Transcript force - Denton ISD

Reassessment Review For Unit 9 & 10
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“Slideshow” and “From Beginning”
As you go through the presentationcomplete the
activities and , take detailed notes on the following
topics.
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Measuring motion Including Speed
Reading Motion Graphs
Forces
Potential & Kinetic Energy
Pulleys
Inclined Planes
Describe Frame of Reference
The object used for comparing
another object to describe its position
is called a…
frame of
reference
Name the frame of
reference in each
of the following
statements.
The dog is beside
the house.
The duck is
swimming in
the pond.
Describe Motion
You can tell that an
object has moved when
its __________
position has
changed.
Motion happens when an object
changes position.
Describe Acceleration
• A change in velocity – which may be:
– A change in speed
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Starting
Stopping
Speeding up
Slowing down
– A change in direction
• Acceleration is caused by unbalanced
forces
The distance an object
moves in a certain amount of
time is called _________.
speed
Describe Speed
• A way to describe motion
– Average speed - calculated by dividing the distance
traveled by the amount of time it takes to travel that
distance
– Constant speed - Speed that does not change
– Instantaneous speed - Speed of an object at any
given time
What is the formula for
calculating speed?
Speed is calculated by dividing distance
by time –
Calculate This Speed
A football field is about 100 m long. If it
takes a person 20 seconds to run its
length, how fast was the football player
running?
Calculate this Speed:
A football field is about 100 m long. If it
takes a person 20 seconds to run its
length, how fast was the football player
running?
Speed = Distance ÷ Time
Speed = 100 m ÷ 20 s
Speed = 5m/s
Remember
to include
the UNITS!!
A ______
line
graph can be
used to show
the change in
an object’s
speed over
time.
Distance Time Graphs
Distance Time Graphs
• Describing a journey made by an object is not exciting if you
just use words. As with much of science, graphs are more
revealing.
• Plotting distance against time can tell you a lot about a journey.
Let's look at the axes:
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Time always runs horizontally (the x-axis). The arrow shows the
direction of time. The further to the right, the longer time from the
start.
Distance runs vertically (the y-axis). The higher up the graph we go,
the further we are from the start.
Not moving? This is what
it looks like…
If something is not moving, a horizontal line is drawn
on a distance-time graph.
• Time is increasing to the right, but its distance does
not change. It is stationary.
Moving….
If something is moving at a constant speed,
it means we expect the same increase in
distance in a given time:
• Time is increasing to the right, and
distance is increasing steadily with time.
It moves at a steady speed.
Can you describe what is going on here?
• For the first part of the journey shown by the graph
below, the object moved at a steady (slow) speed.
• It then suddenly increased its speed, covering a
much larger distance in the same time.
• This sort of motion is not very realistic, but is easy
to understand. It also makes calculations easier!
What is the effect of line
‘Steepness’ or slope?
• Both the lines below show that each object moved the same distance,
but the steeper yellow line got there before the other one:
• A steeper gradient indicates a larger distance moved in a given
time. In other words, higher speed.
• Both lines are of constant gradient, so both speeds are constant.
Speeding Up!
The line below is curving upwards. This shows an
increase in speed, since the gradient is getting
steeper:
In other words, in a given time, the distance the
object moves is larger. It is accelerating.
There are three parts to the journey
shown below:
• Moving at a steady speed, slowly
Not moving for quite some time
Moving again, but at higher speed
• In all the graphs so far, we have not
seen any numbers - it's about time we
did!
Calculate the speeds within a graph…
Stage 1: speed = distance / time = 150 m ÷ 30 s = 5 m/s
Stage 2: speed = distance / time = 100 m ÷ 10 s = 10 m/s
Stage 3: speed = distance / time = 100 m ÷ 50 s = 2 m/s
Unbalanced
forces
______
cause
changes in
motion.
Describing Forces
What is a force?
• A force is push
• or a pull
• Force is measured in Newtons
Combining forces
• Usually, more than one force is exerted on an
object at the same time.
• The sum of all forces acting on an object is called
the net force.
• You need to look at the net force in order to figure
out if or how an object will move.
Balanced Forces
• Net force = zero
• No change in motion
• (object is not moving)
• List 3 examples of
balanced forces:
– Paper on desk
– You in the chair
– Flag not moving
Unbalanced Forces
• There is a net force
acting on an object.
• Causes a change in
motion
• Possible to add the
forces together to
find the size &
direction of the net
force.
Unbalanced forces
• If the forces are
moving in the same
direction, add the
forces.
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Example:
– Girl pushes 25 N to the right
– Boy pulls 20 N to the right
– Net Force = 45 N to the right
– The piano moves to the right
Unbalanced forces
• If the forces are
moving in opposite
directions, subtract
the forces.
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Example:
– Dog #1 pulls 10 N to the left
– Dog #2 pulls 12 N to the right
– Net Force = 2 N to the right
You Try It: Calculating Net Force
How did you do???
To decrease the amount of force
needed to do something. and / or
make work seem easier we use
________________.
Simple
machines
Pulleys
A pulley wheel is a mechanism which helps move or lift objects.
pulley
Exercise machine
Single Pulley
• Changes the direction
of the force needed to
lift something, but
NOT mechanical
advantage.
• It just feels easier to
pull down than to pull
up.
100 lbs
100 lbs
100 ft
100 lbs
100 lbs
Movable Pulley
Movable Pulley:
• Pulley moves along the
rope
• Wheel supports the load
• Effort to lift is in the same
direction as movement
• REDUCES the forces
needed to move an object
In the second
picture, the weight is
held by two ropes
instead of one. That
means the weight is
split equally
between the two
ropes, so each one
holds only half the
weight.
http://www.swe.org/iac/LP/pulley_03.html
COMBINED PULLEY
Combined (double)
Pulley:
•Has at least two wheels
•The more complex the pulley,
the less the effort needed to lift
the object.
• The main disadvantage is it
travels a very long distance.
Whether it is being
pushed or pulled, moving
or still, all things have
energy
______.
The energy of
motion is
kinetic
__________
energy.
The energy of
position is
potential
_____________
energy.
Name the type of
energy in each of
the following
statements.
The ball is rolling down
the hill.
The roller coaster is
sitting at the top of the
track.
The pencil fell off the
edge of the desk.
The skier is at the top of
the ski jump.