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One end of a 10-cm-long spring is attached to the ceiling. When a \(2.0 \mathrm{kg}\) mass is hung from the other end, the spring stretches to a length of \(15 \mathrm{cm}\). a. What is the spring constant? b. How long is the spring when a \(3.0 \mathrm{kg}\) mass is suspended from it?

Short Answer

Expert verified
a. The spring constant k is \(392 \,N/m.\) b. When a 3.0 kg mass is suspended from it, the length of the spring is \(17.5 \,cm.\)

Step by step solution

01

Calculate the spring constant (k)

From Hooke's law, the spring constant k is given by the equation F = kx, where F is the force applied on the spring and x is the displacement of the spring from its equilibrium position. In this case, the force F equals the weight of the 2.0 kg mass, which is \(F = mg = 2.0 \,kg \times 9.8 \,m/s^2 = 19.6 \,N\). The displacement of the spring x is the difference in the lengths of the spring before and after the mass was hung, which is \(x = 15 \,cm - 10 \,cm = 5 \,cm = 0.05 \,m\). We can now plug these values into the equation for Hooke's law and solve for k.
02

Solution for spring constant (k)

Substituting the values of F and x into the equation for Hooke’s law \(F = kx\) and solving for k gives, \(k = F/x = 19.6 \,N / 0.05 \,m = 392 \,N/m.\)
03

Calculate the length of the spring when a 3.0 kg mass is suspended

After finding the spring constant k, it can be used with the weight of the 3.0 kg mass to find the new displacement of the spring when the 3.0 kg mass is hung. The weight of the 3.0 kg mass is \(F = mg = 3.0 \,kg \times 9.8 \,m/s^2 = 29.4 \,N\). Using Hooke's law \(F = kx\), the new displacement can be found by rearranging the equation to \(x = F/k\).
04

Solution for length of the spring when a 3.0 kg mass is suspended

Substituting the values of F and k into the equation \(x = F/k\) and solving for x gives, \(x = 29.4 \,N / 392 \,N/m = 0.075 \,m = 7.5 \,cm.\) As this is the displacement, to get the total length of the spring when the 3.0 kg mass is suspended from it, add the initial un-stretched length of the spring to this displacement, which is \(10 \,cm + 7.5 \,cm = 17.5 \,cm.\)

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Key Concepts

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

Spring Constant
Understanding the spring constant is essential when studying Hooke's law, which describes how a spring stretches or compresses in response to force. The spring constant, denoted by the symbol 'k', is a measure of a spring’s stiffness. It's calculated by dividing the force applied to the spring by the displacement caused by that force, as expressed in the formula \( k = \frac{F}{x} \) where \( F \) is the force in newtons (N) and \( x \) is the displacement in meters (m).

In the exercise, the calculation of the spring constant uses the weight of the mass hanging from the spring as the applied force. Since the spring constant is a ratio that remains constant for a given spring, knowing it allows us to predict how the spring will behave under different forces. This is why after calculating it initially, we can apply it to figure out the displacement for different masses, as the spring constant doesn’t change.
Force and Displacement
The relation between force and displacement in the context of Hooke's law is direct and proportional. This implies that the amount by which a spring stretches (displacement) is directly proportional to the force applied to it. If you double the force, the displacement doubles, provided the spring has not reached its elastic limit.

When materials like springs are deformed, the internal restoring forces try to bring them back to their original shape. In our exercise, the elongation of the spring from its original 10 cm to 15 cm is due to the force exerted by the mass due to gravity, calculated as \( F = mg \) where \( m \) is the mass and \( g \) is the acceleration due to gravity. The change in length (displacement) is then used alongside this force to compute the spring constant using Hooke’s law.
Mass-Spring System
A mass-spring system is a simple harmonic oscillator comprising of a mass attached to a spring that can stretch or compress. When the mass hangs stationary, the system is in equilibrium. However, if the mass is pulled down and released, it will oscillate about the equilibrium point.

Calculating the behavior of a mass-spring system often begins by identifying the spring constant and examining how the system responds to different masses. The application of a mass leads to a force due to gravity which stretches the spring, and the spring exerts an equal and opposite force to support the mass. In our example, when we change the mass from 2.0 kg to 3.0 kg, using the constant 'k' we previously computed, we determine the new displacement and consequently the new length of the spring. The system's predictable nature, governed by Hooke’s law, allows us quantify such scenarios accurately.

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