What is horsepower in Physics?
One horsepower is said to be equal to 33,000 foot-pounds of work per minute. That is said to be the power necessary to lift a total mass of 33,000 pounds one foot in one minute. This value was adopted by the Scottish engineer James Watt in the late 18th century after a few experiments with strong dray horses and is actually about 50 percent more than the rate that an average horse can sustain for a day of working. Here, we are going to discover a few more things about the topic of horsepower.
History of Horsepower
When sir Watt made his improvements to the steam engine then at that time he understandably wanted to convince people to buy it. If he said that his engine is expensive and had the same short-term output as a horse people wouldn't buy it. Instead, he compared it to the amount of work a horse could do in a day, giving it the output of ten horses instead of just one. Suddenly his invention became much more tempting. Although sir Watt may have "bent the truth" to make his engine more appealing that is he was not lying at all. While we can say that it is true that the output that is the maximum of a horse is around 15 horsepower. when we average the output of a horse over the course of a workday it ends up being around a horsepower. In a similar vein, we need to notice that the output of an average human over the course of a day is 100 watts which is where we get the term energy servant from.
Horsepower Standards and Types
By definition, one horsepower is said to be equivalent to 2,545 BTU, that is the British Thermal Units per hour. Another unit which is of power is the metric horsepower equals 4,500 kilogram-metres per minute or 32,549 foot-pounds per minute or 0.9863 horsepower.
There are many different standards and types of horsepower. There are two common definitions that are used today are the mechanical horsepower or imperial horsepower which is about 745.7 watts and the metric horsepower which is approximately 735.5 watts.
Measurement of one Horsepower
The engine power may be measured or it is estimated at several points in the transmission of the power from its generation to its application. A number of names are said to be used for the power developed at various stages in this process. But we can say that none is a clear indicator of either the measurement system or we can say the definition used.
In the case of an engine or dynamometer, the power is said to be measured at the flywheel of the engine.
In General, We Can Say that:
The nominal or the rated horsepower is derived from the size that is of the engine and the speed piston and is only accurate at a steam pressure of 48 kPa that is 7 psi.
Indicated or gross horsepower is the theoretical capability of the engine PLAN/ 33000 minus frictional losses within the engine which is bearing drag and rod and crankshaft windage losses, oil film drag, etc.
Brake or net crankshaft horsepower is delivered directly to and measured at the engine's crankshaft and after that, we need to minus frictional losses in the transmission by bearing gears and the oil drag and the windage, etc..
Use of Horsepower
The term that is horsepower is said to be a unit of power. Power generally describes how fast energy is exchanged, the use of energy is divided by how long it takes to use that energy. Therefore we can say that the measurement of horsepower generally refers to what the sustained output of an engine is.
Limitations of Horsepower
Although we can say that it may seem safe to assume that one horsepower is the output a horse is capable of creating at any one time, that is incorrect. The maximum output of a human is a bit more than a single horsepower. For extreme athletes, this output can be even higher with Tour de France riders outputting around 1.2 horsepower for around 15 seconds and just under 0.9 horsepower that is for a minute.
FAQs on What is horsepower in Physics?
1. What is horsepower (hp) in Physics?
In Physics, horsepower (hp) is a non-SI unit of power, which is defined as the rate at which work is done. It was originally established by the inventor James Watt to compare the power output of steam engines with that of draft horses. One mechanical horsepower is defined as the power required to lift 550 pounds by one foot in one second.
2. How is horsepower related to watts, the SI unit of power?
Horsepower is directly convertible to watts, the standard SI unit of power. The standard conversion used in physics is: 1 mechanical horsepower (hp) is approximately equal to 746 watts (W). This conversion is crucial for standardising calculations and comparing the power of devices rated in different units.
3. What is the fundamental difference between horsepower and torque?
The primary difference lies in what they measure. Torque is a measure of rotational force—the 'twisting' ability of an engine. In contrast, horsepower is a measure of the rate at which that force can be applied over time (power). In simple terms, torque gets the work started (like accelerating a car from a stop), while horsepower determines how fast that work can be done (like maintaining a high speed).
4. Where is the unit horsepower commonly used today?
Despite being a non-SI unit, horsepower is still widely used in specific industries due to historical convention. Its most common applications include:
- The automotive industry to rate the power output of car, truck, and motorcycle engines.
- Measuring the power of electric motors, pumps, and other industrial machinery.
- Aviation, particularly for rating piston and turboprop engines.
5. What is the formula used to calculate horsepower from engine parameters?
For engines, horsepower is typically calculated from torque and rotational speed (RPM). The most common formula in the imperial system is:
Horsepower (hp) = (Torque × RPM) / 5252
In this formula, Torque is measured in pound-feet (lb-ft) and RPM stands for Revolutions Per Minute. This equation shows the direct relationship between the engine's twisting force (torque) and its speed (RPM).
6. Is one horsepower really equal to the power of a single horse?
This is a common misconception. One horsepower is not the exact power output of one horse. James Watt's measurement was an estimate and a conservative one at that, designed for marketing his steam engines. A real horse can produce a peak power of nearly 15 hp for a very short duration, while its sustained power over a full working day is actually less than 1 hp.
7. Are there different types of horsepower?
Yes, it's important to distinguish between the main types of horsepower, as their values differ slightly:
- Mechanical Horsepower (hp): This is the imperial unit, equal to approximately 746 watts. It is the standard in the US and UK.
- Metric Horsepower (PS or cv): Used in Europe and Asia, one metric horsepower is defined as the power to raise 75 kilograms by one meter in one second. It is equal to approximately 735.5 watts, making it slightly less than mechanical horsepower.
8. In which CBSE classes is the concept of horsepower relevant?
The foundational concepts of Work, Energy, and Power are introduced in the CBSE Class 9 Science syllabus. The concept of power and its units, including the watt and the idea of horsepower as a practical unit, are discussed here. The topic is revisited with more mathematical depth and complex problems in the CBSE Class 11 Physics syllabus, within the chapter 'Work, Energy and Power'.
