Damped motion and response time

Key idea: Damping transfers oscillation energy to other stores and reduces amplitude. Critical damping gives the fastest return to equilibrium without oscillation for a release from rest.

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  • GCE A-Level H2 Physics 2027
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A playground swing left alone slowly comes to rest, and a car that goes over a bump stops bouncing almost at once. Both are damped oscillations: resistive forces take energy out of the oscillation. How much damping there is decides whether the motion dies away slowly, settles quickly or creeps back, and a car’s suspension is designed to sit on the boundary between them.

What damping does

In a real oscillator, resistive forces such as air resistance, friction or the viscous drag of oil act against the motion. They do negative work on the oscillator, so its energy is gradually transferred to internal energy of the oscillator and its surroundings. This is damping. The amplitude decreases, because the energy of an oscillation is proportional to the square of its amplitude. The total energy is still conserved: it has moved to other stores, not been destroyed.

Check your understanding 1 Halving the amplitude

The peak amplitude of a lightly damped linear oscillator with unchanged mass and stiffness falls from 4.00 cm to 2.00 cm. What fraction of its energy of oscillation remains? Enter a decimal. Explain the energy destination in the next activity.

Show answer

The energy is proportional to the amplitude squared, so (2.00/4.00)² = 1/4 remains. The other three-quarters has been transferred, mainly to internal energy of the oscillator and the surrounding air.

Check your understanding 2 Account for the missing oscillation energy

Where has the other three-quarters of the energy gone? Explain why total energy is still conserved.

Show answer

Resistive forces do negative work on the oscillation, transferring its energy mainly to internal energy of the oscillator and surroundings. The oscillation has less mechanical energy, but the total energy of the oscillator and surroundings is conserved. Check that you name the larger system rather than saying energy is destroyed.

Light, critical and heavy damping

The graph shows the same oscillator, displaced by the same amount and released from rest, with three degrees of damping.

Damping after the same release from rest

Only the underdamped curve oscillates. Critical damping approaches equilibrium fastest among the non-oscillatory returns for the same mass, stiffness and initial conditions.

Scroll across the figure to read all labels.

