The Forces of the Wind

Clarification Statement: Examples could include that an unbalanced force on one side of a ball can make it start moving and that balanced forces pushing on a box from both sides will not produce any motion at all.


Assessment Boundary: Assessment is limited to one variable at a time: number, size, or direction of forces. Assessment does not include quantitative force size, only qualitative and relative. Assessment is limited to gravity being addressed as a force that pulls objects down.

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Forces and Interactions


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Fig. 1. This model, called Newton's Cradle, shows a ball on one side moving as a result of a force exerted from the motion of a ball on the other side. 

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An understanding of forces is important for describing how the motion of objects changes. 

An individual force is described by its strength and direction. The strengths of forces can be measured and their values compared. Interactions between objects can cause changes in one or both objects (Fig. 1).

All forces between objects arise from a few types of interactions: gravityelectromagnetism, and the strong and weak nuclear interactions. Buouyancy and friction are other common forces.

In 1687, Sir Isaac Newton (Fig. 2) proposed three laws of motion to explain how things move. The three laws state:


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Fig. 2. Sir Isaac Newton, depicted here in an oil painting, was a mathematician and physicist (January 4, 1643- March 31, 1727). 

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Image from Wikimedia Commons

  1. An object won't move, or will continue to move, in a straight line unless acted upon by a force.
  2. The acceleration (a) of an object is directly proportional to the net force (F) exerted and inversely proportional to the object's mass (m); F=ma.
  3. For every action, there is an equal and opposite reaction.

These three laws still make up the basis of classical mechanics and guide our understanding in forces and motion today.

What happens when a force is applied to an object depends not only on that force but also on all the other forces acting on that object. A static object typically has multiple forces acting on it, but they sum to zero. However, if the total (vector sum) force on an object is not zero, its motion will change. For any pair of interacting objects, the force exerted by the first object on the second object is equal in strength to the force that the second object exerts on the first but in the opposite direction (Newton’s third law).

The Power of Wind


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Fig. 3. An inactive wind farm on the Big Island of Hawai'i gives a glimpse into the complexities of storing wind energy.

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Image courtesy of Rebecca Stanek, Wikimedia Commons

Wind is an important natural phenomenon that occurs all over the globe. Wind is a source of renewable energy that helps to power cities, but storing it requires complex engineering and design. Today, we often think of wind power being harnessed and stored through the use of technology like wind turbines (Fig. 3).

And yet, even thousands of years ago, prior to advanced energy storage technologies, humans were harvesting energy from wind through technologies like sail boats. In fact, the earliest recorded use of wind energy appeared in images on an vase found in Egypt, from over 5,000 years ago!

The Science of Sailing

When a sailboat is motionless, the forces acting on the boat are in balance. So, the boat stays still. The two main forces acting on a stationary sailboat are gravity and buoyancy.

In order for the boat to move, the force of wind pushes on the sail and causes the boat to move. The forces are now unbalanced, which is noticeable by the boat's movement. The force of wind on the sail causing the boat to move is an example of Newtons third law of motion: every action has an equal and opposite reaction. 

However, the relationship between force and movement isn't always as simple as wind blowing directly behind the sail to move the boat forward. The shape of the sail is very important for moving the boat when wind is not directly behind where you want to go. The shape of the sail is similar to the shape of an airplane wing. The shape actually causes air moving past the sail to flow faster on one side and slower on the other. Bernoulli's Principle explains how the difference in pressure on either side of the sail creates a driving force from high to low pressure (Figs. 4 & 5). This pressure differential of the wind on opposite sides of the sail creates a push and pull effect.


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Fig. 4. Due to the shape of an airplane wing, air flow above and below it creates a pressure differential, where more pressure is exerted on the bottom of the wing than on the top, leading to upwards lift. 

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Image made by Emily Sesno

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Fig. 5. Similar to an airplane wing (Fig. 4), the shape of a sail allows a pressure differential to create "lift" in the horizontal direction. This simplified diaram shows the counteraction from the keel that allows the boat to have a forward momentum. 

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Image by Emily Sesno


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Fig. 6. A keel on the bottom of the boat helps balance the force from the wind to balance the sideways momentum, prevent it from capsizing, and transfer the force of wind to motion forward.

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Image courtesy of Wikimedia Commons

A sailboat also has a keel to help stabilize it and prevent it from capsizing, or flipping upside down, due to the force on the sails (Fig. 6). The keel counters the force of wind in the sails to allow the boat to move in a more forward direction of travel rather than sideways.



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Fig. 7. This "points-of-sail" diagram shows the wind arrow coming from the top of the image with the corresponding positions of the sail depending on the direction the boat is heading. The area of red dashed lines indicates the region where a sail will not catch any wind. This position is called "in irons" and the sails will remain loose. The letters indicate the names of sail positions: A. No Go Zone, B. Close Hauled, C. Beam Reach, D. Broad Reach, and E. Running.

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Image courtesy of Wikimedia Commons

Even with these various forces and principles at play, the position of the sail in relation to the wind is the most important factor to consider. Sailors will decide what direction they want to go and position their sails in the ideal placement for wind. A "points of sail" guide helps remind sailors of the best position for their sail—based on where the wind is coming from in relation to their direction of travel (Fig. 7).

For more detail on the science of sailing, check out this article from Physics Today.

Sailing in Hawaiʻi

Because of the location of the remote location of the Hawaiian Islands, sailing has been an integral part of transportation and discovery. The polynesians first arrived in Hawaiʻi 1,500 years ago, navigating only by the stars. Now, Hawai'i continues to honor the culture and important role that sailing has played through various voyaging canoes, such as the Hōkūle'a. To read more about this beautiful ship, click on the Special Feature below:


To learn even more about the polynesian voyaging canoes and hear from the voyaging experts that navigate them, check out two episodes of Voice of The Sea in the Special Feature page below:



Forces and Interactions Vocabulary

  • Bernoulli's principle: an increase in the speed of a fluid (including air) occurs simultaneously with a decrease in static pressure.
  • Buoyancy: the force that supports things in a liquid or gas. When a boat is floating in still water, the pressure of water on the boat below the waterline pushes upward, creating a buoyant force.
  • Electromagnetism: the interaction of electric currents or fields and magnetic fields.
  • Force: strength or energy as an attribute of physical action or movement.
  • Friction: the resistance that one surface or object encounters when moving over another.
  • Gravity: the force that attracts a body toward the center of the earth, or toward any other physical body having mass.
  • Newton's first law: An object won't move or continuesto move in a straight line unless acted upon by a force.
    "In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force"
  • Newton's second law: The acceleration of an object is directly proportional to the net force exerted and inversely proportional to the object's mass.
    "In an inertial frame of reference, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma."
  • Newton’s third law: For every action, there is an equal and opposite reaction.
    "When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body."
  • Nuclear: relating to the nucleus of an atom
  • Keel: a structure along the centerline at the bottom of a vessel's hull (boat bottom), in some vessels extended downward as a blade or ridge to increase stability.
  • Renewable Energy: Energy that is collected from naturally replenished sources on a human timescale, such as sunlight, wind, rain, tides, waves, and geothermal heat. 
  • Static: concerned with bodies at rest or forces in equilibrium
  • Vector: a quantity having direction as well as magnitude, especially as determining the position of one point in space relative to another.
  • Wind: The flow of gasses on a large scale, i.e. the movement of across the globe.


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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawai?i, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.