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Symbolic Version of Problem \(75 \mathrm{~A}\) light spring with spring constant \(k_{1}\) hangs from an clevated support. From its lower end hangs a second light spring, which has spring constant \(k_{2}\). An object of mass \(m\) hangs at rest from the lower end of the second spring. (a) Find the total extension distance \(x\) of the pair of springs in terms of the two displacements \(x_{1}\) and \(x_{2}\). (b) Find the effective spring constant of the pair of springs as a system. We describe these springs as being in series.

Short Answer

Expert verified
The total extension distance of the pair of springs is \(x = x_1 + x_2\). The effective spring constant of the pair of springs as a system is given by the harmonic sum of the individual spring constants, \(1/k = 1/k_1 + 1/k_2\).

Step by step solution

01

Define the Spring Constants and Displacements

We have two springs, each with a different spring constant \(k_1\) and \(k_2\). Each spring is extended a distance \(x_1\) and \(x_2\) respectively, due to the weight of an object of mass \(m\). Since the object is at rest, the upward spring force must balance the downward gravitational force.
02

Find the Total Extension Distance

Because the springs are in series, the total extension \(x\) is simply the sum of the extensions of the individual springs. So, \(x = x_1 + x_2\).
03

Equate the Forces on Each Spring

The forces on each spring are equal to their spring constant times their displacement. This gives us two equations \(k_1 x_1 = k_2 x_2\) and \(k_2 x_2 = mg\) (since the downward force due to gravity must equal the total upward spring force).
04

Solve for the Effective Spring Constant

The effective spring constant \(k\) of a system of springs in series is given by \(1/k = 1/k_1 + 1/k_2\). We can solve this equation for \(k\) to find the effective spring constant of the system.

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Key Concepts

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

Spring Constant
The concept of a spring constant is at the heart of understanding how springs behave. A spring constant, often denoted by the symbol \( k \), defines how stiff or flexible a spring is. It measures the force required to compress or extend the spring by a unit distance.
It is a fundamental property of a spring and is expressed in units of Newtons per meter (N/m). The larger the spring constant, the stiffer the spring, which means more force is required to produce the same length of extension compared to a spring with a lower spring constant.
  • Spring constant \( k_1 \) and \( k_2 \): Each spring in our system has unique spring constants.
  • Individual spring extension: Each spring stretches differently depending on its spring constant.
Understanding spring constants is crucial in calculating both the individual extensions of springs (\( x_1 \) and \( x_2 \)) and the total extension when springs are combined.
Effective Spring Constant
When springs are arranged in a series, they combine to create a system with its own collective behavior. The effective spring constant for springs in series can be thought of as a single spring that has the equivalent effect of both springs combined.
The formula for finding the effective spring constant \( k_{eff} \) when two springs are in series is:
\[ \frac{1}{k_{eff}} = \frac{1}{k_1} + \frac{1}{k_2} \]
This equation shows that the effective spring constant of the system is less than the spring constants of the individual springs because the entire system is more flexible than any single spring.
  • The reciprocal nature of the formula emphasizes that longer (and more flexible) spring systems result in lower effective spring constants.
  • This property is important in many applications where the goal might be to achieve a specific flexibility.
By working through this calculation, you gain insight into how the behavior of combined systems diverges from solitary components.
Total Extension
The concept of total extension is straightforward when dealing with springs in series. For a series of springs, the total extension refers to how much the entire system stretches under a given force.
In the problem involving two springs with different spring constants, the total extension \( x \) is given by the sum of the individual extensions of each spring:
\[ x = x_1 + x_2 \]
Here, \( x_1 \) and \( x_2 \) are the extensions of springs with constants \( k_1 \) and \( k_2 \) respectively.
  • Understanding total extension is essential because it affects the system’s ability to absorb force.
  • It depends on how each spring individually responds to the force due to their inherent spring constants.
By calculating the total extension, we learn how the system stretches as a whole, giving us practical insights for engineering and design applications.
Force Equilibrium in Springs
Force equilibrium is a fundamental concept that keeps our spring system balanced. In the context of springs in series, force equilibrium means the forces acting on the springs must balance out. This occurs when the tension in each spring is equal to any external force applied to the system.
The two equations from the problem illustrate how force equilibrium works. These equations are:
  • \( k_1 x_1 = k_2 x_2 \)
  • \( k_2 x_2 = mg \)
In these equations, \( mg \) represents the gravitational force acting downward on the mass attached to the system.
Force equilibrium:
  • Ensures that the sum of forces is zero and the system remains at rest.
  • Helps in verifying that the spring deformations are correctly distributed according to their spring constants.
Recognizing and using force equilibrium allows us to understand how each spring interacts with the applied forces, maintaining system stability.

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Most popular questions from this chapter

A \(7.00-\mathrm{kg}\) bowling ball moves at \(3.00 \mathrm{~m} / \mathrm{s}\). How fast must a \(2.45\) -g Ping-Pong ball move so that the two balls have the same kinetic energy?

A daredevil wishes to bungee-jump from a hot-air balloon \(65.0 \mathrm{~m}\) above a carnival midway (Fig. P5.83). He will use a piece of uniform elastic cord tied to a harness around his body to stop his fall at a point \(10.0 \mathrm{~m}\) above the ground. Model his body as a particle and the cord as having negligible mass and a tension force described by Hooke's force law. In a preliminary test, hanging at rest from a \(5.00-\mathrm{m}\) length of the cord, the jumper finds that his body weight stretches it by \(1.50 \mathrm{~m} .\) He will drop from rest at the point where the top end of a longer section of the cord is attached to the stationary balloon. (a) What length of cord should he use? (b) What maximum acceleration will he experience?

When a \(2.50-\mathrm{kg}\) object is hung vertically on a certain light spring described by Hooke's law, the spring stretches \(2.76 \mathrm{em}\). (a) What is the force constant of the spring? (b) If the \(2.50-\mathrm{kg}\) object is removed, how far will the spring stretch if a \(1.25-\mathrm{kg}\) block is hung on it? (c) How much work must an external agent do to stretch the same spring \(8.00 \mathrm{~cm}\) from its unstretched position?

A toy gun uses a spring to project a \(5.3-g\) soft rubber sphere horizontally, The spring constant is \(8.0 \mathrm{~N} / \mathrm{m}\), the barrel of the gun is \(15 \mathrm{~cm}\) long, and a constant frictional force of \(0.032 \mathrm{~N}\) exists between barrel and projectile. With what speed does the projectile leave the barrel if the spring was compressed \(5.0 \mathrm{~cm}\) for this launch?

On a frozen pond, a \(10-\mathrm{kg}\) sled is given a kick that imparts to it an initial speed of \(v_{0}=2.0 \mathrm{~m} / \mathrm{s}\). The coefficient of kinctic friction between sled and ice is \(\mu_{k}=0.10 .\) Use the work-energy theorem to find the distance the sled moves before coming to rest.

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