__Hooke's Law__

In the diagram below is shown a block attached to a spring. In position (A) the spring is at rest and no external force acts on the block. In position (B) a force F is used to compress the spring by a length equal to $\Delta x $ by pushing the block to the left. In position (C), a force F is used to stretch the spring by a length $\Delta x $ by pulling the block to the right. $\Delta x $ is the change in the length of the spring measured from its position at rest.

In both cases, the relationship between the magnitude of force F used to stretch or compress the spring by a length $\Delta x $ is given by Hooke's law as follows:

| F | = k | $\Delta x $ |

where k is the spring constant.

According to Newton's third law, if a spring is stretched or compressed using force F, as a reaction the spring also react by a force - F.

__Problem 1__
What is the magnitude of the force required to stretch a 20 cm-long spring, with a spring constant of 100 N/m, to a length of 21 cm?

__Solution__

The spring changes from a length of 20 cm to 21 cm, hence it stretches by 1 cm or | $\Delta x $ | = 1 cm = 0.01 m.

| F | = k | $\Delta x $ | = 100 N / m × 0.01 m = 1 N

__Problem 2__
What is the spring constant of a spring that needs a force of 3 N to be compressed from 40 cm to 35 cm?

__Solution__

The spring changes from a length of 40 cm to 35 cm, hence it stretches by 40 cm - 35 cm = 5 cm or | $\Delta x $ | = 5 cm = 0.05 m.

| F | = k | $\Delta x $ | = 3 N

k = | F | / |$\Delta x $| = 3 / 0.05 = 60 N / m

__Spring in Parallel__

Two springs are said to be in parallel when used as in the figure below.

The two springs behave like on spring whose constant k is given by

k = k1 + k2

__Problem 3__
What is the magnitude of the force required to stretch two springs of constants k1 = 100 N / m and k2 = 200 N / m by 6 cm if they are in parallel?

__Solution__

The two springs behave like a spring with constant k given by

k = k1 + k2 = 100 N / m + 300 N / m

| F | = k | $\Delta x $ | = 300 N / m × 0.06 m = 18 N

__Spring in Series__

Two springs are said to be in series when used as in the figure below.

The two springs behave like on spring whose constant k is given by

1 / k = 1 / k1 + 1 / k2

__Problem 4__
What is the magnitude of the force required to stretch two springs of constants 100 N / m and 200 N / m by 6 cm if they are in series?

__Solution__

The two springs behave like a spring with constant k obtained by solving for k the following

1 / k = 1 / 100 + 1 / 300

k = 75 N / m

| F | = k | $\Delta x $ | = 75 N / m × 0.06 m = 4.5 N

__Potential Energy of a Spring__

To stretch or compress a spring by a length | $\Delta x $ |, energy is needed. Once stretched or compressed, the energy is stored in the spring as potential energy P_{e} and is given by:

P_{e} = (1/2) k $(\Delta x)^2 $ , k is the spring constant.

__Problem 5__
How much energy W is needed to compress a spring from 15 cm to 10 cm if the constant of the spring is 150 N / m?

__Solution__

$ \Delta x $ = 10 cm - 15 cm = - 5 cm

The energy W to compress the spring will all be stored as potential energy P_{e} in the spring, hence

W = P_{e} = (1/2) k $(\Delta x)^2 $ = 0.5 × 150 × (-5)^{2} = 1875 J