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2-Sep-10 Physics 101
Chapter 5Friction & Drag
Newton’s Third Law of Motion
For every “action” there is a “reaction”Can’t touch without being touched
Lecture 5
2-Sep-10 Physics 101
Friction and Air ResistanceFriction and air resistance are forces opposing motion.
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2-Sep-10 Physics 101
FrictionOrigin of friction is molecular interaction between
solid surfaces.
Friction is complicated.
Friction depends on support force and on properties of the surface.
Basic properties of friction first established by Leonardo da Vinci.
2-Sep-10 Physics 101
Air Resistance (Drag)Origin of drag is molecules of gas (or liquid) striking
a moving object.Drag force depends on:
•Size (area) of the object•Speed of the object
Larger the size or speed, larger the drag.Also depends on shape of object, density of gas or liquid, etc.
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2-Sep-10 Physics 101
Air Resistance on a Falling ObjectGravity force on an object (i.e., weight) is constant but air resistance depends on an object’s speed.
As a falling object gains speed, the resistance force gets larger so the net force decreases.
Net force is sum of:Weight (downward)Resistance force (upward)
2-Sep-10 Physics 101
Check YourselfGolf ball has more mass than a ping-pong
ball. Force of gravity is greater on: golf ball, ping-pong ball, or the same?
The two balls are the same size; when speeds are equal, drag force is greater on which ball?
Which ball falls faster (which has greater acceleration)?
GolfBall
PingpongBall
Drag
Weight
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2-Sep-10 Physics 101
Terminal SpeedSpeed of falling objects increases until drag force
balances weight. When forces balance, zero acceleration so constant velocity.
Speed for which air resistance balances weight called terminal speed.
High terminal speed
Low terminal speed(large area of chute)
2-Sep-10 Physics 101
Newton’s Third Law of Motion
Whenever an object exerts a force on a second object, the second object exerts a force of equal magnitude in the opposite direction on the first object.
First Object(Hammer)
Second Object(Nail)
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2-Sep-10 Physics 101
Check YourselfA 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20
m.p.h..The force of impact is greatest on which vehicle, the car or
the truck?The change in velocity (the acceleration) is greatest for
which vehicle?By what principle of physics?
2-Sep-10 Physics 101
Arnold Strongman and Suzie Small pull on opposite ends of a rope in a tug of war. The greater force exerted on the rope is by
1. Arnold.2. Suzie.3. Neither. The force is the same.
Check Yourself
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2-Sep-10 Physics 101
1. 10 N,2. 20 N, or3. Zero?
Does the scale read:
10 10
Demo: Pulled in Both Directions
2-Sep-10 Physics 101
Demo: Equal MagnitudesWith spring scales, we verify that action and
reaction forces have equal magnitudes.
0
515
10
0
515
10
Action
HoldPull
Reaction
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2-Sep-10 Physics 101
Action and ReactionCommon expression of 3rd Law is,
To every action there’s an equal and opposite reaction.
What’s an “action”?
How can reaction be “equal” and “opposite”?
2-Sep-10 Physics 101
Action-Reaction
Pairs
????
????
????
????
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2-Sep-10 Physics 101
Demo: Mutual Attraction
What happens when:• Mr. A pulls, Mr. B holds.• Mr. A holds, Mr. B pulls.• Mr. A & Mr. B both pull.
Mr. BMr. A
Mr. B has more mass than Mr. A
2-Sep-10 Physics 101
Demo: Mutual RepulsionSimilar demonstration is to
have Mr. A and Mr. B push away instead of pull together.
Same results; if Mr. A pushes and Mr. B holds then both move apart.
Standing on skateboards
ActionReaction
Mr. AMr. B
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2-Sep-10 Physics 101
Walking, Running & JumpingWhat forces accelerate us into motion when
we walk, run, or jump?
2-Sep-10 Physics 101
Forces when Walking or Jumping
For a person walking, running, or jumping, the three main forces on the person are:
• Gravity (Downward)• Support of the floor (Upward)• Frictional force of the floor (Horizontal)Only these forces can accelerate the person.Gravity is constant but the force exerted by
the floor can increase in reaction to the person exerting a force on the floor.
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2-Sep-10 Physics 101
ReactionAction
Walking ForwardBack foot acts by pushing back on the floor.
Reaction is the friction of the floor, which pushes your body forward
If there were no friction then dancer would fall straight down and could not walk forward
2-Sep-10 Physics 101
Jumping
Jumping is done by pushing downward on the ground (action) so the ground pushes upward on you (reaction).
How high you jump depends on the force and on the distance over which you apply that force. Can only push while in
contact with the ground so squatting helps by increasing distance.
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2-Sep-10 Physics 101
IMPORTANT!!!
Action force & reaction force NEVER cancel, because they act on different objects!
Repeat this to yourself over and over again
2-Sep-10 Physics 101
Check YourselfMiss A pushes the car (action);
car pushes back on her (reaction). Do these forces cancel?
Force on Miss A is to the left; how can she move forward (to the right)?
What if floor had zero friction?
Miss A
Action
Reaction
Action
Reaction
Action-Reaction
Pairs
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2-Sep-10 Physics 101
Check YourselfMiss B also pushes the car;
can she move the car by herself?
In terms of Newton’s laws, why is this not possible?
What other force does Miss B exert on the car besides her hands?
Miss BAction
ReactionReaction
Action
Action-Reaction
Pairs
2-Sep-10 Physics 101
Adding Forces
When two forces or more forces act in different directions, finding the net force is more complicated.
Have to consider the angle for each force.
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2-Sep-10 Physics 101
Parallelogram rule
Vector Addition
Forces are vectors, with magnitude & direction.
Object Force A(20 Newtons)
Force B(10 N)
Net ForceA + B
(25 Newtons)
2-Sep-10 Physics 101
Demo: Straighten the Line
Pull on the line to make it horizontal.
15 poundBowling Ball
PullPull
HORIZONTAL
As the angle gets smaller, must pull muchharder.
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2-Sep-10 Physics 101
Demo: Straighten the Line (II)
10 NewtonWeight
5 N5 N
10 NewtonWeight
15 N15 N
As the angle gets smaller, must pull muchharder.
2-Sep-10 Physics 101
Parallelogram Rule
Weight
pullpullPULL! PULL!
Net Force
Net force is the same in both cases but pulling forces different.
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2-Sep-10 Physics 101
Nellie Newton hangs motionless by one hand from a clothesline as shown—which is on the verge of breaking. Which side of the line is most likely to break?
1. Left side2. Right side3. 50/50 chance
of either side breaking
Check Yourself
Two upward forces must add together to balance Nellie’s weight.
2-Sep-10 Physics 101
Lab: Force Table
Practice addition of forces as vectors in the Physics 102 lab using “force tables.”
Hang weights and adjust angles until forces balance.
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2-Sep-10 Physics 101
Key Points of Lecture 5Key Points of Lecture 5
Before next lecture, read Hewitt through 1st half Chap.6
Homework Assignment #2 (second graded assignment) is due before 11:00 PM on Sunday, Sept. 5.
• Friction• Air drag and terminal speed• Newton’s Third Law of Motion• Combining Force Vectors
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