SECTION 5.1 MEASURING MOTION We have learned all about matter:
what is it made out of, how it forms, different states Now, we are
going to learn about when matter moves Matter moves around you all
the time, can you see something moving? Do you know of something
that is moving right now? We are going to learn how to measure that
movement.
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MOTION When you watch something that is in motion, you are
always watching the object in reference to another object that
appears to stay in place When you are watching the bird fly, it is
in reference to the tree below it that is not moving. The object
that appears to stay in place is called the reference point. In the
example above, the tree is the reference point. Can you give
another example?
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MOTION An objects change in position relative to a reference
point
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Motion You can also describe an objects motion with a reference
direction, such as north, south, east, west, up or down.
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COMMON REFERENCE POINTS The Earth can be used as a common
reference point Nonmoving objects such as buildings and trees can
also be good reference points A moving object can also be used as a
reference point Ex. You are in your car, you see a car zooming past
your car, you would be able to tell that the car is accelerating by
comparing it to yourself in your car.
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SPEED The distance traveled divided by the time interval during
which the motion occurred
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CALCULATING SPEED The speed depends on how far something
traveled and how long it took the object to travel that far. Speed
depends on both distance and time Ex. Supposed the balloon took 10s
to travel 50m. How far would it have traveled? You divide the
distance of the balloon by the time if took (50m)/(10s) = 5m/s You
can measure speed in m/s, km/h, ft/s and mi/h
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AVERAGE SPEED Most of the time, objects do not travel at
constant speeds. Think about when you are in the car, you have stop
lights, stop signs and traffic you do not always go the same speed.
There is a way to calculated how fast you are going on average You
can calculate the average speed by dividing the total distance by
the total time Average speed = (total distance)/(total time)
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SPEED ON A GRAPH Look at Figure 2 from page 119 The blue line
shows the total distance travelled during the trip The distance
travelled each hour is different the graph does not cover the same
distance each hour for all four hours The distance changes because
the speed is not the same throughout the whole trip
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AVERAGE SPEED ON A GRAPH You can find the average speed during
the 4 hour period by using the average speed equation Average speed
= (360km)/(4hours) = 90km/h The red line on figure 2 shows the
average speed this line shows the distance increasing by 90km every
hour. This line shows that if she travelled 90km/h for 4 hours she
would travels 360km and reach her destination right on time The
slope of the red line is the average speed
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THE DIRECTION THE BIRDS ARE TRAVELLING Imagine that 2 birds
leave the same tree at the same time. They both fly at 10km/h for 5
min, 12 km/h for 8 min, and 5km/h for 10 min. Why dont they end up
at the same place?
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VELOCITY The speed of an object in a particular direction
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VELOCITY Velocity is the speed an object is moving AND the
direction is it moving. Knowing the velocity helps us know exactly
where an object is going. What is the difference between speed and
velocity? Speed: how fast an object is moving Velocity: how fast an
object is moving AND the direction
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SOME ARE TRAVELLING SOUTH, AND SOME NORTH If you look at cars
on the highway. You have 2 different directions the cars can travel
North and South. If the cars are travelling at the same speed, how
are their velocities different?
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CHANGING VELOCITY The change of velocity is the change of an
objects position Can the velocity change if the speed remains
constant? Yes! The direction can change. Can the velocity change if
the direction remains constant? Yes! The speed can change.
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EXAMPLE If a bus is travelling 15m/s south and changes to 15m/s
north, does its speed change? No Does its velocity change? Yes If a
bus is travelling 15m/s south and changes to 30m/s south, does its
speed change? Yes Does its velocity change? Yes
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COMBINING VELOCITIES Since velocity needs speed and direction,
there are ways to add and subtract velocities based on the
direction If the two velocities have the same direction, you add
the speeds. If the two velocities have opposite directions, you
subtract the speeds.
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COMBINING VELOCITIES Say you are in a bus travelling 15m/s
East. Suppose you start walking down the bus at 1m/s West. Are you
still traveling at 15m/s? No. What is your new velocity? 15m/s East
1m/s West = 14m/s East Suppose you start walking up the bus at 1m/s
East. Are you still travelling 15m/s? No. What is your new
velocity? 15m/s East + 1m/s East = 16m/s East
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ACCELERATION The rate at which velocity changes over time; an
object accelerates if the speed changes, the direction changes, or
if both change.
