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5

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This question already has an answer here:

• 
  Why does a ping pong ball change direction when I spin it on a table?
  
  4 answers
  
  

I've always thought it was weird that pencils act like this: if one pulls their finger along the side of a pencil until it touches the surface below, the pencil is launched in the opposite direction of the way that the finger moved. Why is this?

newtonian-mechanics forces rotational-dynamics everyday-life

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edited Apr 27 at 18:14

Rishi

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asked Apr 27 at 14:10

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marked as duplicate by Qmechanic♦ Apr 27 at 19:04

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Possible duplicates: physics.stackexchange.com/q/16271/2451 and links therein.
  $endgroup$
  – Qmechanic♦
  Apr 27 at 19:04
  

  
  
  
  

add a comment | 

5

$begingroup$

This question already has an answer here:

• 
  Why does a ping pong ball change direction when I spin it on a table?
  
  4 answers
  
  

I've always thought it was weird that pencils act like this: if one pulls their finger along the side of a pencil until it touches the surface below, the pencil is launched in the opposite direction of the way that the finger moved. Why is this?

newtonian-mechanics forces rotational-dynamics everyday-life

share|cite|improve this question

edited Apr 27 at 18:14

Rishi

3,74972442

asked Apr 27 at 14:10

StormblessedStormblessed

1294

New contributor

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

$endgroup$

marked as duplicate by Qmechanic♦ Apr 27 at 19:04

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Possible duplicates: physics.stackexchange.com/q/16271/2451 and links therein.
  $endgroup$
  – Qmechanic♦
  Apr 27 at 19:04
  

  
  
  
  

add a comment | 

$begingroup$

This question already has an answer here:

• 
  Why does a ping pong ball change direction when I spin it on a table?
  
  4 answers
  
  

I've always thought it was weird that pencils act like this: if one pulls their finger along the side of a pencil until it touches the surface below, the pencil is launched in the opposite direction of the way that the finger moved. Why is this?

newtonian-mechanics forces rotational-dynamics everyday-life

share|cite|improve this question

edited Apr 27 at 18:14

Rishi

3,74972442

asked Apr 27 at 14:10

StormblessedStormblessed

1294

New contributor

Stormblessed 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 Apr 27 at 18:14

Rishi

3,74972442

asked Apr 27 at 14:10

StormblessedStormblessed

1294

New contributor

Stormblessed is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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share|cite|improve this question

edited Apr 27 at 18:14

Rishi

3,74972442

asked Apr 27 at 14:10

StormblessedStormblessed

1294

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edited Apr 27 at 18:14

Rishi

3,74972442

edited Apr 27 at 18:14

Rishi

3,74972442

asked Apr 27 at 14:10

StormblessedStormblessed

1294

New contributor

Stormblessed is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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asked Apr 27 at 14:10

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2 Answers
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3

$begingroup$

The sequence of events is shown below.

Initially the pencil is propelled forward with speed $v_rm A$ but has backspin $omega_rm A$ (anticlockwise rotation) so there is relative movement between the pencil and the surface as $v_rm A ne romega_rm A$ where $r$ is the radius of the pencil.

A kinetic friction force acts which reduces the rotational speed of the pencil $omega_rm B$ until there is no rotation of the pencil $omega_rm C=0$ but the pencil is still moving forward $v_rm C$.

The frictional force then starts the pencil rotating clockwise with increasing angular speed and eventually the no slip condition, $v_rm D = romega_rm D$, is reached.

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answered Apr 27 at 17:08

FarcherFarcher

52.6k340112

$endgroup$

add a comment | 

0

$begingroup$

As the finger comes down the side of the pencil, two things happen:

A compression that imparts a horizontal force taking the pencil away from the finger

And a rotation that causes the pencil to rotate tending to bring the pencil back to the start point

These combine to define how far the pencil travels before it stops.

The use of spin can be seen on a snooker or billiards table : top, bottom and side...

share|cite|improve this answer

answered Apr 27 at 14:18

Solar MikeSolar Mike

1274

New contributor

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

$endgroup$

add a comment | 

3

$begingroup$

The sequence of events is shown below.

