Moment of a force about point O

In summary: The magnitudes of those distances (which are just the length of the legs of the right triangle that you can visually see) are multiplied by the magnitudes of the force components, and by the sine of the angle between the line of action of each force component and the corresponding distance.
  • #1
Jason234578
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Homework Statement
Determine the moment of the force around point O
Relevant Equations
M=fd
G'day.
I have a problem with my statics work, i understand its probably quite simple but I am having trouble in my understanding.
Essentially problems make more sense to me by using the component method, although in saying that i understand this problem would be quite a lot more simple were i to approach it without the component method.
stat.jpg
heres the vector diagram for the problem,
1599285515697.png
this is the answer for the sum of the components, but I am confused why Fx is negative and why Fy is being multiplied by 3+3cos45, i understand the barebones equation for the moment is M=fd(perpendicular), but from my understanding Fx would be acting in the positive (right) direction.
 
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  • #2
Fx as drawn in the diagram pulls clockwise, i.e. in negative direction as angles are counterclockwise by definition.

3+3cos(45 deg) is the horizontal distance between O and the point where the force acts.
 
  • #3
Jason234578 said:
this problem would be quite a lot more simple were i to approach it without the component method.

You can do it without components by remembering the fact that we can find the moment by multiplying the force by the perpendicular distance to the line of action of the force?

What does 'line of action mean'? That is the extension of the force vector in both directions to form a line - it shows the direction along which the force acts. I hope that makes sense

Why is this the case?
$$ \vec M = \vec r \times \vec F = |F||r|sin(\theta) \hat n $$
(EDIT: the ## \hat n ## denotes the direction of the moment vector as explained below by @etotheipi ) where we can interpret ## r sin(\theta) ## as the perpendicular distance (from the origin) to the extended line of action. Just in case this doesn't make sense I have included an illustration below. Apologies I am not used to writing with my mouse. The red line is an extension of the force vector and represents the 'line of action'. As we can see the force is ## F ## and the perpendicular distance to the line of action is ## r sin(\theta) ## and therefore the magnitude of the moment is ##Frsin(\theta) ##
aww-board.png


Now back to the main problem, that means that we don't need to use components if we can find the perpendicular distance to the line of action of the force (we already know what F is) and then we can multiply the two. I would suggest that you extend the force vector line below point ## O ##.

Complete the semicircle that touches the line of action and see whether that helps

If this still doesn't make sense, then I will be happy to clarify where possible.
 
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  • #4
Master1022 said:
$$ M = \vec r \times \vec F = |F||r|sin(\theta) $$

I would just clarify that the moment is a vector orthogonal to ##\vec{r}## and ##\vec{F}##,$$\vec{M} = \vec{r} \times \vec{F} = |\vec{F}| |\vec{r}| \sin{(\theta)} \hat{n}$$The component along the ##\hat{n}## direction is$$M_n = (\vec{r} \times \vec{F})_n = |\vec{F}| |\vec{r}|\sin{(\theta)}$$ And its magnitude is$$|\vec{M}| = |\vec{r} \times \vec{F}| = |\vec{F}| |\vec{r}| |\sin{(\theta)}|$$
 
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  • #5
Jason234578 said:
Homework Statement:: Determine the moment of the force around point O
Relevant Equations:: M=fd

... but I am confused why Fx is negative and why Fy is being multiplied by 3+3cos45, i understand the barebones equation for the moment is M=fd(perpendicular), but from my understanding Fx would be acting in the positive (right) direction.

Welcome, Jason234578 :smile:

In addition to the excelent explanations above:
In moment problems like this one, once you can see the position of each component of force respect to the common fulcrum, it is easy to visually determine the sign of the individual moment that each of those components tend to induce.

In order to do the summation of the individual moments, by convention we consider counter-clockwise "rotations" to be positive and clockwise "rotations" to be negative.

In your specific problem, Fx alone would tend to rotate the body, part or structure clockwise, reason for which moment induced by Fx has that negative sign in the summation equation, even when the direction of Fx itself is positive, as you have properly noted.

The rest is only application of geometry and trigonometry, trying to find the horizontal and vertical distances (as the force components are only horizontal and vertical as well) between the line of action of each force component and the common fulcrum.
 
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Related to Moment of a force about point O

1. What is the moment of a force about point O?

The moment of a force about point O is a measure of the rotational effect of a force around a specific point. It takes into account the magnitude of the force, the direction of the force, and the distance between the point and the line of action of the force.

2. How is the moment of a force about point O calculated?

The moment of a force about point O is calculated by multiplying the magnitude of the force by the perpendicular distance from the point to the line of action of the force. This distance is also known as the lever arm or moment arm.

3. What is the difference between moment of a force and torque?

The moment of a force and torque are often used interchangeably, but there is a subtle difference. The moment of a force is a measure of the rotational effect of a force around a point, while torque is a measure of the rotational effect of a force around an axis. In other words, torque takes into account the distance between the point of rotation and the line of action of the force, while moment of a force does not.

4. How does the direction of the force affect the moment about point O?

The direction of the force affects the moment about point O by determining whether the moment is clockwise or counterclockwise. If the force is applied in a direction perpendicular to the line of action, the moment will be maximum. If the force is applied in a direction parallel to the line of action, the moment will be zero.

5. What are some real-life applications of the moment of a force about point O?

The moment of a force about point O is a fundamental concept in mechanics and is used in various real-life applications. Some examples include calculating the torque and stability of objects, designing machines and structures, and understanding the motion of rotating objects such as wheels and gears.

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