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Chapter 5 – Newton’s Third Law
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Forces and Interactions
Newton’s Third Law
Vectors
Physics 1100 – Spring 2012
Newton’s 3rd Law
For every action (force), there is an equal and opposite reaction
(opposing force).
Force on Newton by Einstein = Force on Einstein by Newton
(But their accelerations need not be the same: Newton tries to
outsmart Einstein by loading his own cart with lead bricks)
Physics 1100 – Spring 2012
Action - Reaction
Physics 1100 – Spring 2012
Action - Reaction
Physics 1100 – Spring 2012
Action - Reaction
Physics 1100 – Spring 2012
Action - Reaction
Physics 1100 – Spring 2012
Why Do We Move?
Physics 1100 – Spring 2012
Why Do We Move?
Physics 1100 – Spring 2012
Action – Reaction and Rockets
Physics 1100 – Spring 2012
Action - Reaction
Physics 1100 – Spring 2012
Rockets
NYT, January 13, 1920
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for after the rocket quits our air and really starts on its longer journey,
its flight would be neither accelerated nor maintained by the
explosion of the charges it then might have left. To claim that it would
be is to deny a fundamental law of dynamics, and only Dr. Einstein
and his chosen dozen, so few and fit, are licensed to do that.
…..That Professor Goddard, with his "chair" in Clark College and the
countenancing of the Smithsonian Institution, does not know the
relation of action to reaction, and of the need to have something
better than a vacuum against which to react--to say that would be
absurd. Of course he only seems to lack the knowledge ladled out
daily in high schools.
NYT, July, 1969
Further investigation and experimentation have confirmed the findings of
Isaac Newton in the 17th century and it is now definitely established that a
rocket can function in a vacuum as well as in an atmosphere. The Times
regrets the error
Physics 1100 – Spring 2012
Newton’s Third Law
Physics 1100 – Spring 2012
Forces are Vectors so Directions are Important
Total Force
Force #1
Force #2
Force #1
Total Force = 0
Force #2
Physics 1100 – Spring 2012
Vectors
Adding Vectors
Physics 1100 – Spring 2012
Graphically Adding Vectors
90
18
0
b
a
0
c
27
0
c
b
a
a
Physics 1100 – Spring 2012
a
b
Vector Addition
Physics 1100 – Spring 2012
Vector Addition of Forces
Physics 1100 – Spring 2012
Vector Addition of Forces
Physics 1100 – Spring 2012
Vectors
Physics 1100 – Spring 2012
Components
Physics 1100 – Spring 2012
Clicker Question
An archer shoots an arrow. Consider the action force to be exerted by the bowstring
against the arrow. The reaction to this force is the
A) friction of the ground against the archer's feet.
B) air resistance against the bow.
C) grip of the archer's hand on the bow.
D) combined weight of the arrow and bowstring.
E) arrow's push against the bowstring.
Arnold Strongman and Suzie Small each pull very hard on opposite ends of a massless
rope in a tug-of-war. The greater force on the rope is exerted by
A) Suzie, surprisingly.
B) Arnold, of course.
C) both the same, interestingly enough.
Physics 1100 – Spring 2012
Clicker Question
The attraction of a person's body toward the Earth is called weight. The reaction to
this force is
A) the person's body pulling on the Earth.
B) the person's body pushing against the Earth's surface.
C) the Earth's surface pushing against the person's body.
D) none of these.
A Mack truck and a Volkswagen traveling at the same speed have a head-on
collision. The vehicle that undergoes the greatest change in velocity will be the
A) Volkswagen. B) Mack truck. C) same for both.
Physics 1100 – Spring 2012
Clicker Question
A car traveling at 100 km/hr strikes an unfortunate bug and splatters it. The
force of impact is
A) greater on the bug. B) greater on the car. C) the same for
both.
A horse exerts 500 N of force on a heavy wagon. The wagon pulls back on the
horse with an equal force. The wagon still accelerates because
A) the horse pulls on the wagon a brief time before the wagon reacts.
B) nevertheless there is still an unbalanced force on the wagon.
C) the wagon does not accelerate because these forces are equal and
opposite.
D) these forces are not an action-reaction pair.
E) the wagon is not alive.
Physics 1100 – Spring 2012