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As part of your daily workout, you lie on your back and push with your feet against a platform attached to two stiff springs arranged side by side so that they are parallel to each other. When you push the platform, you compress the springs. You do 80.0 J of work when you compress the springs 0.200 m from their uncompressed length. (a) What magnitude of force must you apply to hold the platform in this position? (b) How much \(additional\) work must you do to move the platform 0.200 m \(farther\), and what maximum force must you apply?

Short Answer

Expert verified
(a) 800 N (b) Additional work: 240 J; Maximum force: 1600 N

Step by step solution

01

Understanding the Problem

You have two parallel springs and compress them together, which requires 80.0 J of work. You need to find the force required to hold the compression at 0.200 m and the additional work to compress it another 0.200 m, as well as the force needed for this second compression.
02

Relate Work to Spring Force

According to Hooke's Law, the force applied to a spring is proportional to its compression, given by the formula \(F = kx\), where \(k\) is the spring constant and \(x\) is the compression distance. The work done on the spring is given by \(W = \frac{1}{2}kx^2\).
03

Calculate Spring Constant (k)

Using the work-energy principle, where work done is 80.0 J and compression is 0.200 m, calculate the effective spring constant for both springs combined using \(W = \frac{1}{2}kx^2\). Solving, \(80.0 = \frac{1}{2}k(0.200)^2\), results in \(k = 4000\, \text{N/m}\). This is the combined spring constant for the two springs.
04

Calculate Force for First Compression

To find the force required to hold the platform at 0.200 m compression, use Hooke's Law \(F = kx\). Substituting the values found, \(F = 4000 \times 0.200 = 800\, \text{N}\). This is the force needed for initial compression.
05

Calculate Additional Work for Further Compression

For additional 0.200 m compression, total compression distance is 0.400 m. Calculate new work done using \(W = \frac{1}{2}kx^2\) for 0.400 m, so \(W = \frac{1}{2}4000(0.400)^2 = 320.0\, \text{J}\). The additional work is the difference, \(320.0 - 80.0 = 240.0\, \text{J}\).
06

Calculate Maximum Force for Further Compression

Using Hooke's Law again for total 0.400 m compression, \(F = 4000 \times 0.400 = 1600\, \text{N}\). This is the maximum force required for further compression.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Spring Constant
The spring constant is a key part of understanding how springs behave when compressed or stretched. It is commonly denoted by the symbol \(k\) and represents the stiffness of a spring. A larger spring constant indicates a stiffer spring, requiring more force to compress or stretch it by a particular distance.

In our workout example, we have two springs acting together in a parallel arrangement. To determine the spring constant of both springs combined, we use the work-energy principle.

The work done on the springs is given by the equation \(W = \frac{1}{2}kx^2\). Given the work \(W = 80.0 \text{ J}\) and compression \(x = 0.200 \text{ m}\), we can solve for the combined spring constant. The calculation goes like this:\[80.0 = \frac{1}{2}k(0.200)^2\]Solving this yields \(k = 4000 \text{ N/m}\). So, the combined spring constant for these springs is 4000 N/m. This implies the springs are quite stiff, suitable for resisting significant forces.

Understanding the spring constant allows you to calculate the force and energy involved in compressing or stretching the spring, a crucial step for analyzing mechanical systems involving springs.
Work-Energy Principle
The work-energy principle is an essential concept that connects the work done on an object to its energy changes. It's essential for problems involving springs, as it helps us understand the relationship between the force applied, the distance moved, and the energy stored or used.

In the exercise, you perform 80.0 J of work to compress the springs by 0.200 meters. The principle here is that the work done on an object is stored as potential energy in the spring due to its compression or extension. Using the formula \(W = \frac{1}{2}kx^2\), we calculate how much work is needed to hold or further compress the springs.

When further compressing the springs by an additional 0.200 meters, we need to calculate the new work done for a total compression of 0.400 meters. This is calculated as:\[W = \frac{1}{2}4000(0.400)^2 = 320.0 \text{ J}\]The additional work required is the difference between the total work for 0.400 m and the initial 0.200 m compression: \(320.0 \text{ J} - 80.0 \text{ J} = 240.0 \text{ J}\).

This principle elegantly accounts for how energy is transferred and stored, allowing for detailed analysis of mechanical systems.
Force Calculation
Calculating the force needed to compress or stretch the springs involves Hooke's Law, which states that the force \(F\) required to compress or extend a spring is directly proportional to the distance \(x\) it is compressed or extended. The equation is \(F = kx\), where \(k\) is the spring constant.

For the initial compression of 0.200 meters, we calculate as follows: \(F = 4000 \times 0.200 = 800 \, \text{N}\). This is the force needed to hold the compression initially.

For further compression to 0.400 meters, the new force calculation will be: \(F = 4000 \times 0.400 = 1600 \, \text{N}\). This is the maximum force you must apply for the additional compression.

These calculations are crucial in mechanical design and analysis because they tell us precisely how much effort is needed to manipulate springs in systems ranging from simple exercise equipment to complex engineering machinery. By knowing the forces involved, you can ensure systems are designed safely and efficiently.

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Most popular questions from this chapter

A physics student spends part of her day walking between classes or for recreation, during which time she expends energy at an average rate of 280 W. The remainder of the day she is sitting in class, studying, or resting; during these activities, she expends energy at an average rate of 100 W. If she expends a total of 1.1 \(\times\) 10\(^7\) J of energy in a 24-hour day, how much of the day did she spend walking?

Use the work\(-\)energy theorem to solve each of these problems. You can use Newton's laws to check your answers. Neglect air resistance in all cases. (a) A branch falls from the top of a 95.0-m-tall redwood tree, starting from rest. How fast is it moving when it reaches the ground? (b) A volcano ejects a boulder directly upward 525 m into the air. How fast was the boulder moving just as it left the volcano?

All birds, independent of their size, must maintain a power output of 10\(-\)25 watts per kilogram of body mass in order to fly by flapping their wings. (a) The Andean giant hummingbird (\(Patagona gigas\)) has mass 70 g and flaps its wings 10 times per second while hovering. Estimate the amount of work done by such a hummingbird in each wingbeat. (b) A 70-kg athlete can maintain a power output of 1.4 kW for no more than a few seconds; the \(steady\) power output of a typical athlete is only 500 W or so. Is it possible for a human-powered aircraft to fly for extended periods by flapping its wings? Explain.

An ingenious bricklayer builds a device for shooting bricks up to the top of the wall where he is working. He places a brick on a vertical compressed spring with force constant \(k = 450\) N/m and negligible mass. When the spring is released, the brick is propelled upward. If the brick has mass 1.80 kg and is to reach a maximum height of 3.6 m above its initial position on the compressed spring, what distance must the bricklayer compress the spring initially? (The brick loses contact with the spring when the spring returns to its uncompressed length. Why?)

An elevator has mass 600 kg, not including passengers. The elevator is designed to ascend, at constant speed, a vertical distance of 20.0 m (five floors) in 16.0 s, and it is driven by a motor that can provide up to 40 hp to the elevator. What is the maximum number of passengers that can ride in the elevator? Assume that an average passenger has mass 65.0 kg.

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