Questions tagged [torque]

This tag is for questions regarding to torque, which is a measure of the increase in angular momentum over time.

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121 votes
5 answers
19k views

Toilet paper dilemma

There are two ways to orient the toilet paper: "over" (left image), "under" (right image). Each has it's pros and cons. For some reason, it's always easier to tear off the paper ...
Monopole's user avatar
  • 3,454
68 votes
11 answers
72k views

Why is torque not measured in Joules?

Recently, I was doing my homework and I found out that Torque can be calculated using $\tau = rF$. This means the units of torque are Newton meters. Work & Energy are also measured in Newton ...
General Stubbs's user avatar
48 votes
6 answers
19k views

Apparent Violation of Newton's $3^{\text{rd}}$ Law and the Conservation of Momentum (and Angular Momentum) For a Pair of Charged Particles

Consider a system of the two identical positive point charges situated in free space (isolated from the influence of any other external fields) as shown in the attached diagram. Particle $1$ is at $(a,...
user avatar
45 votes
7 answers
21k views

Hanging a hammer from a table and a stick so that its midpoint is outside of the support of the table

I came across this pic on the internet today. At beginning I thought it is just not possible because the centre of mass is way off so gravity will generate torque making the stick and hammer fall. ...
Tanishq Jaiswal's user avatar
40 votes
8 answers
8k views

Is torque as fundamental a concept as force?

I used to think that torque and force were equally “fundamental”. In other words, my understanding was that we usually use Cartesian coordinates in many common problems because it is a convenient ...
StayOnTarget's user avatar
37 votes
6 answers
9k views

Is Nm the same unit of torque as mN?

A couple of days ago, I noticed that the torque unit used by my teachers is $mN$, and while reading on the internet it came to my notice that in all textbooks the official unit is $Nm$. I asked one ...
Abdullah Alhussni's user avatar
31 votes
7 answers
5k views

Why is it easier to handle a cup upside down on the finger tip?

If I try to handle a tumbler or cup on my fingertip (as shown in fig), it is quite hard to do so (and the cup falls most often). And when I did the same experiment but this time the cup is upside ...
Ankit's user avatar
  • 8,306
30 votes
2 answers
11k views

Why are arched bridges stronger than flat bridges?

To keep the question brief: in bridge design, why is the arch structure favoured compared to a simple flat one? In other words, how does the curved platform alter the force decomposition of the load ...
user929304's user avatar
  • 4,625
29 votes
4 answers
8k views

What causes a rotating object to rotate forever without external force—inertia, or something else?

Someone told me that it is not inertia, but I think it is inertia, because it will rotate forever. In my understanding, inertia is the constant motion of an object without external force. Am I wrong?
enbin's user avatar
  • 1,972
28 votes
8 answers
11k views

Why is there a size limitation on animals?

Why is there a size limitation on human/animal growth? Assuming the technology exists for man to grow to 200 feet high, it's pretty much a given that the stress on the skeletal structure and joints ...
SSJGodan's user avatar
  • 383
24 votes
7 answers
24k views

Why does the weighing balance restore when tilted and released

I'm talking about a Weighing Balance shown in the figure: Press & Hold on onside of the horizontal beam and then release it. It makes some oscillations and comes back to equilibrium like shown in ...
claws's user avatar
  • 7,255
23 votes
9 answers
3k views

Are force vectors members of a vector space?

Vectors in vector spaces depend only on their size and direction. Force vectors, for example, depend also on their location. Opposite force at different locations, for example, do not annihilate each ...
user4951's user avatar
  • 591
23 votes
5 answers
20k views

Why does torque point perpendicular to direction of the motion?

I have an intuition problem calculating torque using the cross product formula. As for example let the magnitude of the force be 50 lbs and length of the wrench be one foot and you are exerting ...
user avatar
23 votes
2 answers
3k views

How does a twisted piece of string/yarn wind back on itself? What kinds of forces are responsible for this?

I do a lot of fibrework (carding wool, spinning yarn, weaving, etc.) and I've noticed a funny behaviour that anything long and flexible, which can withstand significant torque exhibits. If I take a ...
Nicholas's user avatar
  • 113
20 votes
7 answers
5k views

Plank overhanging off a building — where does the normal force go?

Suppose there is a (uniform rectangular) plank of mass $m$ and total length $L$ that is on top of a building with some piece overhanging or jutting out past the building. Assume it isn't past the ...
Maximal Ideal's user avatar
20 votes
1 answer
1k views

If the tidal bulge on the earth speeds the moon up, how does the moon move to a higher orbit?

