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A spring with spring constant \(100 \mathrm{~N} / \mathrm{m}\) and unstretched length \(0.4 \mathrm{~m}\) has one end anchored to a wall and a force \(F\) is applied to the other end. If the force \(F\) does \(200 \mathrm{~J}\) of work in stretching out the spring, (a) what is its final length, (b) what is the magnitude of \(F\) at maximum elongation?

Short Answer

Expert verified
(a) The final length is 2.4 m. (b) The force at maximum elongation is 200 N.

Step by step solution

01

Understand the Work-Energy Principle

According to the work-energy principle, the work done on the spring in stretching it results in the change in its elastic potential energy. For a spring, the potential energy is given by \(PE = \frac{1}{2} kx^2\), where \(k\) is the spring constant and \(x\) is the displacement from its unstretched length.
02

Apply the Work-Energy Equation

Given that the work done \(W = 200\) J, and knowing this is equivalent to the change in potential energy, set up the equation: \(200 = \frac{1}{2} \times 100 \times x^2\). Solve for \(x\).
03

Solve for Displacement x

Rearrange the equation \(200 = 50x^2\) to find \(x^2 = \frac{200}{50} = 4\). Solving for \(x\), we have \(x = 2\) meters. This is the displacement from the original length.
04

Calculate Final Length of the Spring

The original length of the spring is \(0.4\) m. The final length \(L_f\) is the original length plus the displacement: \(L_f = 0.4 + 2 = 2.4\) meters.
05

Find the Magnitude of Force at Maximum Elongation

Using Hooke's Law, the force \(F\) on the spring is given by \(F = kx\), where \(k = 100\) N/m and \(x = 2\) m. Thus, \(F = 100 \times 2 = 200\) N.

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Key Concepts

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

Elastic Potential Energy
Elastic potential energy is the stored energy in objects that can be stretched or compressed, like springs. In the context of springs, this energy arises when you apply a force to change the spring's shape. When you release the force, the spring can return to its original form, releasing the stored energy.

The elastic potential energy of a spring is dependent on how much the spring is stretched or compressed from its natural length. It's calculated with the formula:
  • \(PE = \frac{1}{2}kx^2\)
Here, \(PE\) represents the potential energy, \(k\) is the spring constant, and \(x\) denotes the displacement from the spring's equilibrium position.

In our exercise, the work done by the force on the spring, which is 200 J, becomes the elastic potential energy, since it's stored in the stretched spring. Understanding this concept helps in figuring out how energy transformations occur in spring systems.
Hooke's Law
Hooke's Law describes the behavior of springs and is fundamental to understanding spring mechanics. It states that the force needed to extend or compress a spring by some distance \(x\) is proportional to that distance. The equation is:
  • \(F = kx\)
Here, \(F\) is the force applied to the spring, \(k\) is known as the spring constant, and \(x\) is the displacement.

This principle is key to deducing how much force is needed during stretching or compressing. It also helps determine the force at maximum elongation or compression. For the spring in our problem, applying Hooke’s Law means at maximum stretch of 2 meters, the force is 200 N.
Spring Constant
The spring constant, denoted as \(k\), is a measure of a spring's stiffness. It quantifies how resistant a spring is to being compressed or stretched. The larger the spring constant, the stiffer the spring, and a greater force will be required to displace it by one unit of length.

In the exercise given, the spring constant is 100 N/m, illustrating a moderate stiffness. Knowing this value is crucial for any calculations involving Hooke's Law and elastic potential energy, as it directly affects the force and energy calculations.

For example, with a known spring constant, you can accurately determine the force on the spring when it is displaced, as well as the amount of potential energy stored at different displacements.
Displacement in Springs
Displacement in the context of springs refers to the change in length of a spring from its unstretched or relaxed position. This change can be either an extension or a compression.

In our initial exercise, we calculated a displacement of 2 meters. This was derived from the work-energy principle by equating the work done on the spring to the change in elastic potential energy. The displacement is calculated by rearranging the elastic potential energy formula \(PE = \frac{1}{2}kx^2\) to solve for \(x\).

Understanding displacement is important as it tells us how much a spring has been stretched or compressed from its natural state, which is useful for calculating forces and stored energy within the spring system. This understanding is crucial in various applications, from simple gadgets to complex mechanical systems.

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Most popular questions from this chapter

A block of ice with mass \(2.00 \mathrm{~kg}\) slides \(0.750 \mathrm{~m}\) down an inclined plane that slopes downward at an angle of \(36.9^{\circ}\) below the horizontal. If the block of ice starts from rest, what is its final speed? You can ignore friction.

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BIO An exercise program. A \(75 \mathrm{~kg}\) person is put on an exercise program by a physical therapist, the goal being to burn up 500 food calories in each daily session. Recall that human muscles are about \(20 \%\) efficient in converting the energy they use up into mechanical energy. The exercise program consists of a set of consecutive high jumps, each one \(50 \mathrm{~cm}\) into the air (which is pretty good for a human) and lasting \(2.0 \mathrm{~s},\) on the average. How many jumps should the person do per session, and how much time should be set aside for each session? Do you think that this is a physically reasonable exercise session?

II A slingshot obeying Hooke's law is used to launch pebbles vertically into the air. You observe that if you pull a pebble back \(20.0 \mathrm{~cm}\) against the elastic band, the pebble goes \(6.0 \mathrm{~m}\) high. (a) Assuming that air drag is negligible, how high will the pebble go if you pull it back \(40.0 \mathrm{~cm}\) instead? (b) How far must you pull it back so it will reach \(12.0 \mathrm{~m} ?\) (c) If you pull a pebble that is twice as heavy back \(20.0 \mathrm{~cm},\) how high will it \(\mathrm{go} ?\)

Marbles of mass \(m\) are thrown from the edge of a vertical cliff of height \(h\) at speed \(v_{0} .\) Neglecting air resistance, how fast (in terms of \(m, h,\) and \(v_{0}\) ) will these marbles be moving when they reach the bottom of the cliff if they are thrown (a) straight up, (b) straight down, or (c) horizontally away from the cliff? Will the final velocity vectors of the marbles be the same or different for each case?

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