Newton’s Second Law
Chapter 4
Net Force causing acceleration
Friction
Air resistance
Student Learning Outcomes
• Understand when Newton’s law of inertia applies
and when it does not
• Understand what happens to objects when there
is a net force (Newton’s 2nd law)
• Calculate accelerations from the net force
• Understand a conceptual model of friction
– Calculate acceleration when friction acts
• Understand a conceptual model of air resistance
– Calculate acceleration when falling with air resistance
What can happen to an object when the net
force on it equals zero? Select all that apply.
1.
2.
3.
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5.
0
0
0
0
0
Speed up
Slow down
Change direction
Stay at rest
Move in a line with constant speed
Give examples in this room where the net force = 0.
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What can happen to an object when the net
force on it is NOT zero? Select all that apply.
1.
2.
3.
4.
5.
0
0
0
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Speed up
Slow down
Change direction
Stay at rest
Move in a line with constant speed
Give a one word answer to the question, “What happens to an
object when the net force on it is NOT zero?”
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Accelerations
• Newton’s second law:
– ____________ _____________ object ___________
– An object’s _______________________________
– Equation:
• People often say _______ , but the better version is ____________
– Words: ________________________________________
– Same equation written another way:
• Describe this in words.
• Textbook pages ________________
Figure 4.2 – with extra
labels, p. 59
Figure 4.11 – with extra
labels, p. 64
Friction - what
• When does friction occur?
– __________________________________________
_______________________________
• In which direction does friction push?
– ________________________________________.
• Note the word “__________” left out of your textbook!
• Sliding cart with scale
– Start slowly, pull stronger. Maximum friction force?
– Constant speed: how much friction force?
4 examples
• 5-kg box sits on flat floor. Maximum friction force
is 100 N. Draw a picture for each of the 4 cases.
– What is the net force in these cases?
– What happens to the box?
– Calculate acceleration.
1. You push to right with 70 N
2. You push to right with 100 N
3. You push to right with 110 N
4. You push to left with 120 N
Friction - why
• Why does friction occur?
– ________________________________________
______________________________ Figure 4.3:
What does friction depend on?
• Surprising, _________ and _______________
– How do we know?
– Except for “______________ ” (or ____________
____________________)
• In these cases, friction happens because ____________
___________________________)
• ____________________________________
• ___________________________________
___ Types of friction
• ______________________ friction
– When ____________________________________
• ___________ friction
– When ___________________________________
_____________________________________
– The surfaces ___________ if there were no friction.
• ___________ friction (not in your textbook)
– When one object __________ across a surface
Which kind of friction is weakest?
1.
2.
3.
4.
0
0
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0
Static friction
Sliding (kinetic) friction
Rolling friction
All 3 are the same
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Newton’s 3rd law causing ________
accelerations
• Newton’s 3rd law told us
– _____________________________________________
_______________________________.
• Yet we “know” BIG truck “______” and little car
“______” in a collision.
• Why? [Rhetorical]
• On next slide, I’m going to color code car and truck.
In your notes, somehow “color” code them too.
Unequal accelerations, continued
•
• Ftruck on car makes ____________________________
–
• Fcar on truck makes ____________________________
• Summary – putting it all together:
•
•
• Written more graphically:
• And what do people feel?
Simple gravity
• (More complicated version of gravity is our
next chapter, chap. 9 – we’ll come back to 6-8)
• __________________________ , we can say:
– ______________________________
• On Earth: _____________________________________
• On Moon: about 1/6 , 3.7 miles/hr per sec)
• The acceleration number is called _________
– __________________________ (symbol: ____)
– ________________________________________
– _________________________________________
________________________________________
You have two balls of equal size and smoothness, and you can ignore air
resistance. One is heavy, the other much lighter. You hold one in each hand at
the same height above the ground. You release them at the same time. What
will happen?
1. The heavier one will hit the ground first.
2. They will hit the ground at the same time.
3. The lighter one will hit the ground first.
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Same objects. How does the acceleration of gravity compare? Be
careful!
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The heavier one has a larger gravitational acceleration.
They have the same gravitational acceleration.
The lighter one has a larger gravitational acceleration.
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Same objects. How does the force of gravity compare? Be careful!
0 1.
The heavier one has a stronger gravitational force.
They have the same gravitational force.
The lighter one has a stronger gravitational force.
0 2.
0 3.
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Falling with air resistance
• Air resistance depends on _____and “frontal”
__________________ .
• One model of air resistance:
– ________________________________________
• Coffee filter demo
– Regular
– Crumpled
• What forces act after you drop out of a very
high-up helicopter?
What is speed at the instant you drop out of
very high helicopter?
1. Zero
2. 10 m/s
3. Not enough information?
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0
So how strong is air resistance?
What forces act?
What’s the net force?
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What is acceleration at the instant you drop out of very
high helicopter?
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Zero
10 m/s
10 m/s per second
Not enough information
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If wait a long time, what happens to speed?
1. You always speed up
2. You speed up, but then slow down
3. You speed up to a certain speed
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What is acceleration after waiting a long time, BUT
before hitting ground?
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More than g = 10 m/s per second
g
Less than g but more than zero
zero
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What is acceleration in middle, before
achieving terminal speed?
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More than g = 10 m/s per second
G
Less than g but more than zero
zero
What’s happening to the acceleration?
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Jumping out of planes
• People in the movies often jump out of planes
without a parachute, usually after the bad guy
takes the last parachute.
• What do the good guys do to survive?
• Why does this work?
– How does it affect __________________ ?
• What do parachutes do?
– How does it affect ___________________?
Which encounters a greater force of
air resistance if dropped at same time?
1.
2.
3.
4.
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Falling feather
Falling elephant
Same air resistance for both
Not enough information
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Student Learning Outcomes
• Understand when Newton’s law of inertia applies
and when it does not
• Understand what happens to objects when there
is a net force (Newton’s 2nd law)
• Calculate accelerations from the net force
• Understand a conceptual model of friction
– Calculate acceleration when friction acts
• Understand a conceptual model of air resistance
– Calculate acceleration when falling with air resistance