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Why doesn't Newton's third law mean a person bounces back to where they started when they hit the ground?

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Why doesn't Newton's third law mean a person bounces back to where they started when they hit the ground?

With Newton's third law, why are things capable of moving?Clarification regarding Newton's Third Law of Motion and why movement is possibleAccording to Newton's third law, why don't Action and Reaction make equilibrium?Person Pushing a Block vs. People Pushing off Each Other - Newton's Third LawNewton's third law of motion when moving between two surfacesConfused about Newton's 3rd lawHow did tension developed in a string when two equal and opposite forces are applied on the same body?Why does a Ball bounce back if Forces are Equal and Opposite?Why doesn't an object that collides with one that is at rest just do a 180?Newton's $3^rd$ Law of motionWhy does Newton's Third Law work for fields?

8

$begingroup$

Why doesn't a person bounce back after falling down like a ball? If we push a person and he falls down then why doesn't he come back to its initial position. Although according to Newton's 3rd law of motion: To every action there is always equal but opposite reaction. If we take the example of ball then it comes back with the same force as it falls down.But in the case of a human body, this law is not applicable. Why?

newtonian-mechanics forces conservation-laws free-body-diagram

share|cite|improve this question

edited 3 hours ago

knzhou

46.3k11124223

asked 7 hours ago

nameera jabeennameera jabeen

492

New contributor

nameera jabeen is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Related: physics.stackexchange.com/q/45653/2451 and links therein.
  $endgroup$
  – Qmechanic♦
  7 hours ago
  

  
  
  
  


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  1
  

  
  
  
  
  
  

  
  $begingroup$
  A ball does not come back with exactly the same force: youtube.com/watch?v=xXXF2C-vrQE
  $endgroup$
  – StrongBad
  2 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  The law applies to human bodies. But the law doesn't apply to energy. It applies to force. Force isn't the same thing as energy. It isn't force that's gone missing here. It's energy. Ask where the energy went. We have conservation of energy so it's gotta be here somewhere.
  $endgroup$
  – candied_orange
  2 hours ago
  
  

  
  
  
  

add a comment | 

8

$begingroup$

Why doesn't a person bounce back after falling down like a ball? If we push a person and he falls down then why doesn't he come back to its initial position. Although according to Newton's 3rd law of motion: To every action there is always equal but opposite reaction. If we take the example of ball then it comes back with the same force as it falls down.But in the case of a human body, this law is not applicable. Why?

newtonian-mechanics forces conservation-laws free-body-diagram

share|cite|improve this question

edited 3 hours ago

knzhou

46.3k11124223

asked 7 hours ago

nameera jabeennameera jabeen

492

New contributor

nameera jabeen is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Related: physics.stackexchange.com/q/45653/2451 and links therein.
  $endgroup$
  – Qmechanic♦
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  A ball does not come back with exactly the same force: youtube.com/watch?v=xXXF2C-vrQE
  $endgroup$
  – StrongBad
  2 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  The law applies to human bodies. But the law doesn't apply to energy. It applies to force. Force isn't the same thing as energy. It isn't force that's gone missing here. It's energy. Ask where the energy went. We have conservation of energy so it's gotta be here somewhere.
  $endgroup$
  – candied_orange
  2 hours ago
  
  

  
  
  
  

add a comment | 

$begingroup$

Why doesn't a person bounce back after falling down like a ball? If we push a person and he falls down then why doesn't he come back to its initial position. Although according to Newton's 3rd law of motion: To every action there is always equal but opposite reaction. If we take the example of ball then it comes back with the same force as it falls down.But in the case of a human body, this law is not applicable. Why?

newtonian-mechanics forces conservation-laws free-body-diagram

share|cite|improve this question

edited 3 hours ago

knzhou

46.3k11124223

asked 7 hours ago

nameera jabeennameera jabeen

492

New contributor

nameera jabeen is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

share|cite|improve this question

edited 3 hours ago

knzhou

46.3k11124223

asked 7 hours ago

nameera jabeennameera jabeen

492

New contributor

nameera jabeen is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|cite|improve this question

edited 3 hours ago

knzhou

46.3k11124223

asked 7 hours ago

nameera jabeennameera jabeen

492

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nameera jabeen is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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edited 3 hours ago

knzhou

46.3k11124223

edited 3 hours ago

knzhou

46.3k11124223

asked 7 hours ago

nameera jabeennameera jabeen

492

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asked 7 hours ago

nameera jabeennameera jabeen

492

2 Answers
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35

$begingroup$

Newton's third law just says when the person is hitting the floor the force the person exerts on the ground is equal to the force the ground exerts on the person. i.e. all forces are interactions.

Newton's third law does not say that all collisions are elastic, which is what you are proposing. When someone hits the floor most of the energy is absorbed by the person through deformation (as well as the floor, depending on what type of floor it is), but there is barely any rebound since people tend to not be very elastic. i.e. the deformation does not involve storing the energy to be released back into kinetic energy. Contrast this with a bouncy ball where much of the energy goes into deforming the ball, but since it is very elastic it is able to spring back and put energy back into motion. However, it is unlikely the collision is still perfectly elastic, as you seem to suggest in your question.