Only the underdamped curve oscillates. Critical damping approaches equilibrium fastest among the non-oscillatory returns for the same mass, stiffness and initial conditions.Only the underdamped curve oscillates. Critical damping approaches equilibrium fastest among the non-oscillatory returns for the same mass, stiffness and initial conditions.
Only the underdamped curve oscillates. Critical damping approaches equilibrium fastest among the non-oscillatory returns for the same mass, stiffness and initial conditions.
Open full-size graph
What to noticeCalculated linear damping models, not measured traces. All start at x/A = 1 with zero velocity, the same mass and restoring stiffness, and no continuing drive. Time is scaled by the same undamped angular frequency ω₀. ζ labels relative damping; learners need to compare the curves, not memorise their solution formulas.
View figure data
Figure data: Damping after the same release from rest
Scaled time, ω₀t (unitless)Under: ζ = 0.20Critical: ζ = 1Over: ζ = 2
0.001.0000001.000000001.00000
0.050.9987590.998790900.99883
0.100.9950700.995321160.99561
0.150.9889920.989814170.99072
0.200.9805870.982476900.98447
0.250.9699220.973500980.97712
0.300.9570680.963063690.96889
0.350.9421000.951328920.95996
0.400.9250970.938448060.95047
0.450.9061410.924560820.94055
0.500.8853190.909795990.93029
0.550.8627180.894272210.91979
0.600.8384290.878098620.90911
0.650.8125470.861375530.89830
0.700.7851660.844195020.88742
0.750.7563840.826641470.87650
0.800.7263010.808792140.86558
0.850.6950160.790717620.85467
0.900.6626300.772482350.84381
0.950.6292460.754145000.83300
1.000.5949660.735758880.82226
1.050.5598930.717372390.81161
1.100.5241280.699029280.80105
1.150.4877740.680769050.79059
1.200.4509340.662627270.78024
1.250.4137060.644635790.76999
1.300.3761930.626823120.75986
1.350.3384910.609214610.74984
1.400.3006980.591832710.73994
1.450.2629090.574697210.73016
1.500.2252180.557825400.72049
1.550.1877160.541232330.71095
1.600.1504920.524930950.70153
1.650.1136320.508932260.69222
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1.95-0.0953650.419708510.63885
2.00-0.1274840.406005850.63036
2.05-0.1586530.392641460.62198
2.10-0.1888110.379614930.61371
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2.20-0.2458840.354570110.59749
2.25-0.2727010.342547480.58954
2.30-0.2983110.330854180.58170
2.35-0.3226760.319486690.57396
2.40-0.3457600.308441040.56632
2.45-0.3675320.297712870.55879
2.50-0.3879630.287297500.55135
2.55-0.4070290.277189910.54402
2.60-0.4247100.267384880.53678
2.65-0.4409910.257876930.52963
2.70-0.4558570.248660400.52259
2.75-0.4693010.239729480.51563
2.80-0.4813180.231078240.50877
2.85-0.4919050.222700640.50200
2.90-0.5010660.214590560.49532
2.95-0.5088050.206741840.48873
3.00-0.5151320.199148270.48222
3.05-0.5200590.191803640.47581
3.10-0.5236010.184701730.46948
3.15-0.5257780.177836330.46323
3.20-0.5266100.171201260.45706
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3.30-0.5243430.158597620.44498
3.35-0.5213010.152616940.43906
3.40-0.5170290.146842390.43321
3.45-0.5115620.141268080.42745
3.50-0.5049380.135888230.42176
3.55-0.4971950.130697110.41615
3.60-0.4883750.125689120.41061
3.65-0.4785210.120858750.40515
3.70-0.4676780.116200570.39975
3.75-0.4558930.111709290.39443
3.80-0.4432120.107379700.38918
3.85-0.4296860.103206720.38400
3.90-0.4153650.099185370.37889
3.95-0.4002990.095310770.37385
4.00-0.3845410.091578190.36888
4.05-0.3681430.087982990.36397
4.10-0.3511600.084520640.35912
4.15-0.3336430.081186740.35434
4.20-0.3156470.077977000.34963
4.25-0.2972270.074887230.34498
4.30-0.2784360.071913360.34039
4.35-0.2593280.069051450.33586
4.40-0.2399570.066297640.33139
4.45-0.2203750.063648190.32698
4.50-0.2006360.061099480.32262
4.55-0.1807910.058647980.31833
4.60-0.1608920.056290280.31409
4.65-0.1409880.054023050.30991
4.70-0.1211300.051843080.30579
4.75-0.1013660.049747250.30172
4.80-0.0817410.047732530.29771
4.85-0.0623040.045796010.29374
4.90-0.0430960.043934840.28983
4.95-0.0241630.042146280.28598
5.00-0.0055440.040427680.28217
5.050.0127190.038776470.27842
5.100.0305890.037190150.27471
5.150.0480300.035666340.27105
5.200.0650070.034202700.26745
5.250.0814860.032796990.26389
5.300.0974380.031447040.26038
5.350.1128320.030150760.25691
5.400.1276430.028906120.25349
5.450.1418440.027711170.25012
5.500.1554130.026564010.24679
5.550.1683290.025462840.24351
5.600.1805730.024405900.24027
5.650.1921270.023391490.23707
5.700.2029770.022417970.23391
5.750.2131100.021483770.23080
5.800.2225150.020587370.22773
5.850.2311830.019727310.22470
5.900.2391070.018902170.22171
5.950.2462830.018110590.21876
6.000.2527070.017351270.21585
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6.600.2726000.010338800.18379
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6.850.2526900.008316730.17188
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7.700.1069880.003939600.13687
7.750.0965430.003769000.13505
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7.850.0755830.003449300.13148
7.900.0651210.003299620.12973
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8.050.0341090.002887870.12462
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8.55-0.0584030.001848360.10899
8.60-0.0662430.001767420.10754
8.65-0.0737630.001689970.10611
8.70-0.0809520.001615880.10470
8.75-0.0877980.001545000.10331
8.80-0.0942910.001477180.10193
8.85-0.1004230.001412310.10058
8.90-0.1061840.001350250.09924
8.95-0.1115690.001290880.09792
9.00-0.1165710.001234100.09661
9.05-0.1211850.001179780.09533
9.10-0.1254090.001127820.09406
9.15-0.1292380.001078130.09281
9.20-0.1326710.001030600.09157
9.25-0.1357090.000985140.09035
9.30-0.1383500.000941670.08915
9.35-0.1405970.000900090.08796
9.40-0.1424510.000860330.08679
9.45-0.1439160.000822310.08564
9.50-0.1449960.000785940.08450
9.55-0.1456950.000751170.08337
9.60-0.1460210.000717920.08227
9.65-0.1459780.000686130.08117
9.70-0.1455750.000655730.08009
9.75-0.1448200.000626670.07902
9.80-0.1437210.000598880.07797
9.85-0.1422890.000572310.07693
9.90-0.1405320.000546900.07591
9.95-0.1384630.000522620.07490
10.00-0.1360920.000499400.07390
10.05-0.1334310.000477200.07292
10.10-0.1304930.000455980.07195
10.15-0.1272900.000435700.07099
10.20-0.1238350.000416310.07005
10.25-0.1201430.000397770.06912
10.30-0.1162260.000380050.06820
10.35-0.1120990.000363120.06729
10.40-0.1077760.000346930.06639
10.45-0.1032730.000331460.06551
10.50-0.0986020.000316670.06464
10.55-0.0937810.000302530.06378
10.60-0.0888230.000289030.06293
10.65-0.0837430.000276110.06209
10.70-0.0785560.000263780.06126
10.75-0.0732780.000251980.06045
10.80-0.0679230.000240710.05964
10.85-0.0625060.000229940.05885
10.90-0.0570410.000219650.05807
10.95-0.0515440.000209820.05730
11.00-0.0460280.000200420.05653
11.05-0.0405070.000191440.05578
11.10-0.0349950.000182860.05504
11.15-0.0295060.000174660.05431
11.20-0.0240530.000166830.05358
11.25-0.0186480.000159340.05287
11.30-0.0133030.000152190.05217
11.35-0.0080320.000145350.05147
11.40-0.0028450.000138820.05079
11.450.0022460.000132590.05011
11.500.0072310.000126630.04944
11.550.0120990.000120930.04879
11.600.0168400.000115490.04814
11.650.0214470.000110300.04750
11.700.0259090.000105330.04686
11.750.0302180.000100590.04624
11.800.0343680.000096060.04563
11.850.0383500.000091730.04502
11.900.0421580.000087600.04442
11.950.0457870.000083650.04383
12.000.0492300.000079870.04324
  • Light damping (underdamping): the body keeps oscillating, and the amplitude decreases gradually. The period stays almost the same.
  • Critical damping: the body returns to equilibrium in the shortest possible time without oscillating, that is, without overshooting.
  • Heavy damping (overdamping): the body does not oscillate, but it creeps back to equilibrium more slowly than with critical damping.

