Momentum on an Air Track — Watching Conservation Laws Happen in Front of You

There are some experiments in physics where the result is mathematically satisfying but visually rather unimpressive.

Momentum on an air track is not one of them.

Two gliders move towards one another, collide, and separate. Sometimes one stops while the other carries on. Sometimes both rebound. Sometimes they stick together and continue as a single object.

It happens in a fraction of a second.

Yet hidden inside that brief collision is one of the most important ideas in physics:

momentum is conserved.

What makes the air track particularly powerful is that we do not have to accept this merely because a textbook tells us it is true. We can measure the velocities immediately before and after a collision, calculate the momentum, and test the law ourselves.

Add electronic data capture, and the experiment becomes even more compelling. Instead of spending most of the lesson recording numbers, students can see the motion displayed almost immediately and begin asking the more interesting question:

Where did everything go during the collision?

Why Use an Air Track?

The great enemy of almost every school mechanics experiment is friction.

If I push a trolley across an ordinary bench, it begins slowing almost immediately. The wheels have friction, the bearings have friction and the surface itself may not be perfectly level.

An air track attempts to remove much of that problem.

A series of small holes produces a cushion of air underneath the glider. The glider effectively floats just above the track.

Friction is not literally zero, but it can become small enough for us to investigate motion that is much closer to the idealised situations used in physics calculations.

That makes it particularly useful for studying collisions.

First: What Is Momentum?

Momentum is defined as:

p = mv

where:

p = momentum
m = mass
v = velocity

The unit of momentum is:

kg m/s

The important word here is velocity, rather than simply speed.

Momentum has direction.

If we decide that motion towards the right is positive, then:

0.40 m/s to the right = +0.40 m/s

while:

0.40 m/s to the left = -0.40 m/s

Those signs become extremely important when gliders collide and rebound.

The Conservation of Momentum

For an isolated system:

total momentum before a collision = total momentum after the collision

For two objects:

m1u1 + m2u2 = m1v1 + m2v2

where:

u = velocity before the collision
v = velocity after the collision

The equation looks straightforward.

The experiment behind it is much more interesting.

Instead of simply inserting numbers into the formula, students can create the collision themselves and ask whether the equation actually works.

Experiment 1: The Classic Equal-Mass Collision

Start with two gliders of approximately equal mass.

Place one at rest.

Send the other towards it.

Suppose:

Mass of glider A = 0.25 kg

Velocity of A before collision = +0.80 m/s

Mass of glider B = 0.25 kg

Velocity of B before collision = 0 m/s

The initial momentum is therefore:

p = mv

p = 0.25 x 0.80

p = 0.20 kg m/s

Now allow them to collide using a system designed to give a reasonably elastic collision.

With two equal masses, something very satisfying may happen.

The first glider almost stops.

The second glider moves away at approximately the original velocity of the first.

It appears that the motion has been passed from one glider to the other.

The result will never be absolutely perfect in a real laboratory, but it can come remarkably close.

And that immediately raises a question:

Has the momentum been transferred from one object to another?

Yes — but the total momentum of the complete system has remained approximately constant.

PASCO Data Makes the Experiment Much More Powerful

This is where modern data collection can transform what would otherwise be a fairly traditional mechanics practical.

Velocity sensors, photogates or suitable electronic motion measurements can record what happens immediately before and after the collision.

Instead of simply seeing:

Glider A hits Glider B.

students can examine data showing the change.

Imagine seeing one velocity trace suddenly fall while the other suddenly rises.

The collision may have lasted only a tiny fraction of a second, but the computer has captured what happened.

That changes the conversation.

Rather than asking:

"What number did you measure?"

we can begin asking:

Why did the velocity change so rapidly?
Was momentum really conserved?
How close was the experiment to an ideal collision?
Where might the missing energy have gone?
How much difference did friction make?
What happens if the masses are changed?
What happens if both gliders are moving?

That is where a practical becomes an investigation.

Momentum Is Not the Same as Kinetic Energy

This distinction is one of the most important parts of studying collisions.

