objects.
 Air resistance
- The force of air exerted on a
falling object.
 The air pushes up as gravity pulls down.
Ch 7, Lesson 5 Gravity
What is Friction?
Friction = A force that opposes or slows down
motion.
Caused by the physical contact between
moving surfaces.
The amount of friction depends upon the kinds
of surfaces and the force pressing the surfaces
together.
Changes motion into heat.