DiscoverHover CURRICULUM GUIDE #11

© 2004 World Hovercraft Organization


Before taking a closer look at how torque, work, and power are involved in an engine, we need formal definitions of work and power. When we think of the words, “work” and “power”, many different meanings may come to mind. Work is often thought of as physical labor or something we get paid to do. Common meanings for power include the amount of energy something can produce, the electricity that we get out of electrical sockets, or a synonym for strength. According to physics, work and power have specific definitions, and we’re about to learn what those are.

When a force acts to move an object, work is done. This is written as

Work = Force · Distance force acts

When you push something, how much work you do on it depends both on how hard you push it and how far you push it. In the Imperial system, work is measured in foot pounds (ft lb). The equivalent unit in the SI system is the Newton meter (N m), also known as the Joule (J). This is named after James Prescott Joule, a scientist who lived in the 1800’s and made important discoveries in the field of thermodynamics, or the study of heat, work, and other forms of energy.

Example 1: (Using SI units)
You pull a hovercraft with a force of 50 N along a sandbar that is 5 m long. How much work is done on the hovercraft?

The work done is equal to the force applied times the distance it acts across.

Work = Force · Distance
Work = (50 N) · (5 m)
Work = 250 N m

Work = 250 J

In pulling the hovercraft along the sandbar, you performed 250 J [184.4 ft lb] of work on the hovercraft.


Example 2: (Using Imperial units)
Your hovercraft weighs 60 lb and is sitting on the floor. You need to lift it up onto two 3 ft high workhorses so the skirts can be changed. How much work is required to lift the craft?

In this case, we are lifting the craft up, so the force required is the weight of the craft. The calculation, however, is exactly the same.

Work = Force · Distance
Work = (60 lb) · (3 ft)

Work = 180 ft lb

  It will take 180 ft lb [244 J] of work to lift the hovercraft onto the horses.

Remember that torque can be thought of as a turning or twisting force. When we talk about work done by a torque, we multiply the torque by how much it turns, or the angle it rotates through.

Work = Torque · Angle torque acts through

To turn an object, you exert a torque on it. How much it turns is given by the angle it turns through. For example, if you tighten a screw so that the screw twists one full revolution (360°), you exerted a torque on that screw through an angle of 360°.

Before we can do some examples, we need to know what units the angles are in. We are most familiar with angles in terms of degrees. When doing calculations with angles, however, we need to use a unit called radians. 1 radian is equal to about 57.3°. Why would anyone want to make a new unit for angles which is equal to 57.3°? Look at the figure of the circle. It shows both an angle of 1° and of 1 rad (short for radian). You can see that the angles cut out triangular pieces of the circle. The length of the portion of the circle contained in that triangular section is called the arc length. It turns out when the angle is 1 rad, the arc length is equal to the radius of the circle! Another interesting fact is that 1 full revolution (360°) is equal to 2 radians. Half of a revolution (180°) is therefore equal to radians. Remember that is equal to 22/7.

Example 3:
It takes 10 ft lb [13.56 N m] of torque to turn a hovercraft propeller. If we turn the propeller so that it rotates 3 revolutions, how much work did we do on the propeller?

First we must determine the angle of rotation in the right units. This means converting 3 revolutions into radians. Remember that 1 revolution is equal to 2 radians, so 3 revolutions would be equal to 6 radians. Now we can calculate the work done.

Work = Torque · Angle
Work = (10 ft lb) · (6)
Work = (10 ft lb) · (6) · (22 / 7)

Work = 188.5 ft lb

 Turning the propeller 3 revolutions will require 188.5 ft lb [255.6 N m] of work.

Now we need a way to define power. Power is the rate at which work is performed, or the amount of work done per unit time.

Power = Work ÷ Time

Go back to the example where you pull the hovercraft along the sandbar. You could take 10 seconds to pull the craft that far, or you could pull really slowly and take 10 minutes. Either way, you’re still doing the same amount of work. Your power, however, is greater the faster you pull it. In the Imperial system, power is usually measured in horsepower (hp). In the metric system, Joules per second (J/s) has another name, the Watt (W). This was named after James Watt, an engineer who made important contributions to the development of the steam engine. Interestingly, James Watt invented the term horsepower. It is said that he measured a pony producing 22,000 foot pounds of work in a minute while pulling buckets of coal out of a mine. He thought that horses were about 50% more powerful than a pony, so he increased it to 33,000 foot pounds per minute, or 550 foot pounds per second. This is also equal to 746 Watts in SI units.

To get power from torque, multiply torque by the angle it acts through, then divide by the amount of time it takes. This angle per unit time is called an angular velocity. With engines, this is commonly called revolutions per minute (rpm). If we multiply torque by angular velocity, we get power. Often we know the horsepower of an engine, but want to know how much torque it can produce. We now have a simple formula for this.

Power = Torque · Angular velocity
- or -
Torque = Power ÷ Angular velocity

When doing these calculations, it is essential that all the values are in similar units. This often leads to some unit conversions to ensure they are similar.

Example 4:
The Discover Hover One’s engine operates at 12.5 horsepower (hp) [9325 W]. How much torque is being produced when the engine is running at 2500 rpm?

Before solving this problem, convert some units. Convert the angular velocity from rpm to rad /s. There are 2 rad in a revolution and 60 seconds in a minute, so we must multiply by 2 and divide by 60

(2500 revolution/min) · (2 rad/revolution) ÷ (60 sec/min) = 261.8 rad/s

Convert horsepower to either foot pounds per second or watts, depending on if we’re using Imperial or SI units. For this example, we’ll use Imperial units.

(12.5 hp) · (550 ft lb/s / hp) = 6875 ft lb/s

With power in foot pounds per second and angular velocity in radians per second, calculate the torque produced.

Torque = Power ÷ Angular velocity
Torque = (6875 ft lb/s) ÷ (261.8 rad/s)

Torque = 26.3 ft lb

When Discover Hover One’s engine runs at 2500 rpm, it will produce 26.3 ft lb [35.7 N m] of torque.