# Need Help with Physics Moment and Force Questions

• Yossi33
In summary, the conversation discusses various problems related to moments and forces on different structures. The first problem involves understanding how moments are calculated about a point and how to determine if a structure is in equilibrium. The second problem deals with understanding the forces acting on a sliding support and the third problem focuses on determining if a structure is properly constrained. The conversation also touches on the concept of external forces and their direction, and how they affect the calculation of moments.
Yossi33
Homework Statement
classify structures as completely ,partially or improperly constrained
Relevant Equations
first and third law of newton
hello, i struggle with this kind of questions and i don't understand the way of thinking and how to solve this.

in the first picture i don't understand why Moments about point Aare a*F1 because there is the 2 P forces that do moments.?
in the second problem i can't understand why there are 3 forces at B in the left side picture and 2 at the right ?

and the third picture how can i know if this structure is properly constrained or not?

thanks alot

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For case 1: Any force, which extended line does not pass by pivot A will cause a moment about that point.
Based on that, force F1 will induce a moment, but F2 will not.

For case 2:
Left half of the armature: The vertical arrow pointing down and the horizontal arrow pointing left on B are just the reflection of the forces acting on pivot B shown on the right half of the armature.

Therefore, you should consider those forces as external loads, rather than reaction forces at sliding support B (which is unable to oppose any horizontal load).

Sliding support B is only transferring the horizontal force (represented by equal magnitude and opposite directions) between horizontal force P and horizontal support C.

B is inducing a unique vertical reaction, which is represented in both halves.

For case 3: Learning how to determine that is the whole point of that book chapter.
Without doing the calculations, I can see that the left side is properly constrained, but the right side is not, as some of its members have freedom to rotate under the load of vertical force P.
It needs, either a vertical member under P or a horizontal restriction where the slider is now located at.

Last edited:
Yossi33 said:
in the first picture i don't understand why Moments about point Aare a*F1 because there is the 2 P forces that do moments.?
The book's explanation looks inaccurate. The total moment about A is ##aF_1 + aP + 2aP##. This is clockwise, with no counterclockwise moment to balance it, i.e. it is non-zero. But the conclusion that there is no equilibrium is still correct.

But if f1 is compression instead of tension it will be clockwise and counter the p forces moment? why to assume it’s only directions is tension ?

Yossi33 said:
But if f1 is compression instead of tension it will be clockwise and counter the p forces moment? why to assume it’s only directionsI is tension ?
Are you replying to my Post #5? If so, note that ##F_1## is an external force and its direction is given in the diagram.

It acts right and its line of action is above point A. So it must produce a clockwise moment about A.

Steve4Physics said:
If so, note that ##F_1## is an external force and its direction is given in the diagram.
@Yossi33, if ##F_1## is not an external force (with given direction), I can't understand the question and accompanying solution.

## 1. What is a moment in physics?

A moment in physics refers to the turning effect of a force around a point or axis. It is calculated by multiplying the magnitude of the force by the perpendicular distance from the point/axis to the line of action of the force.

## 2. How is moment different from force?

A force is a push or pull on an object, while a moment is the rotational effect of that force. In other words, a force causes linear motion, while a moment causes rotational motion.

## 3. How do you calculate the moment of a force?

The moment of a force can be calculated by multiplying the magnitude of the force by the perpendicular distance from the point/axis to the line of action of the force. The formula is M = F x d, where M is the moment, F is the force, and d is the distance.

## 4. What is the principle of moments?

The principle of moments states that for an object to be in equilibrium, the sum of clockwise moments must be equal to the sum of counterclockwise moments. This principle is based on the fact that a body in equilibrium has no net rotational force acting on it.

## 5. How can moments and forces be applied in real-life situations?

Moments and forces are applied in many real-life situations, such as opening a door, using a wrench to loosen a bolt, or balancing on a seesaw. They are also important in engineering and construction, as they play a crucial role in determining the stability and strength of structures.

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