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# Potential Energy of a Spring

## Potential energy of spring

• The spring force is an example of a variable force, which is conservative.

• In an ideal spring, Fs = − kx , this force law for the spring is called Hooke’s law.

• The constant k is called the spring constant. Its unit is N m-1.

• The spring is said to be stiff if k is large and soft if k is small.

Spring force is position dependent as first stated by Hooke,
Fs = − kx

• Work done by spring force only depends on the initial and final positions. Thus, the spring force is a conservative force.

• We define the potential energy V(x) of the spring to be zero when block and spring system is in the equilibrium position.

• If the extension is xm, the work done by the spring force is

W_s = ∫_0^(x_m) F_s  dx=-∫_0^(x_m) kx  dx
=-(kx_m^2)/2
The same is true when the spring is compressed with a displacement xc (< 0). The spring force does work Ws = -kx2/2 while the external force F does work + kxc^2/2. If the block is moved from an initial displacement xi to a final displacement xf, the work done by the spring force Ws is

W_s = -∫_(x_i)^(x_f) kx  dx = (kx_i^2)/2 - (kx_f^2)/2
Thus the work done by the spring force depends only on the end points. Specifically, if the block is pulled from xi and allowed to return to xi;
W_s = -∫_(x_i)^(x_f) kx  dx = (kx_i^2)/2 - (kx_f^2)/2 = 0
The work done by the spring force in a cyclic process is zero.

We have explicitly demonstrated that the spring force
(i) is position dependent only as first stated by Hooke, (Fs = − kx);
(ii) does work which only depends on the initial and final positions.
Thus, the spring force is a conservative force.

We define the potential energy V(x) of the spring to be zero when block and spring system is in the equilibrium position. For an extension (or compression) x the above analysis suggests that
V(x)=(kx^2)/2

If the block of mass m is extended to xm and released from rest, then its total mechanical energy at any arbitrary point x (where x lies between – xm and + xm) will be given by:
1/2 kx_m^2 = 1/2 kx^2 + 1/2 mv^2
This suggests that the speed and the kinetic energy will be maximum at the equilibrium position, x = 0, i.e.,
1/2 mv_m^2 = 1/2 kx_m^2
where vm is the maximum speed.
Or v_m= sqrt(k/m)  x_m

Note that k/m has the dimensions of [T^(-2)] and our equation is dimensionally correct.
The kinetic energy gets converted to potential energy and vice versa, however, the total mechanical energy remains constant. This is graphically depicted in Fig

Example: A car of mass M travelling with speed v, collides with a spring, having spring constant k. The car comes to rest (momentarily) when spring is compressed to a distance of x_m. Find x_m
Solution: Work done on car = K_f-K_i=0-(mv^2)/2 =-(mv^2)/2
Work done by spring, W_s= -∫_(x_i)^(x_f) kx  dx =-∫_(0)^(x_m) kx  dx = -(kx_m^2)/2
Now work done by spring equals work done on car => -(mv^2)/2 = - (kx_m^2)/2
x_m = sqrt (m/k)  v

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