If linear forces can be transferred to rotational forces then back
again, does that mean we can create a result where two forces are not
in equal and opposite directions?

Eg. If we knock the side of a tea cup, it twists as well as moves;
surely this means that the linear force generated some rotational
force.

Seb

Seb wrote:

If linear forces can be transferred to rotational forces then back
again, does that mean we can create a result where two forces are not
in equal and opposite directions?

Eg. If we knock the side of a tea cup, it twists as well as moves;
surely this means that the linear force generated some rotational
force.


Okay, but so what? What's your point?

I hope that you're not ignoring the force on the cup due to the
contact of the bottom of it with something. If you hit a cup
in free space, it won't twist.

John Anderson

Seb wrote:
If linear forces can be transferred to rotational forces then back
again, does that mean we can create a result where two forces are not
in equal and opposite directions?

No.

[I interpret your context as Newtonian mechanics.]


Eg. If we knock the side of a tea cup, it twists as well as moves;
surely this means that the linear force generated some rotational
force.

There is no such thing as "rotational force" -- forces are vectors.

[Note that "torque" is not "rotational force", either -- it
is a combination of forces (for a wheel, a force from the
rim and another from the axle) on a rigid object.]

Let me consider a teacup floating in space (e.g. in an orbiting space
shuttle).

If you hit the side of a teacup tangentially, the teacup will rotate,
but the force you exert on the side of the teacup will be equal and
opposite to the force it exerts on your hand. The teacup will then move
in the direction you hit it, while rotating.


Tom Roberts

John Anderson wrote:

I hope that you're not ignoring the force on the cup due to the
contact of the bottom of it with something. If you hit a cup
in free space, it won't twist.

Though gravity/friction (etc) on a table is a factor in the example, I
would have still expected it to twist in space. Would not the
acceleration differences between impact side and the opposite side
cause it to twist?

Do you know of any references on this? as this would invalidate my
idea.


Tom Roberts wrote:

If you hit the side of a teacup tangentially, the teacup will
but rotate, the force you exert on the side of the teacup will
be equal and opposite to the force it exerts on your hand. The
teacup will then move in the direction you hit it, while rotating.

Yes, the initial forces are opposite forces, but since the cup is also
left rotating (I'm assuming it would in free space), could we not use
this rotation to create another linear force in a different direction.

(John) the point being if we could create a force with an opposite
force not equal to it, you could potentially accelerate vehicles
through space with no external forces.

Seb

(Seb) wrote in message . com...
If linear forces can be transferred to rotational forces

Such as a piston driving a crankshaft?

then back again,

Such as a wheel driving a car?

does that mean we can create a result where two forces are not
in equal and opposite directions?


It's much easier than that to create two forces that are not equal and
in opposite directions: The force of my butt on my chair is not equal
and opposite the force of ratbert's butt on my monitor.
Note however that in no event will changing directions of forces or
motions allow you to violate Newton's Third Law.

Eg. If we knock the side of a tea cup, it twists as well as moves;
surely this means that the linear force generated some rotational
force.


Not at all. Study torque.

Paul Cardinale

John Anderson wrote:

Seb wrote:


If linear forces can be transferred to rotational forces then back
again, does that mean we can create a result where two forces are not
in equal and opposite directions?

Eg. If we knock the side of a tea cup, it twists as well as moves;
surely this means that the linear force generated some rotational
force.



Okay, but so what? What's your point?

I hope that you're not ignoring the force on the cup due to the
contact of the bottom of it with something. If you hit a cup
in free space, it won't twist.


Not true. If you hit the cup off center (ie. the force vector
does not point through the center of mass), the cup will twist.

-E

John Anderson

Seb wrote:

John Anderson wrote:

I hope that you're not ignoring the force on the cup due to the
contact of the bottom of it with something. If you hit a cup
in free space, it won't twist.


Though gravity/friction (etc) on a table is a factor in the example, I
would have still expected it to twist in space. Would not the
acceleration differences between impact side and the opposite side
cause it to twist?


John Anderson was incorrect, as I'm sure he'll shortly realize.

Do you know of any references on this? as this would invalidate my
idea.


Any introductory physics textbook will cover this topic quite
thoroughly. Just looking at the shelf in front of me:
Tipler
Halliday, Resnick and Walker
Sternheim and Kane
Giancoli
Cutnell and Johnson
and a bunch of others.

