Optimizing Forces in Towing: Minimizing FB for a Given Resultant Force

[haruspex]

For FB perpendicular FA,is there an explanation for this ? why it works ?

It's easy to see geometrically.
Draw the 950N as a line OP with a length representing its magnitude, and a line for the direction of F_A from the same origin.
The magnitude of F_A will be indicated by a point on its line, Q say.
F_B will be represented by a vector from Q to P. What position of Q minimises the length PQ?

 

[ehild]

For F_B perpendicular F_A,is there an explanation for this ? why it works ? and why not perpendicular to F_R ?

Sorry if i ask a lot of questions :)

It IS the explanation. $\vec F_R$ is the sum of the vectors $\vec F_A$ and $\vec F_B$. We expressed their components with the magnitudes F_A, F_B and angles, a, q. Solved the system of equations for F_B. It must be minimum, so its derivative with respect to q has to be zero. From that, we got the equation $\frac{1}{\tan(a)}-\tan(q)=0$, that is, the tangent of the angle q is equal the reciprocal of the tangent of the other angle. Now, remember how the tangent was defined in a right triangle,

$tan(\alpha)=a/b$ and $tan(\beta)=b/a$, the tangents of the two angles are reciprocal to each others in a right triangle. But the sum of the two angles is 90°as the sum of all angles in a triangle makes 180°, one angle is 90° , so $\alpha + \beta = 90°$. Any time when the tangents of two acute angles are reciprocal, their sum is 90°.
Here, the two forces $\vec F_A$ and $\vec F_A$ enclose the angle a+q, and the tangents of the angles are reciprocal to each other, so a+q=90°, the forces are perpendicular to each other.

 

[newbphysic]

It's easy to see geometrically.
Draw the 950N as a line OP with a length representing its magnitude, and a line for the direction of F_A from the same origin.
The magnitude of F_A will be indicated by a point on its line, Q say.
F_B will be represented by a vector from Q to P. What position of Q minimises the length PQ?

when it's perpendicular to F_R ? the red line is the right answer according to calculation

$tan(\alpha)=a/b$ and $tan(\beta)=b/a$, the tangents of the two angles are reciprocal to each others in a right triangle. But the sum of the two angles is 90°as the sum of all angles in a triangle makes 180°, one angle is 90° , so $\alpha + \beta = 90°$. Any time when the tangents of two acute angles are reciprocal, their sum is 90°.
Here, the two forces $\vec F_A$ and $\vec F_A$ enclose the angle a+q, and the tangents of the angles are reciprocal to each other, so a+q=90°, the forces are perpendicular to each other.

OK but what i still don't understand is this, how can you know it must be perpendicular to F_A not to F_R. Here is another example that looks exactly like before but in this example Fb perpendicular Fr. Why it's different ?

I tried to use the your method and i find theta = 60 degrees

 

[ehild]

OK but what i still don't understand is this, how can you know it must be perpendicular to F_A not to F_R. Here is another example that looks exactly like before but in this example Fb perpendicular Fr. Why it's different ?

I tried to use the your method and i find theta = 60 degrees

This is an other problem. Here Fa is given, FR has to be determined, so as its direction is the positive x axis. In the previous problem, FR was given and Fa was unknown.
From the equation Fa sin(30)=Fbsin(θ) ---> 2*1/2 = Fbsin(θ), you get that Fb=1/sin(θ), which is minimum if sin(θ)=1, that is, θ=90°.

 

[newbphysic]

This is an other problem. Here Fa is given, FR has to be determined, so as its direction is the positive x axis. In the previous problem, FR was given and Fa was unknown.
From the equation Fa sin(30)=Fbsin(θ) ---> 2*1/2 = Fbsin(θ), you get that Fb=1/sin(θ), which is minimum if sin(θ)=1, that is, θ=90°.

I'm really confused

Why for the first problem i can't solve it like this one (without substituting F_A)
F_A sin(20) -F_B sin(q) = 0
F_B =F_Asin(20)/sin(q)

I know that we don't know F_A but we know in order for F_B to be minimum sin(q) must equal 1 (the biggest value) but the result will not be 70

 

[haruspex]

when it's perpendicular to F_R ? the red line is the right answer according to calculation

Neither line minimises the distance PQ, keeping P fixed and varying Q. (Or is P supposed to be where the red line meets the baseline?)

 

[newbphysic]

Neither line minimises the distance PQ, keeping P fixed and varying Q. (Or is P supposed to be where the red line meets the baseline?)

P is fixed but Q vary.

Ooh i see so is this mean in order to find minimum value the unknown must perpendicular to each other ?

 

[haruspex]

P is fixed but Q vary.

Ooh i see so is this mean in order to find minimum value the unknown must perpendicular to each other ?

If one has a fixed direction and you are trying to minimise the other, yes.
(I assume Q in the diagram is where the blue line meets the sloping black line.)

 

[ehild]

I'm really confused

Why for the first problem i can't solve it like this one (without substituting F_A)
F_A sin(20) -F_B sin(q) = 0
F_B =F_Asin(20)/sin(q)

I know that we don't know F_A but we know in order for F_B to be minimum sin(q) must equal 1 (the biggest value) but the result will not be 70

In the first problem, FR was given, you knew the point P and you had to draw a line to the line of FA, which length was the shortest. It is the normal from P to the line FA. That line segment corresponded to FB, and the red line segment corresponded to FA.
In the second problem, FA was given, you knew the point Q. You have to draw the shortest line from it to the line of FR. It is perpendicular to FR.
You know, that the shortest distance from a point to a straight line is the perpendicular to the given line.

If you drew a line perpendicular from P, its length would be longer than the real FB.

 

[newbphysic]

If one has a fixed direction and you are trying to minimise the other, yes.
(I assume Q in the diagram is where the blue line meets the sloping black line.)

In the first problem, FR was given, you knew the point P and you had to draw a line to the line of FA, which length was the shortest. It is the normal from P to the line FA. That line segment corresponded to FB, and the red line segment corresponded to FA.
In the second problem, FA was given, you knew the point Q. You have to draw the shortest line from it to the line of FR. It is perpendicular to FR.
You know, that the shortest distance from a point to a straight line is the perpendicular to the given line.
View attachment 97245

If you drew a line perpendicular from P, its length would be longer than the real FB.
View attachment 97246

Thank you for your help