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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