Lecture 5 Chapter 5 Friction & Drag Newton’s Third Law of Motion For every “action” there is a “reaction” Can’t touch without being touched

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Physics 101

Friction and Air Resistance Friction and air resistance are forces opposing motion.

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Friction Origin 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. Physics 101

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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. 2-Sep-10

Physics 101

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Air Resistance on a Falling Object Gravity 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 Yourself Golf 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?

Ping pong Ball

Golf Ball

Drag

Weight

Which ball falls faster (which has greater acceleration)? 2-Sep-10

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Terminal Speed Speed 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.

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First Object (Hammer)

Second Object (Nail)

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Check Yourself A 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?

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Physics 101

Check Yourself 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.

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Physics 101

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Demo: Pulled in Both Directions

10

10

1. 10 N, 2. 20 N, or 3. Zero?

Does the scale read:

Physics 101

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Demo: Equal Magnitudes With spring scales, we verify that action and reaction forces have equal magnitudes. Reaction

Action 5

15

Hold

Pull 10

5

15

0

0

10

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Action and Reaction Common 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”?

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Physics 101

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????

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ActionReaction Pairs 2-Sep-10

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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. B

Mr. A

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Mr. B has more mass than Mr. A

Physics 101

Demo: Mutual Repulsion Similar 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.

Mr. B Mr. A

Reaction

Action

Standing on skateboards

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Physics 101

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Walking, Running & Jumping What forces accelerate us into motion when we walk, run, or jump?

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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. 2-Sep-10

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Walking Forward Back foot acts by pushing back on the floor. Reaction is the friction of the floor, which pushes your body forward

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Action Physics 101

If there were no friction then dancer would fall Reactionstraight down and could not walk forward

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 Can only push while in apply that force. 2-Sep-10

Physics 101

contact with the ground so squatting helps by increasing distance.

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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 Yourself Miss A pushes the car (action); car pushes back on her (reaction). Do these forces cancel?

Miss A Action Reaction

Force on Miss A is to the left; how can she move forward (to the right)? What if floor had zero friction? 2-Sep-10

ActionReaction Pairs

Reaction Action

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Check Yourself Miss B also pushes the car; can she move the car by herself?

ActionReaction Pairs

In terms of Newton’s laws, why is this not possible?

Miss B Action

What other force does Miss B exert on the car besides her hands?

Reaction Reaction Action

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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. 2-Sep-10

Physics 101

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Vector Addition Forces are vectors, with magnitude & direction.

Net Force A+B (25 Newtons)

Force B (10 N) Object

Force A (20 Newtons)

Parallelogram rule

Physics 101

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Demo: Straighten the Line Pull on the line to make it horizontal. HORIZONTAL Pull

Pull 15 pound Bowling Ball

As the angle gets smaller, must pull much harder. 2-Sep-10

Physics 101

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Demo: Straighten the Line (II) 5N

As the angle gets smaller, must pull much harder.

5N

15 N

15 N

10 Newton Weight

10 Newton Weight

Physics 101

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Parallelogram Rule Net force is the same in both cases but pulling forces different. Net Force

PULL!

pull

pull

PULL!

Weight

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Physics 101

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Check Yourself 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 side 2. Right side 3. 50/50 chance of either side breaking

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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Key Points of Lecture 5 • Friction

• Air drag and terminal speed • Newton’s Third Law of Motion • Combining Force Vectors z Before next lecture, read Hewitt through 1st half Chap.6 z Homework Assignment #2 (second graded assignment) is due before 11:00 PM on Sunday, Sept. 5.

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Physics 101

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