Your misunderstanding likely comes from the imprecise usage of the words "action" and "reaction". In this case, these words refer to just forces, not entire processes. You can get some confusing questions if you don't understand this. For example, why is it that when I open my refrigerator that my refrigerator doesn't also open me?

share|cite|improve this answer

edited 1 hour ago

answered 7 hours ago

Aaron StevensAaron Stevens

14.4k42353

$endgroup$

• 
  
  
  

  
  20
  

  
  
  
  
  
  

  
  $begingroup$
  +1 for the last sentence alone!
  $endgroup$
  – knzhou
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  $begingroup$
  Well, I can’t speak for you, but when I open my refrigerator my mouth invariably opens in return—I had always thought it was in anticipation of what is to follow, but perhaps the refrigerator had been returning the gesture after all !
  $endgroup$
  – eggyal
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
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  And, in fact, we cannot make persons more elastic but we can do it with the floor. In general, we call them trampolins. A person falling in a trampoline is very likely to bounce =)
  $endgroup$
  – jean
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @DavidRicherby I suppose it depends on the type of floor as well, but that is a good point. I'll make an edit.
  $endgroup$
  – Aaron Stevens
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  That's deep. I'm going to tweak it and use it to sound wise. "When you take something out of the refrigerator, the refrigerator takes something out of you."
  $endgroup$
  – Lofty Withers
  5 mins ago
  

  
  
  
  

add a comment | 

5

$begingroup$

When you body hits the floor, it does receive an equal and opposite reaction force from the floor. But unlike a ball a body is an complex object. So not all energy is transferred back as kinetic energy. Some energy is used to produce sound, some is used to deform your body... etc. I think you are confusing force with energy. Does every ball bounce back the same amount? Newton's 3rd law talks about force only. More force doesn't always(mostly) equal to more work done.

In your case if all the force was used to change the body's kinetic energy somehow(which is not realistically possible), then it would have bounced back the same amount.

share|cite|improve this answer

answered 7 hours ago

LikhonLikhon

969

$endgroup$

add a comment | 

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35

$begingroup$

Newton's third law just says when the person is hitting the floor the force the person exerts on the ground is equal to the force the ground exerts on the person. i.e. all forces are interactions.

Newton's third law does not say that all collisions are elastic, which is what you are proposing. When someone hits the floor most of the energy is absorbed by the person through deformation (as well as the floor, depending on what type of floor it is), but there is barely any rebound since people tend to not be very elastic. i.e. the deformation does not involve storing the energy to be released back into kinetic energy. Contrast this with a bouncy ball where much of the energy goes into deforming the ball, but since it is very elastic it is able to spring back and put energy back into motion. However, it is unlikely the collision is still perfectly elastic, as you seem to suggest in your question.

Your misunderstanding likely comes from the imprecise usage of the words "action" and "reaction". In this case, these words refer to just forces, not entire processes. You can get some confusing questions if you don't understand this. For example, why is it that when I open my refrigerator that my refrigerator doesn't also open me?

share|cite|improve this answer

edited 1 hour ago

answered 7 hours ago

Aaron StevensAaron Stevens

14.4k42353

$endgroup$

• 
  
  
  

  
  20
  

  
  
  
  
  
  

  
  $begingroup$
  +1 for the last sentence alone!
  $endgroup$
  – knzhou
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  $begingroup$
  Well, I can’t speak for you, but when I open my refrigerator my mouth invariably opens in return—I had always thought it was in anticipation of what is to follow, but perhaps the refrigerator had been returning the gesture after all !
  $endgroup$
  – eggyal
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
  $begingroup$
  And, in fact, we cannot make persons more elastic but we can do it with the floor. In general, we call them trampolins. A person falling in a trampoline is very likely to bounce =)
  $endgroup$
  – jean
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @DavidRicherby I suppose it depends on the type of floor as well, but that is a good point. I'll make an edit.
  $endgroup$
  – Aaron Stevens
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  That's deep. I'm going to tweak it and use it to sound wise. "When you take something out of the refrigerator, the refrigerator takes something out of you."
  $endgroup$
  – Lofty Withers
  5 mins ago
  

  
  
  
  

add a comment | 

35

$begingroup$

Newton's third law just says when the person is hitting the floor the force the person exerts on the ground is equal to the force the ground exerts on the person. i.e. all forces are interactions.

Newton's third law does not say that all collisions are elastic, which is what you are proposing. When someone hits the floor most of the energy is absorbed by the person through deformation (as well as the floor, depending on what type of floor it is), but there is barely any rebound since people tend to not be very elastic. i.e. the deformation does not involve storing the energy to be released back into kinetic energy. Contrast this with a bouncy ball where much of the energy goes into deforming the ball, but since it is very elastic it is able to spring back and put energy back into motion. However, it is unlikely the collision is still perfectly elastic, as you seem to suggest in your question.