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ACCELERATION Unlike normal usage of the word acceleration, in
science acceleration is ANY change of velocity. An increase in
velocity is called a positive acceleration A decrease in velocity
is called a decreased acceleration or a deceleration Acceleration
is both how much the velocity changes and how fast the velocity
changes. The faster the velocity changes, the greater the
acceleration is
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CALCULATE AVERAGE ACCELERATION Average acceleration = (final
velocity starting velocity)/(time it takes to change velocity)
Velocity is expressed in m/s. Acceleration is m/s divided by s
which can be expressed as m/(s^2) Look at the drawing on the board.
How do you calculate the average acceleration?
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ACCELERATION ON A GRAPH When drawing a velocity graph, you have
speed on the y-axis and time on the x-axis When drawing an
acceleration graph, you have velocity on the y-axis and time on the
x-axis Plot the figure on the board on an acceleration graph For
the rest of class, work on the data we collected yesterday in lab.
What do we need to calculate the speed on the balloons?
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ACCELERATION The rate at which velocity changes over time; an
object accelerates if the speed changes, the direction changes, or
if both change.
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VELOCITY The speed of an object in a particular direction
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SPEED The distance traveled divided by the time interval during
which the motion occurred
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DRAW GRAPHS Draw the following graphs. Label the x-axis and the
y-axis. Give units and describe what the slope shows. 1. Graph for
speed 2. Graph for velocity 3. Graph for acceleration
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CIRCULAR MOTION An object travelling in a circular motion is
always changing direction. What does that mean for its velocity?
What does that mean for its acceleration?
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CENTRIPETAL ACCELERATION The acceleration that occurs in
circular motion. What are some examples of centripetal acceleration
in daily life?
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5.2 - WHAT IS A FORCE Use the word force in a sentence So what
exactly does force mean?
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FORCE A push or pull on an object in order to change the motion
of the object; force has size and direction
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FORCE A force can change the acceleration of an object How
would it change the acceleration? Speed or direction; velocity Any
time the motion of an object changes, it is because of force What
are some examples of objects changing motion? Scientists measure
force in a unit called Newton (N)
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FORCES ACTING ON OBJECTS All forces act on objects. For any
push to occur, something has to receive the push. You need to have
something to push in order for you to exert a force. Can you think
of examples of you exerting force on something? Ex. Opening the
book, you put a force on the pages Ex.
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FORCES ACTING ON OBJECTS Just because you exert a force onto an
object, does not mean that the object will move. There are more
forces acting on an object and if those forces are greater than the
one you are putting onto the object, the object wont necessarily
move. Ex. If I try to move the bookcase. I can exert a force, but
the force of all of the books and gravity pushing the bookcase to
the ground will be stronger than my pushing force, and so the
bookcase will not move. Can you think of other examples?
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UNSEEN SOURCES AND RECEIVERS OF FORCES It is not always easy to
see forces. You cannot always tell which forces are acting on
objects There is a magnetic force keeping magnets to refrigerators,
but we cannot see it. We also cannot see the force of gravity, but
we can see how it works around us
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NET FORCE The combination of all the forces acting on an
object. The net force shows us which way the object will move,
since it is the addition of all of the forces on an object.
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DETERMINING NET FORCE When you have 2 forces acting on an
object in the same direction, you can add those two forces to
determine the net force. The net force will be the addition of the
forces and in the same direction of the individual forces. When you
have 2 forces acting on an object in opposite directions, you
subtract the smaller force from the larger force to find the net
force. The direction of the net force will be the same as the
direction of the larger force. Examples!
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DETERMINING NET FORCE When you have 2 forces acting on an
object in the same direction, you can add those two forces to
determine the net force. The net force will be the addition of the
forces and in the same direction of the individual forces. When you
have 2 forces acting on an object in opposite directions, you
subtract the smaller force from the larger force to find the net
force. The direction of the net force will be the same as the
direction of the larger force. Examples!
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NET FORCE So what does the net force show us? If the net forces
are unbalanced, not 0 N, then you know that the object will be
moving Balanced forces: the net force equals 0 N The motion of the
object will not change; if the object is moving, it will continue
to move. If the object is stationary, it will stay stationary.
