Newton's Cart Experiment: Demonstrating Reactive Propulsion and Newton's Laws

Newton's Cart is a compelling physics demonstration that gives a clear, hands-on picture of how a class of engines known as jet or reaction engines actually works. A small vehicle rolls forward with nothing pushing against it from outside, driven only by a jet of steam escaping in the opposite direction — a vivid illustration of Newton's Third Law and the conservation of momentum.

Newton's Cart: reaction motion shown by a simple experiment

The Newton's Cart experiment demonstrates propulsion through reaction forces: when steam and a cork are expelled in one direction, the cart moves the other way. It is a favourite in physics education because it turns an abstract law into something students can watch and measure. The same principle links directly to Newton's third law examples seen in everyday life, from a swimmer pushing off a wall to a rifle recoiling after firing.

Newton's experiment: how it was carried out

According to Isaac Newton's contemporaries, he built a small, lightweight cart and mounted on it a stand holding a test tube, with a little cup positioned beneath it. Newton poured water into the test tube and alcohol into the cup, plugged the mouth of the test tube with a cork, and set the tube at an angle in the stand. He then placed a piece of cotton in the alcohol and lit it.

Equipment and preparation of the cart

The classic apparatus for Newton's Cart is deliberately simple: a light-rolling cart, a vertical stand, a corked test tube of water, a small cup of alcohol as a burner, and a wad of cotton for a wick. The construction and assembly matter — the cart must roll freely with minimal friction so that even a modest reaction impulse produces visible motion. Modern classrooms often replace the open flame with safer sensor-equipped carts, but the layout is unchanged: a mass that can eject a jet, mounted on a low-friction chassis.

Step-by-step method for running the experiment

The data acquisition procedure for the demonstration follows a short, repeatable sequence:

1. Fill the test tube partway with water and seal it with a snug cork.
2. Mount the tube at an incline in the stand so the cork points toward the rear of the cart.
3. Place alcohol-soaked cotton in the cup below the tube and ignite it.
4. Wait one to two minutes for the water to boil and steam pressure to build.
5. Observe the cork ejecting and the cart recoiling; record the direction and, if instrumented, the velocity.

What happens to the cart: describing the motion

After a minute or two the water in the test tube boiled, and the pressure of the newly formed steam blew the cork clear. The cork shot out together with a jet of steam, and at that very instant Newton's Cart rolled off in the opposite direction — even though it rested against nothing and nothing seemed to push it. The motion is the reaction to the momentum carried away by the escaping steam and cork.

The objections of Newton's critics and their refutation

Newton's opponents argued that the cart rolled only because the jet of steam and the flying cork pushed against the surrounding air. Newton refuted this easily. Compared with the thin jet of steam and the little cork, the cart — loaded with its stand, cup and test tube — is very large. Its front face, pressing against the air, meets incomparably greater resistance than the slender jet of steam does, so if air were the cause the cart would resist far more than it gained.

The role of air and the resistance of the medium

Air hinders the cart far more than it helps it. In a vacuum the same cart would roll faster and farther than it does in air, because there would be no drag opposing its forward travel. No one, after all, would claim that a person jumping out of a boat pushes against the air and that this is what sets the boat moving backward — the boat recoils because of the momentum transferred, not because of the air. Reaction engines, unlike every other kind, need no external support: they carry their support within themselves, or rest upon themselves.

Newton's conclusion: the conservation of momentum

Newton drew this striking conclusion from his experiment: the momentum acquired by the cart is exactly equal to the momentum of the steam and cork. The cart has the larger mass and therefore gains the smaller speed; the cork and steam have the smaller mass but move proportionally faster, so the two products of mass and velocity balance.

The cork's momentum points one way and the cart's the other. Added together, the two momenta cancel to zero. In other words, the total momentum does not change: it was zero at the start, when cart and cork were at rest, and it remains zero at the end, when the cart and the cork (with the steam) move apart in opposite directions.

The link to Newton's Second and Third Laws

This principle later became known as the law of conservation of momentum, and it played an enormous role in the history of mechanics. In essence it is simply a consequence of Newton's second law, the fundamental law of dynamics. Newton teaches that a body's momentum can change only under the action of an external force. No external forces acted on Newton's Cart in his experiment, so its total momentum must stay constant — that is, equal to zero, since it was standing still. Treating the cart plus the ejected steam as one isolated system, the momentum of that whole system must remain constant, which is why the momentum gained by the cart and the momentum carried off by the steam are equal and oppositely directed.

Action–reaction force pairs

Newton's Third Law states that every action is met by an equal and opposite reaction, and Newton's Cart is a textbook action–reaction pair. The force pushing the steam and cork backward is matched, at the same instant, by an equal force pushing the cart forward. These matched forces act on different objects — one on the ejected mass, one on the cart — which is why they never cancel on a single body but instead set both in motion. The same paired-force logic explains a rope pull, a hand push against a wall, and a rocket lifting off.

