Ch 09: Work and Kinetic Energy

Knight Calc - Physics for Scientists and Engineers 5th Edition

Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook

Knight Calc 5th Edition

Ch 09: Work and Kinetic Energy

Problem 57b

Chapter 9, Problem 57b

How much work does tension do to pull the mass from the bottom of the hill (θ = 0) to the top at constant speed? To answer this question, write an expression for the work done when the mass moves through a very small distance ds while it has angle θ, replace ds with an equivalent expression involving R and dθ, then integrate.

Verified step by step guidance

Step 1: Recall the formula for work done by a force: \( W = \int F \cdot ds \), where \( F \) is the force and \( ds \) is the infinitesimal displacement. In this case, the force is tension, and the displacement is along the hill.

Step 2: Express \( ds \) in terms of the radius \( R \) and the angle \( \theta \). The arc length \( ds \) for a small angular displacement \( d\theta \) is given by \( ds = R \cdot d\theta \).

Step 3: Write the expression for the work done over a small displacement \( ds \): \( dW = T \cdot ds \), where \( T \) is the tension force. Substitute \( ds = R \cdot d\theta \) into this equation to get \( dW = T \cdot R \cdot d\theta \).

Step 4: Integrate the expression for \( dW \) over the range of \( \theta \) from \( 0 \) to \( \pi \) (assuming the hill spans this angular range): \( W = \int_{0}^{\pi} T \cdot R \cdot d\theta \).

Step 5: Since the tension \( T \) and radius \( R \) are constants, they can be factored out of the integral. The final expression for the work done is \( W = T \cdot R \cdot \int_{0}^{\pi} d\theta \). Evaluate the integral \( \int_{0}^{\pi} d\theta \) to find the total work done.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Work

Work is defined as the product of the force applied to an object and the distance over which that force is applied, in the direction of the force. Mathematically, it is expressed as W = F · d, where W is work, F is the force, and d is the displacement. In this context, understanding how tension contributes to work is crucial, especially since the mass is being moved at a constant speed, indicating that the net work done is zero.

Tension

Tension is the force exerted by a string, rope, or cable when it is pulled tight by forces acting from opposite ends. In this scenario, tension is responsible for moving the mass up the hill against gravitational forces. The angle θ affects the effective component of tension that does work in the direction of the mass's movement, making it essential to analyze how tension varies with the angle during the ascent.

Integration

Integration is a mathematical technique used to find the total accumulation of a quantity, such as work done over a distance. In this problem, we need to integrate the expression for work done over a small distance ds, which is expressed in terms of the radius R and the angle dθ. This process allows us to calculate the total work done as the mass moves from the bottom to the top of the hill, taking into account the changing angle of tension.

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