Of course, those are college texts. Any high school textbook
would "invalidate your idea".



Tom Roberts wrote:

If you hit the side of a teacup tangentially, the teacup will
but rotate, the force you exert on the side of the teacup will
be equal and opposite to the force it exerts on your hand. The
teacup will then move in the direction you hit it, while rotating.


Yes, the initial forces are opposite forces, but since the cup is also
left rotating (I'm assuming it would in free space), could we not use
this rotation to create another linear force in a different direction.


If you apply a force to a body in free space, in general you will
induce both translational and rotational motion (unless of course
the force vector points right through the center of mass).

(John) the point being if we could create a force with an opposite
force not equal to it, you could potentially accelerate vehicles
through space with no external forces.


In a word, no. In the future, before setting out to revolutionize
physics, you might want to at least open a physics book.

-E



Seb

Seb wrote:

John Anderson wrote:

I hope that you're not ignoring the force on the cup due to the
contact of the bottom of it with something. If you hit a cup
in free space, it won't twist.

Though gravity/friction (etc) on a table is a factor in the example, I
would have still expected it to twist in space. Would not the
acceleration differences between impact side and the opposite side
cause it to twist?

Do you know of any references on this? as this would invalidate my
idea.


Try any reference on Newtonian Mechanics. The only way to exert a
torqueon something in free space is to apply a force that's different at
different
parts of the object.

If you just apply a force at one point, it will accelerate but not start
rotating.

Tom Roberts wrote:

If you hit the side of a teacup tangentially, the teacup will
but rotate, the force you exert on the side of the teacup will
be equal and opposite to the force it exerts on your hand. The
teacup will then move in the direction you hit it, while rotating.


Tom Roberts assumed another force, the one from the hand.

That isn't there in free space.

Yes, the initial forces are opposite forces, but since the cup is also
left rotating (I'm assuming it would in free space), could we not use
this rotation to create another linear force in a different direction.

Your assumption is wrong.

John Anderson

EjP wrote:

John Anderson wrote:

Seb wrote:


If linear forces can be transferred to rotational forces then back
again, does that mean we can create a result where two forces are not
in equal and opposite directions?

Eg. If we knock the side of a tea cup, it twists as well as moves;
surely this means that the linear force generated some rotational
force.



Okay, but so what? What's your point?

I hope that you're not ignoring the force on the cup due to the
contact of the bottom of it with something. If you hit a cup
in free space, it won't twist.


Not true. If you hit the cup off center (ie. the force vector
does not point through the center of mass), the cup will twist.


That's not correct. Hitting it off center will not cause it to
twist unless there is another force to produce a torque.

If you hit something off center in free space, it will move off
at an angle to the collision direction without spinning.

You're thinking about things like billiard balls which can
be made to spin when hit off center because of the friction
force of the billiard table. That ain't free space.

John Anderson

John Anderson wrote in message ...
EjP wrote:

John Anderson wrote:

Seb wrote:


If linear forces can be transferred to rotational forces then back
again, does that mean we can create a result where two forces are not
in equal and opposite directions?

Eg. If we knock the side of a tea cup, it twists as well as moves;
surely this means that the linear force generated some rotational
force.



Okay, but so what? What's your point?

I hope that you're not ignoring the force on the cup due to the
contact of the bottom of it with something. If you hit a cup
in free space, it won't twist.


Not true. If you hit the cup off center (ie. the force vector
does not point through the center of mass), the cup will twist.


That's not correct. Hitting it off center will not cause it to
twist unless there is another force to produce a torque.
If you hit something off center in free space, it will move off
at an angle to the collision direction without spinning.

Can't the Space Shuttle pitch it's nose up and down
using only nose thrusters?

You're thinking about things like billiard balls which can
be made to spin when hit off center because of the friction
force of the billiard table. That ain't free space.
John Anderson

I did a GR experiment to test this.

Drop a horizontally pointed pencil, it's in free fall.
Have one end strike your finger, that's the off center
applied force to the pencil in "free space". Using
Equivalence this is the same as being weightless.
Of course the pencil rotates from horizontal,
would that rotation qualify as torque?
Regards Ken S. Tucker