…and how does it relate to horsepower in cars, trucks, etc…?
Twisting force. Word definitions can be found here…
The rest is pretty self explainitory.
(It’s the Second definition, unless you really meant old french neckwear.)
In the words of Enzo Ferrari,
“Horsepower is what puts you into a wall. Torque is what puts you through it”
Yes, torque is force applied in an angular direction. Or, twisting force. It doesn’t necessarily need to be related to an axle, but in your case it is. Torque is a fundamental principle in most all static and dynamic structures courses.
It applies to cars in several ways, such as the amount of force the engine applies to the rotating drive shaft, or the amount of rotating force the axle applies to the wheels. I believe that if you’re asking based on statistics mentioned in your typical automobile spec sheet by a manufacturer or by a auto magazine it is considering the latter. Sometimes its an estimated value based on the power of the engine and the gear ratios, but good auto magazines will physically test it on a specialized apparatus, the name of which eludes me at the moment.
Waiting for someone who has more time to piss away to swing in with some cites.
particlewill is correct. To get the idea of twisting force think of this:
You now those little battery powered handheld fans that some people use in hot weather? Let’s say they spin at 200 RPM (I don’t know how fast they really spin). Grab the spindle with you thumb and forefinger and you can bring the whole thing to a stop practically immediately.
Now consider one of the drive shafts on an escalator. It spins at maybe 20 RPM (again…just guessing but it doesn’t matter for this illustration). Grab on to that shaft with your hand as hard as you can. It’ll either slip under your hand, break your wrist, dislocate your elbow or shoulder or flip you over the top (depending on how hard and how determindely you grab the shaft).
Our little fan has over 10x the RPM of the escalator shaft but the escalator has a LOT more torque despite its lower rotation speed.
And one of the Monkees had a very painful sounding name …
This is actually the best that anyone has explained torque vs horse power to me, nice one 
Touque can be considered as what happens when you apply a force to an object NOT in line with its center of mass.
Consider the mass on the frictionless surface oh-so-common in physics problems: If I apply a force on it, it will move in the direction of the force and accelerate proportionately. (the direction of motion is the same as the direction of the applied force.) This holds true if the force is applied “through” the center of mass of the object. In fact, when you draw out physics problems (as you always should) you usually draw the force acting on the center of mass.
Now Consider the force NOT in line with the center of mass: some proportion of of the force can be in line with the center of mass, depending on the angle of the applied force, and some proportion is not (it is perpendicular - remember deconstruction of vectors into x and y components). The “portion” of the force that is perpendicular to the center of mass causes the mass not to move in a straight line (as the in-line portion does) but causes it to spin. thus, the angle is important.
To determine the angle of application, draw a radius from the center of mass of the object to the point where the force is applied. The deconstructed component perpendicular to this line is the portion that will provice torque.
The amount of torque provided is proportional to the radius (also called lever arm). The further from the center of mass the perpendicular portion of applied, the more torque. Think about a door. Though we aren’t spinning it about its center of mass, we can see that application of a perpendicular force close to the hinge will not have the same effect as if we apply the same force further from the hinge.
Direct effect in the masculine world: You come to a door to a building and push on it to open it. You “instantly” realize that you are pushing on the wrong side from the hinges. What you should do from a physics standpoint is to stop pushing, move your hand further from the hinges and push again. What you actually do, however, is push harder. You never know, there may be girls watching.
Which gives us the third thing proportional to torque - the amount of applied force.
This gives us:
t = R*F(sin[theta]) where,
t is torque
R is radius (lever arm length)
F is the applied force
theta is the angle between the applied force and the lever arm.
sin is because it maxes when the two are perpendicular. Just be careful how you measure the angle; if you measure the compliment, you use cos.
In terms of automobiles, torque is generally a measure of the “ability” or “power” available to twist the drive shaft or axle.
there. more than you asked for; more than you need.
Excellent, Spritle. Now two more questions.
If we use a screwdriver, is ‘R’ the length of the screwdriver? And what effect does the radius of the screwdriver (at the slotted end) have on the torque?
Actually, ‘R’ must be the radius of the slotted end of the screwdriver. No?
Well, for the torque applied to the screw, the radius of the screw is “R”. But for the torque applied to the screw, “R” is the radius of the handle. Note that since the latter is larger than the former, but the torque is the same, the force in the latter must be smaller than in the former. Thus, the force you put on a screwdriver is less than the force the screw feels. Which is the whole point of a screwdriver.
