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Acceleration and Force
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Galileo’s Investigation
Engagement
What are the similarities and differences between a basketball and a tennis ball?
Make a T-Chart
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Galileo’s Investigation
Engagement
What will happen if the balls are released from the same height above the floor?
Try to use Physics vocabulary in your response.
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Galileo’s Investigation
Engagement
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.
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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
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Newton’s Universal Law of Gravitation HO 3.2
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.
Explanation
Force Gm 1 m 2
d2
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Newton’s First Law
Objects require a force to change their velocity.
– Another term for this property of matter is inertia
What is the principle property of an object that determines its inertia?
– mass
Explanation
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Newton’s First Law Example
If we apply the same force (push) to a bus and a bicycle, which object will experience a greater change in velocity?
Explanation
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Newton’s Second Law
Force causes a change in motion
– Change in motion means change in velocity
– Change in velocity means???
A change in velocity occurs in a certain amount of time.
AHA! That’s a rate.
Explanation
F = ma
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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
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Newton’s Second Law Example #1
Important note: the acceleration vector is always in the same direction as the force vector.
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?
Explanation
F = ma
100 N2 m/s2
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Newton’s Second Law Example #2
What happens when the acceleration vector points opposite to the velocity vector?
Explanation
30 m/s
F -5m/s2
How long will it take Mighty Mikey to stop the train?
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Newton’s Third Law
No single isolated force can exist
Equal and Opposite forces exist only between two objects
Explanation
Push on water
WRONG!
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Newton’s Third Law Example
You push on the water The water pushes back on you with an equal force in the
opposite direction So why do you move?
Explanation
Push on water
CORRECT!
Push on swimmer
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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.
Weight = mass X gravity (W=mg).
Explanation
Wbb>Wtb
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Acceleration Due to Gravity
Explanation
F = weightF = weight
m = massm= mass
F/m=a F/m = a
a = gregardless of mass!
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Elaboration
Is there a Starbuck’s in the Leaning Tower of Pisa?
Observe the instructor drop a new set of objects.
As a class, discuss the similarities and differences in their motion.
Why is this different?
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Elaboration
Paper Drop Challenge HO 3.3
Materials per group: 2 sheets of paper, stopwatch, 8 paperclips, 30cm of tape, scissors, meter stick.
Have Materials Manager distribute supplies.
You have 10 minutes for this activity.
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.)
GO!
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Elaboration
Paper Drop Challenge
As a group, describe your strategy for making the piece of paper:
– Drop the slowest
– Drop the fastest
Make a table of your drop times for comparison with the class
Explain why your two pieces of paper accelerate at different rates.
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Free Body Diagram HO 3.4
A free body diagram is a pictorial representation of all forces acting on an object.
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
Explanation
Label the forces acting on the block.
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Free Body Diagram Example
Explanation
Force on the block by the ground
Force on the block by the earth
Force on the block by Antonio
Force on the block by the rope.
Force on the block by the ground
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Free Body Diagram Example 2
Forces on coffee filter before release (balanced forces)
Explanation
Weight
Support from my
hand
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Free Body Diagram Example 2
Forces on coffee filter immediately after release (unbalanced force acceleration)
Explanation
Weight
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Free Body Diagram Example 2
Forces on coffee filter while falling
Explanation
Weight
Air resistance
What factors influence the magnitude of
the force of air resistance?
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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
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Net Force
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
Explanation
Net Force
F
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Net Force
The force of air resistance is greater for the basketball than the coffee filter.
However, the force of air resistance is negligible compared to the weight of the basketball, therefore the Net Force is toward the floor.
Explanation
Air resistance
Weight
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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.
In fact, at some point, the force of air resistance is equal to the weight of the coffee filter.
When this is the case:
– There is NO net force
– Therefore there is no acceleration
– Therefore the object moves at a constant speed
This equilibrium condition is called terminal velocity
Explanation
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Evaluation
Sky Diver Physics HO 3.5
A skydiver jumped out of a plane 1 sec ago.
– Label the forces on the skydiver
After some time, the skydiver has zero acceleration
– What does this indicate about her speed?
– What is the name for this equilibrium condition of a falling object?
– Label the forces on the skydiver
What factors influence the force of air resistance on the skydiver?
BONUS: What is the terminal velocity of a human?
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Summary
Force
Gravity & Weight
Newton’s Laws
Air resistance
Free Body Diagrams
Net Force
So what?
– This applies to transportation, moving objects, sports, etc.
Summary
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Homework
Reading Assignments HO 3.6
NSES
– p. 123, 126-127;
149, 154-155; 176-181
BSL
– 87-92
Integrated Science
– Forces: p. 26-27, Falling Objects:p. 29-31, Laws of Motion:p. 32-38
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Homework
Concepts and Questions
Integrated Science, Ch. 2
– Applying the Concepts 1-6, 8-9
– Questions for Thought 1, 5, 8
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