Transcript Acceleration and Force

Physics
:acceleration and force
Acceleration and Force
Engagement
Physics
:acceleration and force
Galileo’s Investigation

What are the
similarities and
differences between
a basketball and a
tennis ball?

Make a T-Chart
Engagement
Physics
:acceleration and force
Galileo’s Investigation

What will happen if
the balls are
released from the
same height above
the floor?

Try to use Physics
vocabulary in your
response.
Engagement
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Galileo’s Investigation

What’s going on here,
why did they move?

How would you
describe the motion
of the balls?

–
Path
–
Speed
Again, try to use
Physics vocabulary in
your response.
Explanation
Physics
:acceleration and force
Force of Gravity: Pull or Push?

Force can be described as a
push or pull one object
exerts on another.

One specific force is weight.
Weight = the force of gravity
on an object.

But what causes gravity?
–
The presence of mass
Explanation
Newton’s Universal Law of Gravitation HO 3.2
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
Masses attract one another
about their centers with a
force proportional to the
product of their masses and
inversely proportional to the
square of the distance
between them (whew!).
Try to write that in an
equation form.
Force  G
m 1 m 2
d
2
Explanation
Newton’s First Law

Objects require a force to change their velocity.
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:acceleration and force
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
Another term for this property of matter is
inertia
What is the principle property of an object that
determines its inertia?
–
mass
Explanation
Newton’s First Law Example
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If we apply the same force (push) to a bus and a
bicycle, which object will experience a greater
change in velocity?
Explanation
Newton’s Second Law
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Force causes a change in motion
–
Change in motion means
change in velocity
–
Change in velocity means???
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A change in velocity occurs in a
certain amount of time.
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AHA! That’s a rate.
SF = ma
Explanation
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Acceleration is a Rate

The speedometer in your car
indicates what property of
motion?

What happens to the needle if
you speed up or slow down?

The speed of the needle is a
measure of your acceleration
(in a straight line).

Acceleration is the rate of
change in velocity.
Explanation
Newton’s Second Law Example #1
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SF = ma
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Important note: the acceleration vector is always
in the same direction as the force vector.
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If we apply a 100N force to a wagon full of rocks
initially at rest with a mass of 50 kg, what is its
acceleration?

What does this answer tell us?

What is the velocity of the object after 1 second?
–
2 seconds?
–
5 seconds?
2 m/s2
100 N
Explanation
Newton’s Second Law Example #2
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What happens when the acceleration vector points
opposite to the velocity vector?
30 m/s
F
-5m/s2
How long will it take Mighty Mikey to stop the train?
Explanation
Physics
:acceleration and force
Newton’s Third Law

No single isolated force can exist

Equal and Opposite forces exist only between two
objects
WRONG!
Push on
water
Explanation
Physics
:acceleration and force
Newton’s Third Law Example

You push on the water
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The water pushes back on you with an equal force in
the opposite direction
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So why do you move?
Push on
swimmer
Push on
water
CORRECT!
Explanation
Physics
:acceleration and force
Acceleration Due to Gravity

The Tennis Ball and Basketball hit the ground at the
same time because they accelerated at the same
rate (~10 m/s2).

What force changed their velocities?...Weight.
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Weight = mass X gravity (W=mg).
Wbb>W
tb
Explanation
Acceleration Due to Gravity
F
m
F/m a
F = weight
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= weight
m= mass
= mass
=
F/m
a=g
regardless of mass!
=a
Elaboration
Physics
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Is there a Starbuck’s in the Leaning
Tower of Pisa?
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Observe the instructor drop a
new set of objects.

As a class, discuss the
similarities and differences in
their motion.

Why is this different?
Elaboration
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Paper Drop Challenge HO 3.3
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Materials per group: 2 sheets of paper, stopwatch, 8
paperclips, 30cm of tape, scissors, meter stick.
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Have Materials Manager distribute supplies.
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You have 10 minutes for this activity.
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Make one whole piece of paper drop to the floor as
fast as possible from a height of 2 meters and the
other drop as slowly as possible from the same
height. (Do not attach the scissors or meter stick to
your paper.)
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GO!
Elaboration
Paper Drop Challenge
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As a group, describe your strategy for making
the piece of paper:
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Drop the slowest
–
Drop the fastest
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Make a table of your drop times for comparison
with the class
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Explain why your two pieces of paper accelerate
at different rates.
Explanation
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Free Body Diagram HO 3.4
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A free body diagram is a pictorial
representation of all forces acting on
an object.
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We consider the object to be isolated
from the physical system and choose
to examine only the forces directly
acting ON the object, not forces
applied BY the object.
Explanation
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Free Body Diagram Example
Label the forces acting on the block.
Explanation
Free Body Diagram Example
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Force on the block by
the ground
Force on the block
by the ground
Force on the
block by Antonio
Force on the block
by the rope.
Force on the block by
the earth
Explanation
Free Body Diagram Example 2
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Forces on coffee filter before release (balanced
forces)
Support
from my
hand
Weight
Explanation
Free Body Diagram Example 2
Forces on coffee filter immediately after release
(unbalanced force  acceleration)
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Weight
Explanation
Free Body Diagram Example 2
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Forces on coffee filter while falling
What factors
influence the
magnitude of
the force of air
resistance?
Air
resistance
Weight
Explanation
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So Why Different Times?

Force due to gravity is different for the two
objects

Their masses are different and therefore they
have different inertias

They do not accelerate at the same rate
…WHY???
–
We must examine ALL the forces acting on
the objects in order to determine the Net
Force.
Explanation
Net Force
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SF
If you earn $2000/mo but taxes, ins, retirement
and other reductions total $350/mo then your:
–
Gross Pay = $2000/mo
–
Net Pay = $1650/mo
Net Force
Explanation
Physics
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Net Force
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The force of air
resistance is greater for
the basketball than the
coffee filter.
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However, the force of air
resistance is negligible
compared to the weight
of the basketball,
therefore the Net Force
is toward the floor.
Air
resistance
Weight
Explanation
Physics
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So Why Different Times?

The force of air resistance is also acting on the coffee
filter and is comparable in size to the weight.
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In fact, at some point, the force of air resistance is
equal to the weight of the coffee filter.
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When this is the case:
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There is NO net force
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Therefore there is no acceleration
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Therefore the object moves at a constant speed
This equilibrium condition is called terminal velocity
Evaluation
Sky Diver Physics HO 3.5
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A skydiver jumped out of a plane 1 sec ago.
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Label the forces on the skydiver
After some time, the skydiver has zero acceleration
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What does this indicate about her speed?
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What is the name for this equilibrium condition of a
falling object?
–
Label the forces on the skydiver
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What factors influence the force of air resistance on the
skydiver?
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BONUS: What is the terminal velocity of a human?
Summary
Physics
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Summary
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Force
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Gravity & Weight
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Newton’s Laws
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Air resistance
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Free Body Diagrams
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Net Force
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So what?
–
This applies to transportation, moving objects,
sports, etc.
Homework
Reading Assignments HO 3.6
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NSES
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p. 123, 126-127;
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149, 154-155; 176-181
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BSL
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87-92
Integrated Science
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Forces: p. 26-27, Falling
Objects:p. 29-31, Laws of
Motion:p. 32-38
Homework
Concepts and Questions
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Integrated Science, Ch. 2
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Applying the Concepts
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1-6, 8-9
Questions for Thought
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1, 5, 8