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  • Understand Newton's third law of motion.
  • Apply Newton's third law to define systems and solve problems of motion.

There is a passage in the musical Man of la Mancha that relates to Newton’s third law of motion. Sancho, in describing a fight with his wife to Don Quixote, says, “Of course I hit her back, Your Grace, but she’s a lot harder than me and you know what they say, ‘Whether the stone hits the pitcher or the pitcher hits the stone, it’s going to be bad for the pitcher.’” This is exactly what happens whenever one body exerts a force on another—the first also experiences a force (equal in magnitude and opposite in direction). Numerous common experiences, such as stubbing a toe or throwing a ball, confirm this. It is precisely stated in Newton’s third law of motion    .

Newton’s third law of motion

Whenever one body exerts a force on a second body, the first body experiences a force that is equal in magnitude and opposite in direction to the force that it exerts.

This law represents a certain symmetry in nature : Forces always occur in pairs, and one body cannot exert a force on another without experiencing a force itself. We sometimes refer to this law loosely as “action-reaction,” where the force exerted is the action and the force experienced as a consequence is the reaction. Newton’s third law has practical uses in analyzing the origin of forces and understanding which forces are external to a system.

We can readily see Newton’s third law at work by taking a look at how people move about. Consider a swimmer pushing off from the side of a pool, as illustrated in [link] . She pushes against the pool wall with her feet and accelerates in the direction opposite to that of her push. The wall has exerted an equal and opposite force back on the swimmer. You might think that two equal and opposite forces would cancel, but they do not because they act on different systems . In this case, there are two systems that we could investigate: the swimmer or the wall. If we select the swimmer to be the system of interest, as in the figure, then F wall on feet size 12{F rSub { size 8{"wall on feet"} } } {} is an external force on this system and affects its motion. The swimmer moves in the direction of F wall on feet size 12{F rSub { size 8{"wall on feet"} } } {} . In contrast, the force F feet on wall size 12{F rSub { size 8{"feet on wall"} } } {} acts on the wall and not on our system of interest. Thus F feet on wall size 12{F rSub { size 8{"feet on wall"} } } {} does not directly affect the motion of the system and does not cancel F wall on feet size 12{F rSub { size 8{"wall on feet"} } } {} . Note that the swimmer pushes in the direction opposite to that in which she wishes to move. The reaction to her push is thus in the desired direction.

A swimmer is exerting a force with her feet on a wall inside a swimming pool represented by an arrow labeled as vector F sub Feet on wall, pointing toward the right, and the wall is also exerting an equal force on her feet, represented by an arrow labeled as vector F sub Wall on feet, having the same length but pointing toward the left. The direction of acceleration of the swimmer is toward the left, shown by an arrow toward the left.
When the swimmer exerts a force F feet on wall size 12{F rSub { size 8{"feet on wall"} } } {} on the wall, she accelerates in the direction opposite to that of her push. This means the net external force on her is in the direction opposite to F feet on wall size 12{F rSub { size 8{"feet on wall"} } } {} . This opposition occurs because, in accordance with Newton’s third law of motion, the wall exerts a force F wall on feet size 12{F rSub { size 8{"wall on feet"} } } {} on her, equal in magnitude but in the direction opposite to the one she exerts on it. The line around the swimmer indicates the system of interest. Note that F feet on wall size 12{F rSub { size 8{"feet on wall"} } } {} does not act on this system (the swimmer) and, thus, does not cancel F wall on feet size 12{F rSub { size 8{"wall on feet"} } } {} . Thus the free-body diagram shows only F wall on feet size 12{F rSub { size 8{"wall on feet"} } } {} , w size 12{w} {} , the gravitational force, and BF size 12{ ital "BF"} {} , the buoyant force of the water supporting the swimmer’s weight. The vertical forces w size 12{w} {} and BF size 12{ ital "BF"} {} cancel since there is no vertical motion.

Questions & Answers

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Muhammad
is the branch of biology that deals with the study of microorganisms.
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What is microbiology
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studies of microbes
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How bacteria create energy to survive?
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Bacteria doesn't produce energy they are dependent upon their substrate in case of lack of nutrients they are able to make spores which helps them to sustain in harsh environments
_Adnan
But not all bacteria make spores, l mean Eukaryotic cells have Mitochondria which acts as powerhouse for them, since bacteria don't have it, what is the substitution for it?
Muhamad
they make spores
Louisiaste
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the significance of food webs for disease transmission
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food webs brings about an infection as an individual depends on number of diseased foods or carriers dully.
Mark
explain assimilatory nitrate reduction
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Assimilatory nitrate reduction is a process that occurs in some microorganisms, such as bacteria and archaea, in which nitrate (NO3-) is reduced to nitrite (NO2-), and then further reduced to ammonia (NH3).
Elkana
This process is called assimilatory nitrate reduction because the nitrogen that is produced is incorporated in the cells of microorganisms where it can be used in the synthesis of amino acids and other nitrogen products
Elkana
Examples of thermophilic organisms
Shu Reply
Give Examples of thermophilic organisms
Shu
advantages of normal Flora to the host
Micheal Reply
Prevent foreign microbes to the host
Abubakar
they provide healthier benefits to their hosts
ayesha
They are friends to host only when Host immune system is strong and become enemies when the host immune system is weakened . very bad relationship!
Mark
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faisal Reply
cell is the smallest unit of life
Fauziya
cell is the smallest unit of life
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ok
Innocent
cell is the structural and functional unit of life
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is the fundamental units of Life
Musa
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Micheal Reply
There are nothing like emergency disease but there are some common medical emergency which can occur simultaneously like Bleeding,heart attack,Breathing difficulties,severe pain heart stock.Hope you will get my point .Have a nice day ❣️
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I think infection prevention and control is the avoidance of all things we do that gives out break of infections and promotion of health practices that promote life
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ESTHER Reply
Many sites of the body have it Skin Nasal cavity Oral cavity Gastro intestinal tract
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skin
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skin,Oral,Nasal,GIt
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Shelly Reply
part of a tissue or an organ being wounded or bruised.
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Binomial nomenclature
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Source:  OpenStax, College physics arranged for cpslo phys141. OpenStax CNX. Dec 23, 2014 Download for free at http://legacy.cnx.org/content/col11718/1.4
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