For an isolated system, total momentum is conserved.

But kinetic energy does not necessarily remain constant.

Kinetic energy is:

KE = 1/2 mv^2

Notice the squared velocity.

This produces very different behaviour from momentum.

And it allows us to divide collisions into different types.

Elastic Collisions

In an ideal elastic collision:

momentum is conserved

and

kinetic energy is conserved

Perfectly elastic collisions are idealisations, although some physical collisions can get reasonably close.

Air-track gliders with suitable spring or magnetic collision systems can provide a good approximation.

Suppose our 0.25 kg glider is travelling at 0.80 m/s.

Its kinetic energy is:

KE = 1/2 x 0.25 x 0.80^2

KE = 0.08 J

If that energy is effectively transferred to another identical glider, the kinetic energy afterwards should also be close to:

0.08 J

But real experiments rarely produce precisely the theoretical result.

That is not a failure.

That is where the science becomes interesting.

Inelastic Collisions

Now change the collision mechanism.

Instead of allowing the gliders to bounce apart, arrange for them to attach to one another.

Velcro is an obvious simple method.

The two gliders collide and continue moving together.

This is an example of a perfectly inelastic collision.

Momentum is still conserved.

Kinetic energy is not.

A Surprisingly Good Numerical Example

Return to our original experiment.

Glider A:

m = 0.25 kg
u = +0.80 m/s

Glider B:

m = 0.25 kg
u = 0 m/s

Initial momentum:

p = 0.25 x 0.80

p = 0.20 kg m/s

Now suppose they stick together.

Their combined mass becomes:

0.25 + 0.25 = 0.50 kg

Using conservation of momentum:

0.20 = 0.50v

Therefore:

v = 0.40 m/s

The two gliders should move away together at approximately 0.40 m/s.

That prediction can now be tested experimentally.

But What Happened to the Energy?

This is where the experiment becomes much more interesting.

Before the collision:

KE = 1/2 x 0.25 x 0.80^2

KE = 0.08 J

Afterwards:

KE = 1/2 x 0.50 x 0.40^2

KE = 0.04 J

We appear to have "lost":

0.08 - 0.04 = 0.04 J

Half the kinetic energy has disappeared.

Except, of course, it has not actually disappeared.

Energy is still conserved overall.

Some of that kinetic energy has been transferred into other forms.

It may become:

sound;
vibration;
thermal energy;
deformation of the collision surfaces;
internal movement within the gliders.

This is an important distinction:

Momentum can be conserved even when kinetic energy is not conserved.

Students sometimes find that surprising.

Where the Air Track Becomes an Investigation

The basic demonstration is excellent.

But I think the air track becomes much more educational when we stop giving students one collision to perform and instead allow them to change the conditions.

There are dozens of possibilities.

Investigation 1: Change the Mass

Add masses to one glider.

What happens when:

a light glider hits a heavy stationary glider?
a heavy glider hits a light stationary glider?
two unequal gliders move towards one another?

Students can make predictions before releasing anything.

Sometimes the results are counter-intuitive.

A very light object bouncing from a much heavier one behaves quite differently from a heavy object striking something light.

Investigation 2: Make Both Objects Move

Instead of having one stationary object, start both gliders moving.

Try:

both travelling in the same direction;
moving towards one another;
one travelling much faster than the other;
equal masses moving at equal speeds in opposite directions.

That final case is particularly interesting.

Suppose:

Glider A momentum = +0.20 kg m/s

Glider B momentum = -0.20 kg m/s

Total momentum is:

0 kg m/s

There is plenty of movement.

There is plenty of kinetic energy.

Yet the total momentum is zero.

That is a wonderful reminder that zero momentum does not mean nothing is moving.

Investigation 3: Elastic Versus Inelastic

Perform approximately the same collision twice.

First allow the gliders to rebound.

Then arrange for them to stick together.

Measure:

total momentum before;
total momentum after;
total kinetic energy before;
total kinetic energy after.