So long as you understand that RPM is not linearly proportional to BHP.
Horsepower is a measure of power.
Power is a measure of work over time.
Work is a measure of force over distance.
Torque is a type of force.
RPM, while derived from the above, isn’t really dependant on it.
Um, actually, on a screwdriver, the torque you apply is 2rf, because you apply a “couple” to it.
Let’s first consider tightening a Chevrolet head bolt with a wrench.
When you’re using a wrench, you apply a force (in, say, the North direction) to the end of the wrench handle.
Unbalanced forces result in linear acceleration. If the force applied to the end of the wrench handle wasn’t counteracted by a force of equal magnitude but opposite direction, the wrench would be accelerating in a Northward direction.
The counterforce on the wrench (in the South direction) is provided by the bolt you’re tightening (and ultimately by the friction of your Goodyears against the pavement).
The torque you’re applying with the wrench is (moment arm) * force, where the moment arm is the wrench length (or “r”). The counterforce, since it acts through the center of the bolt and has no moment arm, does not result in a torque.
Now, using a screwdriver is like using two wrenches at once. Not only is your thumb, on one side of the screwdriver handle, applying a force, but your fingers, on the other side of the handle, are applying a force of the same magnitude in the opposite direction. These two forces, equal in magnitude but opposite in direction, separated by some distance (the diameter of the screwdriver handle)are a “Couple”.
So the total torque your hand applies to the screwdriver is your thumb force * radius of the handle, plus your finger force * radius of the handle. Since your thumb force is equal to your finger force (if it wasn’t, the screwdriver would be accelerating sideways), the total torque is 2radiusforce.
As I understand it, Horsepower is related to Torque and RPM. If you have the same torque, double the RPM’s, you get double the horesepower. Similarly, double the torque at the same RPM, double horsepower. Generally, torque will not stay constant in a vehicle when the RPM changes, that’s why RPM isn’t directly proportional to horsepower.
Your torque curve (torque measured at various RPM) is very important to determine the performance of a vehicle. A high, narrow curve will generally give great pulling power at low RPM, but not great acceleration, since the torque drops off quickly. Vehicles with great acceleration will have a broad torque curve, allowing for high torque at high RPM, thus very high horsepower.
Dammit, finally a thread in my area of expertise and all you engineers and physicists beat me to it, and with more eloquent explanations than I probably would’ve been able to come up with (I need to draw pictures for stuff like this). However, I still feel I can contribute. Here is an article that discusses force, torque, horsepower, and energy and ties them all together using neat little animated gifs. God I love that website.
Cheesesteak, of course they are all related in some way, but they are not directly dependant on one another. In cars the Horsepower is applied to the driveshaft, RPM is the rotation of the crankshaft, Torque is the force on the wheels. Yeah, in a very complex free body diagram they’ll all manage to be related, and when you plot each one you’ll see some correlation, but they are by no means “related” in the engineering sense. They are all also important in evaluating performance of a vehicle.
I am simply pointing out that Pushkin’s comment is misguided, however it is outside the scope of this OP.
Now, if you just consider the torque exerted by the crankshaft, the RPM of the crankshaft, you can determine the Horsepower output by the crankshaft. But those numbers you see in the car’s specs don’t illustrate these values. There are hundreds of other elements drawing from that horsepower long before the torque gets to the wheel.
Good explanation, Whack-a-Mole. Being a EE, I have always found that rotational torque is best “visualized” with an electrical analogy.
Let’s take a battery… The open-circuit battery voltage is analogous to no-load RPM, and the source resistance is analogous to torque. The lower the source resistance, the more “torque” you have.
For example: let’s say I have two 9V batteries in series (18V), and a car battery. Let’s put a variable load (variable resistor) on each. As I increase the load (i.e. decrease the resistance) on the 18V battery, the battery voltage decreases rather rapidly. This is because, as more current is drawn, there is a significant voltage drop across the source resistance, which itself is relatively high. Now do the same for the car battery… As I increase the load, the voltage drops very little. This is because the car battery has a very small source resistance. In other words, the car battery has a hell of a lot more “torque” than the 18V battery, and thus its voltage decreases very little as the load increases.