Your misunderstanding likely comes from the imprecise usage of the words "action" and "reaction". In this case, these words refer to just forces, not entire processes. You can get some confusing questions if you don't understand this. For example, why is it that when I open my refrigerator that my refrigerator doesn't also open me?

share|cite|improve this answer

edited 1 hour ago

answered 7 hours ago

Aaron StevensAaron Stevens

14.4k42353

$endgroup$

• 
  
  
  

  
  20
  

  
  
  
  
  
  

  
  $begingroup$
  +1 for the last sentence alone!
  $endgroup$
  – knzhou
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  $begingroup$
  Well, I can’t speak for you, but when I open my refrigerator my mouth invariably opens in return—I had always thought it was in anticipation of what is to follow, but perhaps the refrigerator had been returning the gesture after all !
  $endgroup$
  – eggyal
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
  $begingroup$
  And, in fact, we cannot make persons more elastic but we can do it with the floor. In general, we call them trampolins. A person falling in a trampoline is very likely to bounce =)
  $endgroup$
  – jean
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @DavidRicherby I suppose it depends on the type of floor as well, but that is a good point. I'll make an edit.
  $endgroup$
  – Aaron Stevens
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  That's deep. I'm going to tweak it and use it to sound wise. "When you take something out of the refrigerator, the refrigerator takes something out of you."
  $endgroup$
  – Lofty Withers
  5 mins ago
  

  
  
  
  

add a comment | 

$begingroup$

Newton's third law just says when the person is hitting the floor the force the person exerts on the ground is equal to the force the ground exerts on the person. i.e. all forces are interactions.

Newton's third law does not say that all collisions are elastic, which is what you are proposing. When someone hits the floor most of the energy is absorbed by the person through deformation (as well as the floor, depending on what type of floor it is), but there is barely any rebound since people tend to not be very elastic. i.e. the deformation does not involve storing the energy to be released back into kinetic energy. Contrast this with a bouncy ball where much of the energy goes into deforming the ball, but since it is very elastic it is able to spring back and put energy back into motion. However, it is unlikely the collision is still perfectly elastic, as you seem to suggest in your question.

Your misunderstanding likely comes from the imprecise usage of the words "action" and "reaction". In this case, these words refer to just forces, not entire processes. You can get some confusing questions if you don't understand this. For example, why is it that when I open my refrigerator that my refrigerator doesn't also open me?

share|cite|improve this answer

edited 1 hour ago

answered 7 hours ago

Aaron StevensAaron Stevens

14.4k42353

$endgroup$

share|cite|improve this answer

edited 1 hour ago

answered 7 hours ago

Aaron StevensAaron Stevens

14.4k42353

answered 7 hours ago

Aaron StevensAaron Stevens

14.4k42353

answered 7 hours ago

Aaron StevensAaron Stevens

14.4k42353

5

$begingroup$

When you body hits the floor, it does receive an equal and opposite reaction force from the floor. But unlike a ball a body is an complex object. So not all energy is transferred back as kinetic energy. Some energy is used to produce sound, some is used to deform your body... etc. I think you are confusing force with energy. Does every ball bounce back the same amount? Newton's 3rd law talks about force only. More force doesn't always(mostly) equal to more work done.

In your case if all the force was used to change the body's kinetic energy somehow(which is not realistically possible), then it would have bounced back the same amount.

share|cite|improve this answer

answered 7 hours ago

LikhonLikhon

969

$endgroup$

add a comment | 

5

$begingroup$

When you body hits the floor, it does receive an equal and opposite reaction force from the floor. But unlike a ball a body is an complex object. So not all energy is transferred back as kinetic energy. Some energy is used to produce sound, some is used to deform your body... etc. I think you are confusing force with energy. Does every ball bounce back the same amount? Newton's 3rd law talks about force only. More force doesn't always(mostly) equal to more work done.

In your case if all the force was used to change the body's kinetic energy somehow(which is not realistically possible), then it would have bounced back the same amount.

share|cite|improve this answer

answered 7 hours ago

LikhonLikhon

969

$endgroup$

add a comment | 

$begingroup$

When you body hits the floor, it does receive an equal and opposite reaction force from the floor. But unlike a ball a body is an complex object. So not all energy is transferred back as kinetic energy. Some energy is used to produce sound, some is used to deform your body... etc. I think you are confusing force with energy. Does every ball bounce back the same amount? Newton's 3rd law talks about force only. More force doesn't always(mostly) equal to more work done.

In your case if all the force was used to change the body's kinetic energy somehow(which is not realistically possible), then it would have bounced back the same amount.

share|cite|improve this answer

answered 7 hours ago

LikhonLikhon

969

$endgroup$

share|cite|improve this answer

answered 7 hours ago

LikhonLikhon

969

answered 7 hours ago

LikhonLikhon

969

answered 7 hours ago

LikhonLikhon

969