Students should discover something fundamental.

Momentum behaves similarly in both experiments.

Kinetic energy does not.

That is a much stronger way of learning the distinction than memorising a definition.

Investigation 4: How Elastic Is "Elastic"?

Real collisions lie on a spectrum.

We often describe a collision using the coefficient of restitution.

For a one-dimensional collision:

e = relative speed of separation / relative speed of approach

A perfectly elastic collision has:

e = 1

A perfectly inelastic collision in which the objects stick together has:

e = 0

Real collisions normally lie somewhere between these extremes.

This makes an excellent extension for stronger GCSE students or A-level students.

Rather than simply saying a collision is "elastic", ask:

How elastic is it?

Students can compare different collision materials.

For example:

spring bumpers;
magnets;
rubber;
foam;
Velcro.

Suddenly the experiment has become quantitative.

Try Predicting the Result Before the Collision

One improvement I particularly like is not showing students the experimental answer immediately.

Give them:

the two masses;
their starting velocities;
the collision type.

Then ask them to predict what will happen.

Will one glider stop?

Will both continue forwards?

Will one rebound?

If they stick together, what will their final velocity be?

Only then perform the collision.

Physics becomes much more satisfying when the apparatus appears to answer a question that has already been asked.

The Graph Is Often More Interesting Than the Final Number

With data capture it is tempting to look only at the values immediately before and after the collision.

But the shape of the graph can reveal much more.

Look at the velocity against time trace.

Before impact there should be a fairly steady region.

Then comes the collision.

The velocity changes dramatically.

Afterwards another relatively steady region appears.

That invites questions about the collision itself.

How long did it last?

Was the acceleration enormous?

Could we investigate force?

And that leads directly towards impulse.

From Momentum to Impulse

The change in momentum is called impulse.

Impulse = change in momentum

So:

J = delta p

It is also related to force:

J = F delta t

or, more accurately when force varies:

Impulse = area under a force-time graph

Now our collision experiment connects several important topics:

force → acceleration → impulse → momentum

If suitable force sensors are available, an even more sophisticated experiment becomes possible.

Measure the force during the collision.

Then compare:

area under the force-time graph

with:

change in momentum

Two apparently different measurements should give approximately the same answer.

That is a particularly elegant experiment.

Newton's Third Law Appears Too

Imagine placing force sensors so that the interaction forces between the two objects can be measured.

During the collision, glider A pushes glider B.

But glider B simultaneously pushes glider A.

Newton's Third Law predicts that these forces are equal in magnitude and opposite in direction.

If the data are displayed together, students can see the two force curves.

One positive.

One negative.

Almost mirror images of each other.

So a single air-track collision can connect:

momentum;
conservation laws;
kinetic energy;
impulse;
Newton's laws;
velocity;
acceleration;
experimental uncertainty.

That is remarkable for an event that may last only a fraction of a second.

Why Real Results Never Match Perfectly

One of the great benefits of doing this experimentally is discovering that physics does not produce perfectly neat numbers simply because the equation is correct.

Suppose you calculate:

Momentum before = 0.200 kg m/s

Momentum after = 0.193 kg m/s

Has the conservation of momentum failed?

Almost certainly not.

Instead we need to consider the experiment.

Possible sources of discrepancy include:

residual friction;
the track not being perfectly level;
uncertainty in velocity measurement;
uncertainty in mass;
glider rotation;
air resistance;
vibrations;
imperfect collision alignment;
external forces acting during the measurement period.

The more interesting question therefore becomes:

Is the difference greater than we would reasonably expect from experimental uncertainty?

That is much closer to the way real experimental science works.

Calculate the Percentage Difference

A simple comparison might be:

percentage difference = |p_after - p_before| / |p_before| x 100

If:

p_before = 0.200 kg m/s

and:

p_after = 0.193 kg m/s

then:

percentage difference = 0.007 / 0.200 x 100

percentage difference = 3.5%

Students can then ask whether 3.5% is reasonable for the apparatus being used.