I understand that the moon causes a tidal bulge on Earth, and this tidal bulge moves slightly ahead of the moon due to Earth's rotation. When reading about why the moon is moving away from the Earth, ...
WillowRook's user avatar
19 votes
3 answers
7k views

Why does an unhinged body rotate about its centre of mass?

If a force is applied to a body which does not act through its centre of mass, it rotates about its centre of mass and not any other point. Why?
jerry's user avatar
  • 485
18 votes
3 answers
4k views

How does a lever arm increase force?

A lever and a fulcrum is able to to multiply the input force to the output force and trade moving distance for increased force. The same happens in automobile transmissions, taking advantage of high ...
Erik's user avatar
  • 347
18 votes
7 answers
30k views

What determines the direction of precession of a gyroscope?

I understand how torque mathematically causes a change to the direction of angular momentum, thus precessing the gyroscope. However, the direction, either clockwise or counterclockwise, of this ...
high schooler's user avatar
18 votes
5 answers
28k views

Why are bicycle pedal threads' handedness left on the left and right on the right?

I understand the reason that bicycle pedals are oppositely threaded on either side. What I don't understand is why it works because I'm missing something. Take the right pedal for example. It's ...
JustcallmeDrago's user avatar
17 votes
8 answers
7k views

Why is it possible to keep an object, say a book, horizontal while holding its corner?

When holding a book from its corner with two fingers in a pinched position, the fingers act as a sort of hinge, and the book is free to rotate about this corner only. The external forces acting on the ...
brainfreeze's user avatar
17 votes
2 answers
4k views

Why do speed skaters use their right hand to accelerate?

I was watching the speed skating event, and was wondering, why do the athletes use their right hand while accelerating? If they want to increase the moment of inertia, then they should move the hand ...
Saurabh Raje's user avatar
  • 1,128
16 votes
8 answers
5k views

Why is the concept of torque necessary?

In my physics textbook (“Physics for Scientists and Engineers” by Serway and Jewett) it says that “the cause of changes in the rotational motion of an object about some axis is measured by a quantity ...
Bruno Nowak's user avatar
15 votes
3 answers
1k views

Does the earth’s rotational angular velocity change?

This is what is written in The Feynman Lectures on Physics, Vol. 1 (ch.5) We now believe that, for various reasons, some days are longer than others, some days are shorter, and on the average the ...
Ankit's user avatar
  • 8,306
15 votes
6 answers
7k views

How do levers amplify forces?

This is really bothering me for a long time, because the math is easy to do, but it's still unintuitive for me. I understand the "law of the lever" and I can do the math and use the torques, or ...
fiftyeight's user avatar
  • 1,075
14 votes
3 answers
2k views

Can the value of friction force ever exceed value of applied force?

My teacher taught me that the value of friction force can never be greater than the applied force. But recently, when I was studying rotational motion, I got a dilemma… Suppose I made a stand (from ...
TPL's user avatar
  • 434
14 votes
3 answers
2k views

Is it possible to apply a torque without a moment arm?

In some statics problems, the question may say something like "a torque is applied about Point B". I've always assumed this was a simplification and the torque was created using a force and a moment ...
James's user avatar
  • 421
14 votes
4 answers
45k views

Torque vs Moment

I was wondering, why in Newtonian physics torque is called "torque" while in static mechanics they call it "moment"? I prefer by far the term "torque", for not only it sounds strong, but also ...
Severo Raz's user avatar
14 votes
6 answers
11k views

Torque direction meaning

I apologize if this question is dumb, but I've looked all over for a straightforward answer, and either I can't find one or either the terms are too complex for me to understand. I have only a ...
xxe's user avatar
  • 143
14 votes
2 answers
48k views

Ball Rolling Down An Inclined Plane - Where does the torque come from?

There is a ball rolling down an incline, with no slipping. If we consider the point of contact between the ball and the inclined plane to be the pivot point (for our torque calculations), then I have ...
user3904840's user avatar
13 votes
2 answers
2k views

Why does rotation occur? [closed]

So it sounds like a dumb question, as it is very intuitive why rotation occurs. However, can you give me a scientific explanation as to why whenever I exert a force on a body it tends to move, but ...
Prabhat's user avatar
  • 666
12 votes
8 answers
1k views

Why do object rotate at all?

