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Torque (article) Khan Academy
Bachelor of Science in Biology (BSBiol)
Saint Louis University (Philippines)
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Learn how to find the torque exerted by a force.
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What is torque?
Torque is a measure of the force that can cause an object to rotate about an axis. Just as force is what causes an object to accelerate in linear kinematics, torque is what causes an object to acquire angular acceleration.
Torque is a vector quantity. The direction of the torque vector depends on the direction of the force on the axis.
Anyone who has ever opened a door has an intuitive understanding of torque. When a person opens a door, they push on the side of the door farthest from the hinges. Pushing on the side closest to the hinges requires considerably more force. Although the work done is the same in both cases (the larger force would be applied over a smaller distance) people generally prefer to apply less force, hence the usual location of the door handle.
Torque, moments, and angular momentum
Science Physics library Torque and angular momentum Torque, moments, and angular momentum
Introduction to torque
Moments
Moments (part 2)
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Rotational version of Newton's second law
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Torque
Science Physics library Torque and angular momentum Torque, moments, and angular momentum
Opening a door with maximum torque.
Torque can be either or.
A is one which does not produce an angular acceleration. Someone pushing on a closed door is applying a static torque to the door because the door is not rotating about its hinges, despite the force applied. Someone pedaling a bicycle at constant speed is also applying a static torque because they are not accelerating.
The drive shaft in a racing car accelerating from the start line is carrying a because it must be producing an angular acceleration of the wheels given that the car is accelerating along the track.
The terminology used when describing torque can be confusing. Engineers sometimes use the term , or interchangeably with torque. The radius at which the force acts is sometimes called the.
momentum when no net torque
Angular momentum of an extended object
Ball hits rod angular momentum example
Cross product and torque
Direction of the torque vector found with the right-hand rule.
How is torque measured?
The SI unit for torque is the Newton-meter.
In imperial units, the Foot-pound is often used. This is confusing because colloquially the pound is sometimes used as a unit of mass and sometimes force. What is meant here is , the force due to earth gravity on a one-pound object. The magnitude of these units is often similar as.
Measuring a static torque in a non-rotating system is usually quite easy, and done by measuring a force. Given the length of the moment arm, the torque can be found directly. Measuring torque in a rotating system is considerably more difûcult. One method
works by measuring strain within the metal of a drive shaft which is transmitting torque and sending this information wirelessly.
What role does torque play in
rotaonal kinemacs?
In rotational kinematics, torque takes the place of force in linear kinematics. There is a direct equivalent to Newton’s 2ⁿᵈ law of motion (),
.
Here, is the angular acceleration. is the rotational inertia, a property of a rotating system which depends on the mass distribution of the system. The larger , the harder it is for an object to acquire angular acceleration. We derive this expression in our article on rotational inertia.
What is rotaonal equilibrium?
The concept of rotational equilibrium is an equivalent to Newton’s 1ˢᵗ law for a rotational system. An object which is not rotating remains not rotating unless acted on by an external torque. Similarly, an object rotating at constant angular velocity remains rotating unless acted on by an external torque.
There is considerable confusion between torque, power and energy. For example, the torque of an engine is sometimes incorrectly described as its 'turning power'.
Torque and energy have the same dimensions ( they can be written in the same fundamental units), but they are not a measure of the same thing. They differ in that torque is a vector quantity deûned only for a rotatable system.
Power however, can be calculated from torque if the rotational speed is known. In fact, the horsepower of an engine is not typically measured directly, but calculated from measured torque and rotational speed. The relationship is:
Along with horsepower, the peak torque produced by a vehicle engine is an important and commonly quoted speciûcation. Practically speaking, peak torque is relevant for generally describing how quickly a vehicle will accelerate and its ability to pull a load. Horsepower (relative to weight) on the other hand is more relevant to the maximum speed of a vehicle.
It is important to recognize that while maximum torque and horsepower are useful general speciûcations, they are of limited use when making calculations involving the overall motion of a vehicle. This is because in practice both vary as a
function of rotational speed. The general relationship can be and differs for different types of motor as shown in Figure 4.
Relationship of available torque to rotational speed for different sources.
How can we increase or decrease
torque?
It is often necessary to increase or decrease the torque produced by a motor to suit different applications. Recall that the length of a lever can increase or decrease the force on an object at the expense of the distance through which the lever must be pushed. Similarly, the torque produced by a motor can be increased or decreased through the use of. An increase in torque comes with a proportional decrease in rotational speed. The
achieve a useful speed when driving a wheel directly (unless one is cycling a penny-farthing).
Adjustable gearing is not typically required in vehicles powered by steam engines or electric motors. In both cases, high torque is available at low speeds and is relatively constant over a wide range of speeds.
Exercise 2a:
A gasoline engine producing of torque at a rotational speed of is used to drive a winch and lift a weight as shown in ûgure 6. The winch drum has a radius of 0 m and is driven from the engine via a 1:50 speed reduction gear. What mass could be raised with this setup? (Assume the winch is in rotational equilibrium, the mass is traveling up at constant velocity).
Engine-driven winch used to lift a mass (exercise 2).
Exercise 2b:
At what speed would the weight be traveling upward?
Data sources
Cyclist : Hansen, E, Smith G. Factors affecting cadence choice during submaximal cycling and cadence inüuence on performance. International Journal of Sports Physiology and Performance. March 2009; 4(1):3-17.
Diesel engine: Mercedes 250 CDI
Otto cycle engine: Mercedes E
Electric motor: Tesla Model S 85
Steam locomotive: 2-8-0 "Consolidation" Locomotive at 70% boiler capacity
Penny-farthing : Wikimedia Commons
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Torque (article) Khan Academy
Course: Bachelor of Science in Biology (BSBiol)
University: Saint Louis University (Philippines)
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