# Moment of a force about a point

• jonnyboy
In summary, the problem involves determining the value of alpha when a 2.9 lb force is applied to a lever and the moment of the force about point A is 17 lb-in counterclockwise. The equation for moment about point A is given and the solution involves using a trig identity or rearranging the equation to solve for cosα.
jonnyboy

## Homework Statement

This one's easy. A 2.9 lb force (P) is applied to a lever. Determine the value of alpha knowing that the moment of P about A is counterclockwise (+) and has a magnitude of 17 lb-in.

## Homework Equations

Moment about a = (Force of x-component)(distance from a to Force point) + (F_y-comp)(dist. to foce)

M_a = (F_x)(d_y) + (F_y)(d_x)

Answer: alpha = 49.9 degrees or 59.4 degrees

## The Attempt at a Solution

17 = (2.9sin(alpha))(4.8) + (2.9cos(alpha))(3.4)

how do I solve for alpha?

#### Attachments

• untitled.bmp
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Use a trig identity to express sinα in terms of cosα (or vice versa). Then you can solve for cosα.

can I use sin A = cos(pi/2 - A)?

Use sin2α + cos2α = 1. (Rearrange it.)

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

The moment of a force about a point is a measure of the tendency of a force to rotate an object about a specific point. It is the product of the magnitude of the force and the perpendicular distance from the point to the line of action of the force.

## 2. How is the moment of a force calculated?

The moment of a force can be calculated by multiplying the magnitude of the force by the distance from the point to the line of action of the force, and then by the sine of the angle between the force and the line connecting the point and the line of action.

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

Moment of a force and torque are often used interchangeably, but technically, torque refers to the rotational effect of a force on a specific axis, while moment of a force refers to the rotational effect about a specific point.

## 4. How does the direction of the force affect the moment?

The direction of the force relative to the point of rotation will determine the sign of the moment. A force acting clockwise will have a negative moment, while a force acting counterclockwise will have a positive moment.

## 5. Why is the moment of a force important in engineering?

The moment of a force is important in engineering because it is a crucial factor in determining the stability and balance of structures and machines. It is also used in the design and analysis of mechanisms and structures to ensure their safe and efficient operation.

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