Examples of balanced forces?
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NET FORCE Unbalanced forces: the net force does not equal 0 N
They produce a change in motion or direction You NEED to have an
unbalanced force if the object is moving An object can continue to
move even if the force has stopped Ex. Use the golf ball and wooden
ball to explain some forces Do worksheet
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GRAVITY: A FORCE OF ATTRACTION Watch vacuum video What is going
on in that video? Was the result what you were expecting? Why/why
not?
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GRAVITY A force of attraction between objects that is due to
their masses
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Since gravity is a force, what can it change about the object?
The speed, direction or both!
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GRAVITY ON MATTER All matter has mass! Gravity is a result of
mass! Therefore, all matter is affected by gravity! This means that
all matter has attraction forces to all other matter. This means
that you are being pulled, by gravity, towards you pen, the desk,
and each other! And they are being pulled towards you. Can you feel
it? No. Of course you dont get physically pulled towards your desk.
You arent stuck to your desk because the mass of the desk is too
small, your mass is also too small to pull the book towards you.
HOWEVER, the Earth is big enough to pull you to it!
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THE EARTH IS THE BIGGEST THING ON EARTH!! Due to the above
statement, of all of the things we will come into contact with on
Earth, the Earth is by far the largest. Since gravitation force is
related to mass, the Earth has the greatest gravitational force. We
are all being pulled towards the Earth. Every time we try to
separate ourselves, or another object, from the Earth, we need to
use forces!
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NEWTON AND THE STUDY OF GRAVITY Why do objects fall towards the
Earth? How do the planets keep in their orbit? These two questions
are related! Newton was the first person to document how these two
are related The legend is that he came up with the answer to both
questions by tossing an apple and watching it fall back to Earth.
We know that unbalanced forces cause things to move. What is the
force that causes the apple to fall and the force that keeps the
moon moving around the Earth are the same! Gravity!
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THE LAW OF UNIVERSAL GRAVITATION All objects in the universe
attract each other through gravitational force. The size of the
force depends on the masses of the objects and the distance between
the objects. This law can be understood in 2 parts: Part 1:
Gravitational force increases as mass increases Part 2:
Gravitational force decreases as distance increases
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GRAVITATIONAL FORCE INCREASES AS MASS INCREASES Think of an
elephant and a cat. The elephant is so much larger that the cat,
which means it has a greater gravitational pull to the Earth, which
is why it is harder to pick up an elephant than a cat. Since both
the elephant and the cat have mass, they also have a pull towards
each other. However, since the Earth is so big in comparison, you
cannot see that force. In the same way, when an astronaut is on the
moon, they are able to jump and move easier because the moon has
less mass than the Earth, and so there is less gravitational pull
on the astronauts.
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GRAVITATIONAL FORCE DECREASES AS DISTANCE INCREASES Why doesnt
the giant sun pull you towards itself more than the Earth pulls us
to it? The answer lies in the fact that the sun is so far away! We
are about 150million km from the sun. Due to this distance, the
gravitational force is very small. If we were to stand on the Sun,
the gravitational pull would be so great due to the Suns size, that
we would not be able to move. It is the gravitational pull from the
Sun on the Earth that keeps the Earth in orbit. This gravitational
pull on the planets is large because of the mass of both the Sun
and the Earth.
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MASS A measure of the amount of matter in an object
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WEIGHT A measure of the gravitational force exerted on an
object; its value can change with the location of the object in the
universe
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WEIGHT Mass does not change whether you are on Earth or the
Moon or the Sun Weight will change in each of these different
places. As long as the gravitational pull remains the same, mass
and weight can be used interchangeably. If you moved to somewhere
with greater gravitational pull, the weight will greatly increase,
but the mass would remain the same. Look at Figure 6 on page
139
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UNITS OF WEIGHT AND MASS Force Newton (N) Weight is the measure
of gravitational force Newton (N) Mass grams (g) 100g = 1N When you
stand on a scale, you measure the gravitational force between you
and the Earth. So it should be measured in Newtons. However, it is
normally expressed in colloquial terms, pounds or kilograms.