The physics of reaction motion

Reaction motion arises whenever a system expels mass in one direction and is propelled in the other, without needing anything external to push against. Aristotle had once assumed motion required continuous contact with a medium; the Newton's Cart experiment shows the opposite, undermining that intuitive but incorrect view of how objects move.

How reaction engines work

A reaction engine works by throwing mass rearward at high speed and receiving forward thrust in return, exactly as the cart is thrust forward by escaping steam. Because the momentum carried away by the exhaust must be balanced by momentum gained by the engine, no ground, track, or air is required for propulsion. This is why such engines function in the vacuum of space, where wheels and propellers would be useless.

Modern applications of the principle

The reaction principle demonstrated by Newton's Cart underlies a wide range of modern technology and everyday devices:

• Rocket and jet propulsion — spacecraft and aircraft engines that expel exhaust to gain thrust.
• Classroom analogues such as a Fan Cart, a Fire Extinguisher Rocket, and balloon-powered vehicles.
• Recoil phenomena like a firing gun, a Dropped Slinky, or a person stepping off a small boat.
• Watercraft and pumps that push fluid one way to drive a vessel the other.

Calculations and measurements in the experiment

Turning Newton's Cart into a quantitative laboratory experiment lets students verify the conservation of momentum with real numbers rather than accepting it on trust. With sensors and graphing tools the demonstration becomes a full data-collection and analysis exercise suitable for high school and college physics curricula.

How to calculate the acceleration of the cart

Acceleration of the cart is found from Newton's Second Law in the form a = F/m, where F is the reaction force from the ejected steam and m is the cart's mass. Measured directly, acceleration is the change in velocity divided by the time over which it occurs. A Low-g Accelerometer or an Accelerometer built into a Wireless Smart Cart records this change, and a force-versus-time and acceleration-versus-time graph reveals the brief impulse at the moment of ejection.

Measuring velocity and momentum

Velocity is measured with photogate technology, which times the cart passing between two gates to compute speed, or with the encoder in a sensor cart. Momentum is then the product of the cart's measured mass and its velocity. Comparing the cart's momentum against the estimated momentum of the steam and cork lets students calculate a percent difference and judge how closely the experiment confirms momentum conservation.

Analysing the applied force and the cart's motion

A Force Sensor or Dual-Range Force Sensor captures the reaction force driving the cart, and a free-body diagram helps separate that thrust from friction and air resistance. Resolving forces into x, y and z components clarifies which act along the direction of travel. Spring Scales offer a simpler, low-cost route to force measurement for classes without electronic sensors, supporting spring scale experiments alongside the reaction demonstration.

Collecting and analysing experimental data

Data from sensor carts flow into software such as Capstone, DataStudio, or Excel, where a linear regression fit of force against acceleration yields the cart's mass as the slope. Plotting velocity and momentum over time makes the transfer of momentum between the cart and the ejected mass explicit, and the load-displacement style curve reading familiar from a Materials Testing System transfers directly to interpreting these motion graphs.

Newton's Cart in the school and university physics course

Newton's Cart fits naturally into mechanics education as an activity-based, hands-on demonstration of Newton's Laws of Motion. It supports conceptual understanding of the First, Second and Third Laws in a single setup, and it slots into science-curriculum units on force, friction, and momentum for both high school and introductory college classes.

Experiment variants and equipment options

Teachers can stage the reaction demonstration with a range of equipment configurations, from historical apparatus to fully instrumented modern carts:

• The classic steam-and-cork cart, cataloguing under demonstration reference codes such as 1H10.10.
• A PASCO Wireless Smart Cart or Go Direct Sensor Carts for wireless force, position, and acceleration data.
• A Dynamics Cart and Track System paired with a photogate for precise velocity work.
• Physics with Vernier and PASCO sensor bundles for guided lab activities.
• Related demonstrations — Atwood's Machine, Inertia Ball, Friction Battle, and Double Cone on Rails — to extend the mechanics unit.

Practical tasks for students

Student tasks turn Newton's Cart into project-based STEM learning: measure the cart's acceleration and predict it from F/m; calculate the momentum of the cart and compare it with the ejected mass; draw free-body diagrams of the cart during ejection; investigate how added mass changes the cart's final speed; and graph force versus time to identify the propulsion impulse. Each task reinforces the mass-and-acceleration relationship and the idea that forces come in matched, opposite pairs.

Safety precautions when running the experiment

The traditional Newton's Cart involves boiling water, alcohol, and an open flame, so it demands careful handling. Observe these precautions:

• Perform the demonstration only with adequate ventilation and a clear, non-flammable surface.
• Keep hands and clothing clear of the flame and the path of the ejected cork.
• Never over-tighten the cork or seal the tube fully — steam must be able to escape.
• Wear eye protection, since the cork leaves the tube at speed.
• Where possible, substitute a sensor-based cart to obtain the same momentum data without an open flame.