Aiming a hosepipe, intuition told me a 45-degree angle would maximise the distance travelled by the water, as it would for a projectile. But when I did this, the water actually went a shorter distance. Why?
Graham Smith
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Melbourne, Victoria, Australia
Isaac Newton used calculus to prove that, in the absence of air resistance, a fired cannonball travels in a symmetrical arch shape, or parabolic path, and he showed that maximum range is achieved by launching the ball at a 45-degree angle. As a cannonball has a very high momentum and experiences a relatively low air resistance, artillery crews have used this principle for 300 years.
But for water coming from a hosepipe, the atmosphere can’t be ignored because the stream of water quickly breaks up into droplets that encounter higher levels of air resistance relative to their momentum, compared with a cannonball. This slows the droplets down quickly. What’s more, water droplets at the front generate turbulence that affects the paths of droplets behind them.
A hosepipe should therefore be held at an angle of less than 45 degrees to allow the water to cover as much horizontal distance as possible before the velocity of the water is too degraded.
This is evident during water fights with my grandchildren, where I can achieve maximum water-spraying range when I hold the hosepipe at an angle of about 20 degrees. When they are hiding behind a fence, I am able to hit them with a near-vertical stream of water if I hold the hosepipe at a 60-degree angle.
Richard Swifte
Darmstadt, Germany
School pupils learn in their physics classes that launching a projectile at a 45-degree angle maximises the horizontal distance travelled by the propelled object, according to equations of motion. But this ignores an importance factor: air resistance.
Taking drag into account vastly complicates the calculations, since the distance travelled by an object now depends on other variables, such as the direction and strength of the wind, as well as the weight and shape of the projectile.
While ammunition from a tank is aerodynamic and should be fired at an angle of about 42 degrees to achieve maximum range, the optimal angle would be less for increasingly light objects, such as the jet or spray of water from a hosepipe.
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