Water jet packs are an exciting way to see physics in action. Even though they look fun and easy to use, flying one is more challenging than it seems. To understand how these devices work, we need to explore some basic ideas from rocket science.
Rocket science might sound complicated, but its main idea is simple: Newton’s Third Law of Motion. This law states that for every action, there is an equal and opposite reaction. Imagine using a fire extinguisher while standing on a skateboard. When the extinguisher releases carbon dioxide, it pushes the skateboard backward. If the gas doesn’t come out fast enough, the skateboard won’t move because the force isn’t strong enough to overcome friction.
The force created by the expelled gas depends on two things: the mass flow rate (how much gas is released per second) and the speed of the gas. For example, during a space shuttle launch, gases exit at speeds of 3 to 4 km/s, with a mass flow rate of 9,000 kg/s. This creates a thrust of about 30 million Newtons, which is like the force of around 2 million fire extinguishers!
Astronauts face unique challenges during a launch. They lie down horizontally to reduce the effects of acceleration on their bodies. As the shuttle speeds up, the thrust stays the same, but the mass decreases, causing acceleration to increase. This can reach up to three times the force of gravity (3 g’s), which has a big impact on their bodies.
Water jet packs work differently from rockets because they don’t carry their own fuel. Instead, they use water from a lake, pumped through a hose. The water shoots out through nozzles, creating thrust by changing direction. This thrust, about 1,800 Newtons (like the force of 150 fire extinguishers), lets the user accelerate at about 1.5 g’s. Steering is done with hand movements, controlling how you go up, move forward, or turn.
Many people think you need a force to keep moving at a constant speed. Actually, balanced forces allow for constant motion. Another myth is that rockets need air to push against. In reality, they work by expelling propellant, so they can function even without air.
If you’re interested in using a jet pack, it’s important to stay calm and not overreact with the controls. Trust the jet pack’s abilities, as it can handle different situations without big adjustments. The feeling of water rushing past you while flying is thrilling and feels a lot like flying.
Water jet packs are a thrilling example of physics at work. Understanding the principles behind them not only makes the experience better but also shows the amazing connection between force, motion, and human creativity.
Conduct a simple experiment to observe Newton’s Third Law of Motion. Use a balloon and a piece of string. Inflate the balloon and tape it to a straw threaded on the string. Release the balloon and watch it move in the opposite direction of the air being expelled. Discuss how this relates to the operation of water jet packs.
Using the formula for thrust, $F = dot{m} cdot v$, where $dot{m}$ is the mass flow rate and $v$ is the velocity of the expelled fluid, calculate the thrust produced by a water jet pack. Assume a mass flow rate of 50 kg/s and a water velocity of 40 m/s. Discuss how this compares to the thrust of a rocket.
Create a simple simulation using a fan and a lightweight object like a paper cup. Use the fan to simulate the thrust and practice steering the object by changing the direction of the airflow. Discuss how hand movements control the direction of a water jet pack.
Research and present on how astronauts experience g-forces during a launch. Compare this to the acceleration experienced by a water jet pack user, which is about 1.5 g’s. Discuss how the body reacts to these forces and the importance of positioning during high acceleration.
In groups, list common misconceptions about motion and forces, such as the need for a force to maintain constant speed. Use examples from the article to explain why these are incorrect. Present your findings to the class to help reinforce correct scientific concepts.
Water – A transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth’s streams, lakes, and oceans, and the fluids of most living organisms. – In physics, the study of water’s properties helps us understand concepts like buoyancy and density.
Jet – A stream of fluid that is forced out of a small opening at high speed. – The jet of water from the hose demonstrated the principle of conservation of momentum.
Packs – In physics, this term can refer to a collection or group of items, often used in experiments or demonstrations. – The science teacher used packs of different materials to show how mass affects acceleration.
Physics – The branch of science concerned with the nature and properties of matter and energy. – Physics explains how forces like gravity and electromagnetism affect the motion of objects.
Thrust – A force applied to an object to move it in a specific direction, often used in the context of propulsion. – The rocket’s engines provide thrust to overcome Earth’s gravity and launch into space.
Acceleration – The rate of change of velocity of an object with respect to time. – According to Newton’s second law, acceleration is directly proportional to the net force acting on an object and inversely proportional to its mass, expressed as $a = frac{F}{m}$.
Force – An interaction that, when unopposed, will change the motion of an object. – The force applied to the car was enough to overcome friction and set it in motion.
Motion – The change in position of an object over time. – The study of motion involves understanding concepts like velocity, acceleration, and displacement.
Newtons – The unit of force in the International System of Units (SI), symbolized as N. – A force of 10 newtons was required to push the box across the floor.
Astronauts – People trained to travel and perform tasks in space. – Astronauts experience microgravity, which affects their bodies differently than the gravity on Earth.