More damping is not always quicker. Beyond critical damping, extra damping slows the return.

Worked example 1

Three settings of one damper

Problem

The same spring and mass are pulled down 4.00 cm and released from rest, with three settings P, Q and R of an adjustable damper. P passes through equilibrium several times. Q and R do not pass through equilibrium, and Q returns sooner than R. What can you say about the damping in each case?

Show full solution
  1. Setting P

    Method

    Look for oscillation.

    Reason

    Only a lightly damped body oscillates.

    Working

    Answer

    P is lightly damped.

  2. Settings Q and R

    Reason

    Neither oscillates, and the same system was released in the same way, so their return times can be compared.

    Working

    Answer

    Both are critically or heavily damped, and R is more heavily damped than Q, because it is slower.

  3. How far you can go

    Working

    Answer

    You cannot say Q is exactly critical. To find critical damping, reduce the damping step by step: the last setting before the body starts to overshoot is critical.

Check your understanding 3 Two door closers

A door closer returns without overshooting. When its damping is reduced slightly, the door overshoots its resting position. What does this suggest about the original setting? How would you test the suggestion using the same door and starting position?

Show answer

The original setting is close to critical damping, because a slightly smaller damping allows oscillation. To locate the boundary more precisely, vary the damping in small steps while using the same door, spring and starting position. Find the smallest setting that returns without overshooting. Repeat with smaller changes to locate the setting more closely.

Why critical damping matters

When a car goes over a bump, its springs start the body oscillating. Shock absorbers damp the oscillation. Close to critical damping, the car returns to its normal level quickly without bouncing up and down. Light damping would leave the car bouncing; heavy damping would make the suspension slow to recover, so the car would ride on compressed springs after a series of bumps. The pointers of analogue meters are damped close to critically for the same reason: they settle on the reading quickly without swinging past it.

Spot the mistake 2

As much damping as possible?

About 3 min

Problem

A learner says a car suspension should have as much damping as possible, because more damping always makes the car settle faster after a bump. Which degree of damping should the suspension have?

Choose the damping

Degree of damping

Show solution step by step
  1. Too much damping

    Reason

    Beyond critical damping, the extra resistance slows the return to equilibrium.

    Working

    Answer

    Very heavy damping makes the car slow to recover from a bump.

  2. The right amount

    Working

    Answer

    Critical damping returns the car to its level in the shortest time without bouncing.

Check your understanding 4 A slow meter pointer

A meter pointer released from rest returns to its new equilibrium without overshooting. Reducing the damping makes it return faster, still without overshooting. What does this tell you about the original setting? Explain how to choose a setting for rapid readings without repeated swings.

Show answer

The original setting is heavily damped: a smaller damping still prevents overshoot and returns faster. Tune towards the boundary between overshooting and non-overshooting motion, keeping the same pointer and starting displacement. Close to critical damping gives the fastest non-oscillatory return; very heavy damping delays the reading. Check that your inference uses the changed setting, not just the absence of overshoot.

Common mistakes

  • Saying damping destroys energy; it transfers energy to internal energy.
  • Thinking more damping always means a quicker return.
  • Identifying critical damping only from the absence of oscillation.
  • Comparing damping in two different systems released in different ways.

Spotted an error? Report a correction

Syllabus and review details

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