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Bellringer
At which point in this diagram is the field the
strongest?
Motion & Forces
Notes
Describing and
Measuring
Forces
• An object is said to be in motion if it
changes position relative to a reference
point.
•
•
Motion means moving. To find out if an
object is in motion, you MUST compare it
to another object or place. This is known
as relativity.
An object is in motion if its distance from
another object or place is changing.
Describing and
Measuring Forces
• A reference point is an object or place
that you can use to tell if an object is in
motion. A tree, a sign, or a building
make good reference points.
Describing and
• HaveMeasuring
you ever been in a busForces
or car sitting beside
another one? Suddenly, you think that your
vehicle is moving backward. Upon closer
inspection, you actually find that it’s the other
vehicle that is moving forward. Because you were
using a moving object as a reference point, your
sense of motion was inverted.
•
Now, imagine astronomers when they saw the
sun. Because they thought that Earth was the
stationary reference point, they mistakenly thought
that it was the universe that revolved, and not
Earth.
Describing and
Measuring Motion
• Motion is measured as distance. A
meter is a unit of length.
• This can be modified by adding a metric
prefix such as kilo-, nano-, cent-, etc.
• 1mm = .001 meters
• 1nm = .000001 meters
• 1km = 1000 meters
Calculating Speed
• If you know the distance an object
travels in a certain amount of time, you
can calculate the speed of an object.
• Speed is a rate.
It tells how far
something moves in a certain amount of
time. For example, 1 meter per second
is speed.
Calculating Speed
D = Distance
D
S T
S = Speed
T =Time
Calculating Speed
• On a car ride to school, you speed up
and slow down.
• Average speed tells the total distance
you rode divided by the total time it
took.
• Instantaneous speed is the speed you
were moving at an instant in time.
during the car ride.
It’s ten miles from 79th Street to
5300N. It takes your school bus
20 minutes to drive this
distance.
What is it’s average speed?
But note the railroad crossings.
At each crossing, the bus is required
to stop.
Therefore, as the bus approaches
each crossing, it must slow down, and
it’s instantaneous speed will
decrease.
Another way to determine
instantaneous speed is to just look at
Calculating Speed
• Suppose that a cyclist travels 32
kilometers in the first 2 hours of her
trek. She then travels another 13
kilometers during the next hour.
• What is her average speed?
Describing Velocity
• To completely describe the motion of an
object, you need to not only know its
speed, but also its direction.
• When you know both speed AND
direction, then you can calculate
velocity
• Velocity is speed in a given direction.
•
•
•
Describing Velocity
For example, you hear that a thunderstorm
is traveling at 25 km/h. Should you fear for
your life?
Well, that depends on its direction of
motion.
If you’re an air traffic controller, if you just
give the speeds of the other planes to the
pilots, then they still might crash into each
other. The planes’ directions are vitally
important.
24 hours of United States Air
Traffic
What is a Force?
• Like velocity, force is described by its
strength and by the direction in which it
acts.
• A force is a push or pull.
What is a Force?
YES
Not Quite
What is a Force?
• To tell about a force, you must tell how
strong the force is. The SI unit for
strength of a force is the newton.
• To tell about a force, you must also tell
the direction the force is pushing or
pulling.
• Arrows can be used to show forces.
The point of the arrow shows the
direction of the force. The length of the
arrow shows how strong the force is.
Combining Forces
• Unbalanced forces acting on an object
result in a net force and cause a change
in the object’s motion. Balanced forces
acting on an object do not change the
object’s motion.
• Often there is more than once force
acting on an object. The total of all the
forces acting on an object is called the
net force.
Combining Forces
• Sometimes the net force on an object is
0. This means there are balanced
forces acting on the object. The
object’s motion does not change.
• In space, this means it could be
stationary, or it’s traveling at constant
velocity.
Combining Forces
• Sometimes, the net force does not
equal 0. This means there are
unbalanced forces acting on the
object. The object’s motion changes.
• In space, this means the object will
curve, speed up, or slow down. In other
words, it’s velocity will change.