Initially the pencil is propelled forward with speed $v_rm A$ but has backspin $omega_rm A$ (anticlockwise rotation) so there is relative movement between the pencil and the surface as $v_rm A ne romega_rm A$ where $r$ is the radius of the pencil.

A kinetic friction force acts which reduces the rotational speed of the pencil $omega_rm B$ until there is no rotation of the pencil $omega_rm C=0$ but the pencil is still moving forward $v_rm C$.

The frictional force then starts the pencil rotating clockwise with increasing angular speed and eventually the no slip condition, $v_rm D = romega_rm D$, is reached.

share|cite|improve this answer

answered Apr 27 at 17:08

FarcherFarcher

52.6k340112

$endgroup$

add a comment | 

3

$begingroup$

The sequence of events is shown below.

Initially the pencil is propelled forward with speed $v_rm A$ but has backspin $omega_rm A$ (anticlockwise rotation) so there is relative movement between the pencil and the surface as $v_rm A ne romega_rm A$ where $r$ is the radius of the pencil.

A kinetic friction force acts which reduces the rotational speed of the pencil $omega_rm B$ until there is no rotation of the pencil $omega_rm C=0$ but the pencil is still moving forward $v_rm C$.

The frictional force then starts the pencil rotating clockwise with increasing angular speed and eventually the no slip condition, $v_rm D = romega_rm D$, is reached.

share|cite|improve this answer

answered Apr 27 at 17:08

FarcherFarcher

52.6k340112

$endgroup$

add a comment | 

$begingroup$

The sequence of events is shown below.

Initially the pencil is propelled forward with speed $v_rm A$ but has backspin $omega_rm A$ (anticlockwise rotation) so there is relative movement between the pencil and the surface as $v_rm A ne romega_rm A$ where $r$ is the radius of the pencil.

A kinetic friction force acts which reduces the rotational speed of the pencil $omega_rm B$ until there is no rotation of the pencil $omega_rm C=0$ but the pencil is still moving forward $v_rm C$.

The frictional force then starts the pencil rotating clockwise with increasing angular speed and eventually the no slip condition, $v_rm D = romega_rm D$, is reached.

share|cite|improve this answer

answered Apr 27 at 17:08

FarcherFarcher

52.6k340112

$endgroup$

share|cite|improve this answer

answered Apr 27 at 17:08

FarcherFarcher

52.6k340112

answered Apr 27 at 17:08

FarcherFarcher

52.6k340112

answered Apr 27 at 17:08

FarcherFarcher

52.6k340112

0

$begingroup$

As the finger comes down the side of the pencil, two things happen:

A compression that imparts a horizontal force taking the pencil away from the finger

And a rotation that causes the pencil to rotate tending to bring the pencil back to the start point

These combine to define how far the pencil travels before it stops.

The use of spin can be seen on a snooker or billiards table : top, bottom and side...

share|cite|improve this answer

answered Apr 27 at 14:18

Solar MikeSolar Mike

1274

New contributor

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

$endgroup$

add a comment | 

0

$begingroup$

As the finger comes down the side of the pencil, two things happen:

A compression that imparts a horizontal force taking the pencil away from the finger

And a rotation that causes the pencil to rotate tending to bring the pencil back to the start point

These combine to define how far the pencil travels before it stops.

The use of spin can be seen on a snooker or billiards table : top, bottom and side...

share|cite|improve this answer

answered Apr 27 at 14:18

Solar MikeSolar Mike

1274

New contributor

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

$endgroup$

add a comment | 

$begingroup$

As the finger comes down the side of the pencil, two things happen:

A compression that imparts a horizontal force taking the pencil away from the finger

And a rotation that causes the pencil to rotate tending to bring the pencil back to the start point

These combine to define how far the pencil travels before it stops.

The use of spin can be seen on a snooker or billiards table : top, bottom and side...

share|cite|improve this answer

answered Apr 27 at 14:18

Solar MikeSolar Mike

1274

New contributor

Solar Mike 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 answer

answered Apr 27 at 14:18

Solar MikeSolar Mike

1274

New contributor

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

answered Apr 27 at 14:18

Solar MikeSolar Mike

1274

New contributor

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

answered Apr 27 at 14:18

Solar MikeSolar Mike

1274