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Question

Two identical spring balances S1 and S2 are connected one after the other and are held vertically as shown in the figure. A mass of 10 kg is hanging from S2. If the readings on S1 and S2 are W1 and W2 respectively, then :

This question was previously asked in
NDA I 2023 GAT Previous Year Paper (16-Apr-2023)
The correct answer is

W1 = 10 kg and W2 = 10 kg

Concept:

Spring - A spring is a mechanical component or device typically made of elastic material that can deform under the application of force and return to its original shape when the force is removed. It is commonly used to store and release mechanical energy.

  • Springs work based on Hooke's Law, which states that the force needed to extend or compress a spring is directly proportional to the displacement (change in length) from its equilibrium position. Mathematically, this can be represented as:
  • F = -k × x

where:

  • F is the force applied to the spring,
  • k is the spring constant (also known as stiffness or force constant), which represents the stiffness of the spring and how much force it takes to stretch or compress the spring by a certain amount,
  • x is the displacement (elongation or compression) from the spring's equilibrium position.

Spring Connection in Parallel:

  • When springs are connected in parallel, they share the same load or force applied to the system.
  • The springs are aligned in such a way that they are connected at both ends, and the force is distributed among all the springs.
  • In this arrangement, the total effective spring constant (stiffness) is the sum of the individual spring constants.
  • If we have springs with spring constants k1, k2, k3, ..., kn connected in parallel, the total effective spring constant (keff) can be calculated as:
    • keff = k1 + k2 + k3 + ... + kn
  • The benefit of connecting springs in parallel is that it increases the overall stiffness of the system, enabling it to handle larger forces.

Spring Connection in Series:

  • When springs are connected in series, they are arranged end-to-end so that the force is applied to one spring, and then that force is transmitted to the next spring in line.
  • In this arrangement, the total effective spring constant is less than the individual spring constants.
  • If we have springs with spring constants k1, k2, k3, ..., kn connected in series, the total effective spring constant (keff) can be calculated as:
  • \(\frac1k_{eff} = \frac1k_1 + \frac1k_2 + \frac1k_3 + ... +\frac 1k_n\)
  • The benefit of connecting springs in series is that it increases the overall displacement or elongation that the system can undergo. However, the overall stiffness of the system is reduced compared to individual springs.

Explanation:

  • If two identical spring balances, S1 and S2, are connected one after the other and held vertically, with a mass of 10 kg hanging from S2, then the readings on S1 and S2 should be equal.
  • This is because the spring balances are connected in series, and the tension in the connecting string is the same throughout.
  • When connected in series, the tension in the string is constant, so the force measured by both spring balances will be the same. So, W1 should be equal to W2.

If the mass is 10 kg, then W1 = W2 = 10 kg.

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