Time Period of a Spring-Mass System

Physics · Oscillations · NEET

The time period of a spring-mass system is T = 2 pi root(m/k), where m is the hanging mass and k is the spring constant. Heavier mass means slower oscillation (larger T); a stiffer spring (bigger k) means faster oscillation (smaller T). Memory hook: "Mass on top, k below" - m is inside the square root on top, k is on the bottom.
Spring-Mass System: T = 2 pi root(m / k)mmeanpositionfixedm larger -> T larger (slower)k larger -> T smaller (faster)gravity: shifts mean by mg/k onlyg is NOT in the T formula
A mass m on a spring of constant k oscillates about its mean position with T = 2 pi root(m/k). Gravity only shifts the mean position by mg/k; it does not enter the time period.

Your doubts, answered

Does g (gravity) affect the time period of a vertical spring-mass system?

No. The formula is T = 2 pi root(m/k) for both horizontal and vertical springs. Gravity only shifts the mean position downward by an amount x0 = mg/k. Once the mass sits at this new equilibrium, it oscillates about that point with the same T = 2 pi root(m/k). So g decides where the block rests, not how fast it oscillates.

How do I find k when the question only gives a stretch under a force?

Use Hooke's law first: k = F/x, where x is the extension in metres. Example: a force of 10 N stretches the spring by 5 cm (0.05 m), so k = 10/0.05 = 200 N/m. Then put this k into T = 2 pi root(m/k). This is the exact two-step trick tested in NEET 2021.

What happens to the time period if I double the mass?

Because T depends on root(m), doubling m multiplies T by root(2) (about 1.41 times), not by 2. To double the time period you must make the mass 4 times bigger, since T is proportional to root(m). Many students wrongly assume T doubles when m doubles.

Is the time period different for a horizontal spring versus a vertical spring?

No, it is the same: T = 2 pi root(m/k). The only difference is the equilibrium position. A horizontal spring rests at its natural length; a vertical spring rests stretched by mg/k due to gravity. Both give identical time periods for the same m and k.

How do I find the unknown mass when two time periods are given?

Use T proportional to root(m). Write T1/T2 = root(m1/m2) and square both sides. In NEET 2016, T = 3 s for mass m and T = 5 s for mass (m+1). So (3/5)^2 = m/(m+1) gives 9/25 = m/(m+1), which solves to m = 9/16 kg.

Does the amplitude of oscillation change the time period?

No. For an ideal spring obeying Hooke's law, T = 2 pi root(m/k) is independent of amplitude. Whether you pull the block 2 cm or 5 cm, the time for one full oscillation stays the same. This property is called isochronism and is true for all simple harmonic motion.

⚠️ The NEET trap
Adding a g term or writing T = 2 pi root(m/(k+mg)) because the spring hangs vertically and gravity 'must' appear.
For any spring-mass system, horizontal or vertical, T = 2 pi root(m/k). Gravity only shifts the mean position by mg/k and never enters the time period formula.
🧠 NTA loves the vertical spring with gravity.

Real NEET questions

2021

A spring is stretched by 5 cm under a force of 10 N. The time period of oscillation when a mass of 2 kg is suspended from it is

A · 3.14 s
B · 0.628 s
C · 0.0628 s
D · 6.28 s
Solution: Step 1: Find k from Hooke's law. k = F/x = 10 / 0.05 = 200 N/m (5 cm = 0.05 m). Step 2: Apply the formula. T = 2 pi root(m/k) = 2 pi root(2/200) = 2 pi root(0.01). Step 3: root(0.01) = 0.1, so T = 2 pi (0.1) = 0.628 s. Answer: B.
2016

A body of mass m hangs from a spring of negligible mass. When pulled down and released it oscillates with a time period of 3 s. When the mass is increased by 1 kg, the time period becomes 5 s. The value of m (in kg) is

A · 3/4
B · 4/3
C · 16/9
D · 9/16
Solution: Step 1: T is proportional to root(m), so T1/T2 = root(m/(m+1)). Step 2: (3/5) = root(m/(m+1)). Square both sides: 9/25 = m/(m+1). Step 3: 9(m+1) = 25m gives 9 = 16m, so m = 9/16 kg. Answer: D.

Solved Oscillations NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

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Frequently asked

What is the formula for the time period of a spring-mass system?

T = 2 pi root(m/k), where m is the attached mass in kg and k is the spring constant in N/m. The frequency is f = 1/T = (1/2 pi) root(k/m).

Why does gravity not appear in the spring time period formula?

Gravity is a constant force that only shifts the equilibrium position down by mg/k. It does not change the restoring behaviour about that new mean position, so T = 2 pi root(m/k) stays the same for vertical springs.

Does the time period depend on amplitude?

No. For an ideal spring, the time period is independent of amplitude. This is a key feature of simple harmonic motion.

How does the time period change if the spring is made stiffer?

A stiffer spring has a larger k. Since T is proportional to 1/root(k), a bigger k gives a smaller time period, so the block oscillates faster.

What is the effect of doubling the mass on the time period?

Since T is proportional to root(m), doubling the mass multiplies the time period by root(2), about 1.41 times, not by 2.