In a PWC, the impeller has three blades made of stainless steel and it's about 15cm (~5inches) in diameter. An impeller is like a propeller fitted completely within a pipe so it sucks water in at one end of the pipe and blows it out of the other end as a high-speed jet. In a PWC, the engine's job is to power the water pump and its impeller. In a car or a motorbike, the engine drives the wheels.It has a large 75 liter (20 gallon) fuel tank-to reduce the risk of running out of fuel in the middle of the ocean! Power is provided by a medium-sized gasoline engine fired by electric ignition (you switch on by turning a key).Ī large PWC might have a 1500cc, four-stroke, four-cylinder engine, which is roughly as big as you'd get in a subcompact car (a small hatchback) or a large motorbike.Water is sucked in through a large intake grate on the bottom of the craft.Here's a very simplified cutaway of what's going on inside a typical PWC: That's why PWCs need really powerful engines. To get the PWC moving quickly, the water jet has to exit at immense speed. (mass × velocity) of the water jet firing backward must be equal to the momentum of the craft (and its passengers) going forward, so Why does the jet need to exit at such high speed? A large PWC can weigh up to aboutĤ50kg (1000lbs-about as much as six adults), which is much more than the weight of the water shooting out of it.Ī law of physics called the conservation of momentum tells us that the momentum When you crank the throttle, the pump sucks in water through a grate underneath the craft and the impeller blasts it out of a hole at the back, so the force of the jet pushing backward (action) drives the whole craft forward (reaction). The key to a PWC is a small pump with a rotating part called an impeller. The whole of the lower section (colored black) sits beneath the water.Īction and reaction explains how a PWC works. Note also how much biggerĪ PWC looks when it's on land. Note the motorcycle handlebars and wing mirrors. Photo: A Sea-Doo Personal Water Craft (PWC) sitting on a trailer The forces don't cancel out because they act on different things. The action is a force acting backwards on the water the reaction is a force acting forwards on your body. The reaction is your body moving through the water. The action is you pulling back on the water. How come you go anywhere at all? The answer is that the action and the reaction act on different things. Now there's clearly an action force here (you pull backwards on the water),īut if there's an equal and opposite reaction force, why don't these two forces simply cancel out? Let's say you're swimming freestyle in the oceanĪnd you pull backwards on the water with your arms. People find the idea of action and reaction very confusing. Photo: Science in action: This Sea-Doo is using basic laws of physics (Newton's laws and the conservation of momentum) to propel itself through the water. In each case, the force of the hot gas rushingīackward from the engine hurls the rocket or airplane forward through The backward pulling force of yourĪrms (the "action") makes you go forward (the equal and oppositeĪction and reaction. "action") makes you go forward (the equal and opposite "reaction"). Is always an equal and opposite reaction (a force of the same size Sometimes see it written like this: for every action (or force), there Newton's third law is also called "action and reaction" and you
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