Calculate the moment mechanics problem

In summary, when calculating the moment of a force, it is important to consider the direction of the force and use the appropriate sign convention. In this problem, the force is split into horizontal and vertical components and multiplied by the distance to find the moment. The negative sign is added to the vertical component because it is in the opposite direction of the clockwise rotation. It is also important to pay attention to units and convert them accordingly.
  • #1
engineer_dave
35
0

Homework Statement



Calculate the moment of the 90N force about point O for the condition theta=15 degrees.

Homework Equations



Moment= Force* perpendicular distance


The Attempt at a Solution



I am doing the first step

Fx= 90sin theta
Fy= 90costheta

What i got was Moment= (Fx*0.8) + (Fy*0.6)= 72sintheta + 54costhetha

When I looked at the worked solution, they got a -Fx*0.8, i don't understand how the negative sign came about. Can you help out? Thanks
 

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  • #2
Clockwise moments are consider +, counterclockwise moments are considered -. Righthand rule.
 
  • #3
You need to look at your units as well. As an early student, write your units out for everything. Degrees/radians are unitless. All you have right now is a force times a unitless quantity, your resultant is N. You need Nm, which means you need to still multiply by a distance.

You were correct to take the sin/cos to find the horizontal and vertical components of the force, but you still need to multiply it by the distance from the center.

So, for the vertical direction:

M = -90N*cos(theta)*600mm*(1 m / 1000 mm) = x Nm

As Astro said, negative because of sign convention. Do the same for the other component, making sure its in the direction that you think.
 

1. How do I calculate the moment of a force?

The moment of a force is calculated by multiplying the magnitude of the force by the perpendicular distance from the point of rotation to the line of action of the force. This can be represented mathematically as M = F x d, where M is the moment, F is the force, and d is the distance.

2. What is the difference between a couple and a moment?

A couple is a pair of equal and opposite forces that act along parallel lines but in opposite directions, resulting in a net moment of zero. A moment, on the other hand, is a single force that causes a rotational motion around a point. In other words, a couple has no net force but does have a net moment, while a moment has both a force and a moment.

3. How do I calculate the moment of a force about a point?

The moment of a force about a point can be calculated by taking the cross product of the position vector from the point to the line of action of the force and the force vector itself. This can be represented mathematically as M = r x F, where M is the moment, r is the position vector, and F is the force vector.

4. Can moments be negative?

Yes, moments can be negative. A negative moment indicates that the force is causing a clockwise rotation, while a positive moment indicates a counterclockwise rotation. Whether a moment is positive or negative depends on the direction of the force and the orientation of the coordinate system.

5. Do I need to consider the direction of the force when calculating moments?

Yes, the direction of the force is an important factor to consider when calculating moments. The direction of the force will determine whether the resulting moment is positive or negative, and thus whether the force is causing a counterclockwise or clockwise rotation. It is important to use the correct sign convention and consider the direction of the force when calculating moments.

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