Problem 54 - Trigonometry

Question

Find the work performed when a force \(\mathbf{F}=15 \mathbf{i}-9 \mathbf{j}\) is applied to an object whose resulting motion is represented by displacement vector \(\mathbf{d}=80 \mathrm{i}+12 \mathrm{j}\). Assume the force is measured in pounds and the displacement in feet. a. \(1,415 \mathrm{ft}-\mathrm{lb}\) b. \(552 \mathrm{ft}-\mathrm{lb}\) c. \(1,092 \mathrm{ft}-\mathrm{lb}\) d. \(1,308 \mathrm{ft}-\mathrm{lb}\)

Step-by-Step Solution

Answer

The work done is 1,092 ft-lb, which is option c.

Step 1: Understand the Problem

We need to find the work done when force \(\mathbf{F}=15 \mathbf{i}-9 \mathbf{j}\) acts on an object causing displacement \(\mathbf{d}=80 \mathbf{i}+12 \mathbf{j}\). Work is calculated as the dot product of the force vector and the displacement vector.

Step 2: Calculate the Dot Product

The dot product \(\mathbf{F} \cdot \mathbf{d}\) is calculated by multiplying corresponding components of the vectors and adding them:\[\mathbf{F} \cdot \mathbf{d} = (15)(80) + (-9)(12)\]

Step 3: Perform Multiplications

Calculate the product of the components:\((15)(80) = 1200\) and \((-9)(12) = -108\).

Step 4: Sum the Products

Add the results from the multiplication to find the work done:\[\mathbf{F} \cdot \mathbf{d} = 1200 - 108 = 1092\, \text{ft-lb}\]

Step 5: Choose the Correct Answer

The calculated work \(1092\, \text{ft-lb}\) matches option c.

Key Concepts

Dot Product

The dot product is a fundamental operation in vector calculus. It provides a way to multiply two vectors and results in a scalar (a single number). This operation is particularly useful for work calculations. The dot product is computed by multiplying corresponding components of two vectors and then summing these products. For example, if we have two vectors \( \mathbf{F} = 15 \mathbf{i} - 9 \mathbf{j} \) and \( \mathbf{d} = 80 \mathbf{i} + 12 \mathbf{j} \), the dot product is calculated as follows:

• Multiply the \(\mathbf{i}\) components: \(15 \times 80\)
• Multiply the \(\mathbf{j}\) components: \(-9 \times 12\)
• Sum the results: \( (15 \times 80) + (-9 \times 12) = 1200 - 108 = 1092 \)

This result, 1092, is a scalar quantity representing the work done. The dot product is crucial for finding this because it accounts for the direction and magnitude of the vectors involved.

Work Calculation

Work is a measure of energy transfer that occurs when an object is moved over a distance by an external force. In vector calculus, calculating work involves finding the dot product of a force vector and a displacement vector. Work, in this context, is defined as:

• \( W = \mathbf{F} \cdot \mathbf{d} \)

This formula merges both direction and magnitude. For an example with a force vector \( \mathbf{F} = 15 \mathbf{i} - 9 \mathbf{j} \) acting through a displacement vector \( \mathbf{d} = 80 \mathbf{i} + 12 \mathbf{j} \):

• Compute the dot product as previously shown: \( 1092 \text{ ft-lb} \)
• This value represents the work performed, here measured in foot-pounds, since force is in pounds and displacement in feet.

Understanding this principle helps comprehend how forces are utilized to accomplish physical tasks through movement.

Force and Displacement Vectors

Vectors play a substantial role in physics and engineering as they describe quantities having both magnitude and direction. When determining the work performed, it's important to consider both the force exerted and the displacement incurred through vectors.A force vector, such as \( \mathbf{F} = 15 \mathbf{i} - 9 \mathbf{j} \), includes components that indicate the force applied in horizontal (\(\mathbf{i}\)) and vertical (\(\mathbf{j}\)) directions. Displacement vectors like \( \mathbf{d} = 80 \mathbf{i} + 12 \mathbf{j} \) tell us how far an object moves in these directions.Both types of vectors help describe real-world situations. They define the amount and direction of force applied and movement achieved, respectively. By understanding vectors, students can solve more complex physics problems, such as those involving work. The ability to use vectors effectively makes it possible to analyze and predict how different forces impact physical systems, leading to a better grasp of practical and theoretical applications.