# Determine the magnitude of F.

• celtics777
In summary, to raise the lamp post, a counterclockwise moment of 1500 lb*ft must be created at point A. The magnitude of the force on the cable, F, can be determined using the cross product rule for determining moments. This involves finding the magnitude of the position vector between the base of the post and the point of application of the force, as well as using the law of sines and cosines to find theta, the included angle between the force and position vectors.

## Homework Statement

In order to raise the lamp post from the position shown, the force on the cable must create a counterclockwise moment of 1500 lb*ft about point A. Determine the magnitude of F that must be applied to the cable.

## Homework Equations

I can find the magnitude of a vector when I know the vector. Not in this problem.

## The Attempt at a Solution

I know a fair amount to get started, such as definitions, but I have no idea how to actually do calculations.

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Are you familiar with the cross product rule for determining moments?

I'm familiar with the cross product, but not sure how to use it to determine moment.

M = r X F =rFsintheta, wher r is the magnitude of the position vector between the base of the post and the point of application of the force(the length of the post), F is the magnitude of the applied tensile force, ant theta is the included angle between the force and position vectors. You'll have to use the law of sines and cosines to first find theta.

I would approach this problem by first identifying the relevant physical principles and equations that can help us solve for the unknown magnitude of F. In this case, we can use the concept of torque, which is the product of force and distance from the pivot point, to determine the magnitude of F.

First, we can set up the equation for torque as T= F*d, where T is the torque, F is the force applied, and d is the distance from the pivot point. In this case, we know that the torque is 1500 lb*ft and the distance d is unknown. Therefore, we can rearrange the equation to solve for F as F= T/d.

Next, we need to determine the distance d. Looking at the diagram, we can see that the distance from point A to the point where the force is applied is the same as the length of the cable. Therefore, we can use the Pythagorean theorem to find the length of the cable, which is the hypotenuse of a right triangle formed by the lamp post and the ground. This can be expressed as d= √(20^2 + 10^2) = √500 = 22.36 ft.

Now, we can plug in the known values into the equation F= T/d to solve for the magnitude of F. This gives us F= 1500 lb*ft/22.36 ft = 67.1 lb.

Therefore, the magnitude of F that must be applied to the cable to raise the lamp post is approximately 67.1 lb.

## 1. What is the formula for determining the magnitude of F?

The formula for determining the magnitude of F is |F| = √(Fx² + Fy² + Fz²), where Fx, Fy, and Fz are the components of F in the x, y, and z directions, respectively.

## 2. How do I calculate the magnitude of F if I only have the vector components?

If you only have the vector components of F, you can use the Pythagorean theorem to find the magnitude. The formula is |F| = √(Fx² + Fy² + Fz²).

## 3. Can the magnitude of F be negative?

No, the magnitude of a vector is always positive. It represents the size or length of the vector, not its direction.

## 4. What is the unit of measurement for the magnitude of F?

The unit of measurement for the magnitude of F will depend on the units used for the vector components. For example, if the components are in meters, the magnitude of F will be in meters as well.

## 5. Why is it important to determine the magnitude of F?

Determining the magnitude of F is important because it gives information about the strength or intensity of the vector. It can also be used to find the distance traveled or the force applied in a given direction.