Repeat the experiment several times and the discussion becomes even better.

Do Not Hide the Imperfections

It is tempting in school practical work to want results that reproduce the textbook exactly.

I think that misses something important.

If momentum before is 0.200 kg m/s and momentum after is 0.200 kg m/s every single time, I would actually become suspicious.

Real measurements contain uncertainty.

The educational value lies partly in discovering that the laws of physics emerge through experimental uncertainty rather than because the uncertainty somehow disappears.

Students should become comfortable saying:

"Our measurements support conservation of momentum within the uncertainty of the experiment."

That is a much more scientifically mature conclusion than:

"They were nearly the same, so momentum is conserved."

Take It Further: Can You Identify the Collision?

Give students only the velocity data.

Do not tell them how the collision was arranged.

Ask them to determine whether it was:

approximately elastic;
partially inelastic;
perfectly inelastic.

They must calculate both momentum and kinetic energy.

If momentum remains approximately constant but kinetic energy decreases, the collision is inelastic.

If momentum and kinetic energy are both approximately conserved, it is close to elastic.

If the two objects have the same final velocity, they have probably coupled together.

Now students are analysing evidence rather than following instructions.

A Collision Competition

Another enjoyable version is to turn the experiment into a prediction challenge.

Give students a target.

For example:

Can you arrange a collision so that Glider A stops after the collision?

Or:

Can you make the two gliders move away with equal speeds?

Or:

Can you produce a collision where the total momentum is zero before and after, despite both gliders moving?

Or:

Can you arrange the masses so that the lighter glider rebounds?

Students have to use the theory to design the experiment.

That reverses the usual practical lesson.

Instead of:

apparatus → measurements → equation

we get:

equation → prediction → apparatus → test

That is much closer to genuine scientific thinking.

From Air Tracks to the Real World

The apparatus may look artificial, but the principle certainly is not.

Momentum is enormously important in understanding:

vehicle collisions;
rockets;
recoil;
sports;
ball games;
particle physics;
spacecraft manoeuvres;
explosions;
railway wagons;
collisions between astronomical bodies.

Even catching a ball involves momentum.

A fast-moving ball has momentum. Your hands must change that momentum to zero.

If you move your hands backwards while catching it, you increase the stopping time.

Since:

F = delta p / delta t

increasing the stopping time reduces the average force.

The same idea lies behind:

airbags;
crumple zones;
crash mats;
helmets;
padded surfaces.

The small gliders on an air track are demonstrating physics that matters far beyond the laboratory.

One Apparatus, Many Levels of Physics

That is perhaps why I like the air track so much.

At GCSE level, the experiment can simply demonstrate:

p = mv

and:

total momentum before = total momentum after

At A-level, the same apparatus can investigate:

elastic and inelastic collisions;
kinetic energy changes;
impulse;
force-time graphs;
coefficients of restitution;
experimental uncertainty;
mathematical modelling.

And students who want to go further can begin exploring centre-of-mass frames and more sophisticated collision mechanics.

The apparatus has not changed.

Only the depth of the question has.

Conclusion: Conservation Laws You Can Actually See

Conservation of momentum can easily become another formula students memorise for an examination.

But an air track makes it physical.

Two objects move.

They collide.

Their individual momenta change dramatically.

Yet when we consider the complete system, something remarkable emerges.

The total momentum remains.

Sometimes the gliders bounce apart.

Sometimes they continue together.

Sometimes kinetic energy remains almost unchanged.

Sometimes a substantial fraction is transformed into other forms of energy.

And with electronic data capture, all of this can be turned into measurements and graphs almost immediately.

That is what makes this such a satisfying experiment.

We are not merely telling students that momentum is conserved.

We are giving them the opportunity to try to prove us wrong.

And when repeated collisions, different masses and different collision types continue to reveal the same underlying conservation law, the equation:

m1u1 + m2u2 = m1v1 + m2v2

stops being something written on a formula sheet.

It becomes a description of something they have actually watched happen.

25 August 2026