Why does flipping a pencil across a table make it move like this?
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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?
kinematics
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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?
kinematics
New contributor
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may be -> linear momentum dies, still rotational momentum is there and hence 'launched in opposite direction' :)
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– aranyak
3 hours ago
add a comment |
$begingroup$
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?
kinematics
New contributor
$endgroup$
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?
kinematics
kinematics
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New contributor
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asked 3 hours ago
StormblessedStormblessed
1214
1214
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may be -> linear momentum dies, still rotational momentum is there and hence 'launched in opposite direction' :)
$endgroup$
– aranyak
3 hours ago
add a comment |
$begingroup$
may be -> linear momentum dies, still rotational momentum is there and hence 'launched in opposite direction' :)
$endgroup$
– aranyak
3 hours ago
$begingroup$
may be -> linear momentum dies, still rotational momentum is there and hence 'launched in opposite direction' :)
$endgroup$
– aranyak
3 hours ago
$begingroup$
may be -> linear momentum dies, still rotational momentum is there and hence 'launched in opposite direction' :)
$endgroup$
– aranyak
3 hours ago
add a comment |
2 Answers
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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...
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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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2 Answers
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2 Answers
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$begingroup$
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...
New contributor
$endgroup$
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...
New contributor
$endgroup$
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...
New contributor
$endgroup$
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...
New contributor
New contributor
answered 3 hours ago
Solar MikeSolar Mike
1313
1313
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New contributor
add a comment |
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.
$endgroup$
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.
$endgroup$
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.
$endgroup$
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.
answered 43 mins ago
FarcherFarcher
52.5k340112
52.5k340112
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$begingroup$
may be -> linear momentum dies, still rotational momentum is there and hence 'launched in opposite direction' :)
$endgroup$
– aranyak
3 hours ago