So I realize I am asking a pretty obvious, intuitive question. All of us have witnessed object rotating in everyday life when we nudge/push them from the side (not from the centre of mass of the ...
TLo's user avatar
  • 813
11 votes
3 answers
3k views

Derivative of angular momentum in a rotating frame of reference

I keep seeing that when taking the time derivative of the angular momentum in a rotating reference frame, we get: $$\frac{d\vec{L}}{dt} = \vec{\tau} + \vec{\omega} \times \vec{L}$$ meaning the torque ...
Darkenin's user avatar
  • 998
11 votes
3 answers
12k views

Is torque independent of choice of the point of rotation?

Is torque or moment of a couple independent of the choice of point of rotation? Torque is given by $\tau =\vec r\times\vec F$. It depends on $\vec r$, which is the position vector from the axis of ...
pcforgeek's user avatar
  • 1,406
11 votes
4 answers
7k views

Which is the axis of rotation?

This should be simple, but it keeps bothering me. If a rigid body has no fixed axis, and a torque (defined relative to a point $A$) is applied, it will rotate around $A$. But often I can also ...
yarnamc's user avatar
  • 1,201
11 votes
7 answers
5k views

What is the difference when we measure torque/angular momentum about a point and about an axis?

When do we measure torque about an axis and when do we measure torque about a point? What's the difference between measuring torque about an axis or a point. I tried searching this on google but did ...
Infinity's user avatar
  • 111
10 votes
6 answers
7k views

Why does a rolling sphere slow down if work done by friction on it is zero?

When we push a sphere on a rough horizontal surface, it slows down because there is a net torque on it (by normal force and friction acting in opposite directions) and this causes its angular speed to ...
Mr. Wayne's user avatar
  • 343
10 votes
3 answers
2k views

Apparent contradiction between moment of inertia and Archimedes's law of the lever

Suppose you have a (nearly massless) lever you are using to lift something up. On the other end there is a $2\,\text{kg}$ object at $1\,\text{m}$ away from the pivot. Let's say I input some force on ...
Maximal Ideal's user avatar
10 votes
3 answers
12k views

Why if the torque equals zero measured from one point in space it equals zero measured from any other point?

I've heard it from two teachers and saw a task with a solution based on this assumption: If the net torque is zero when measured from one frame of reference, it is equal to zero in all other ...
neverneve's user avatar
  • 763
9 votes
5 answers
6k views

Why does jumping off a merry go round in the radial direction cause no change in rotation speed?

Suppose a man is standing on a merry-go-round which is rotating with some speed, if he jumps out in the radial direction, then the rotation speed of the merry-go-round is unaffected. But I don't ...
tryst with freedom's user avatar
9 votes
4 answers
8k views

Why does Torque exist?

Specifically, why does force increase with the moment arm? What about making a perpendicular force farther away from the axis of rotation increases it? How does the moment arm cause that?
BoddTaxter's user avatar
  • 2,858
9 votes
5 answers
15k views

How much force is required to propel a bicycle?

A friend of mine had an idea to attach a motor to a bicycle with the idea to be able to turn it on and have it maintain average bike speed, apparently 15-20 mph. Neither of us have taken physics ...
John Galt's user avatar
9 votes
3 answers
2k views

Can torque relations actually be *derived* from Newton's Laws, or is it something extra?

For a long time I have wondered if there is a way to show that the rotational analogs of Newton's Laws are a direct consequence of just those laws, or are we adding more to them? I understand ...
user55515's user avatar
  • 514
9 votes
4 answers
10k views

What causes this corkscrew-shaped object to rise up in a stream of water falling down?

A corkscrew-shaped object is floating in a tub of water. When water is poured into the tub from some height, the object starts rising up vertically in the stream of water. The water stream is falling ...
schhugani's user avatar
8 votes
7 answers
5k views

Why is torque a cross product?

If I'm not wrong, torque is perpendicular to both the radius and force i. e. It is along the axis of rotation. Questions that arise are- why do we consider the length between the axis/point of ...
Plato's user avatar
  • 129
8 votes
5 answers
518 views

Why is torque defined as $\vec{r} \times F$?

Here I cannot convince myself myself that it is units because the torque is defined to be in units of Newton meter is a reiteration of the law stated above. Why was it not $r^2 \times F$ or $r^3 \...
realanswers's user avatar
8 votes
4 answers
1k views

Paradox in applying Newton's second law [duplicate]

Suppose I apply two vertical but opposite forces with the same magnitude in a body like is shown in the picture: According to newton's second law, the center of mass shouldn't accelerate, since the ...
JLagana's user avatar
  • 307
8 votes
7 answers
20k views

Difference between torque and moment

What is the difference between torque and moment? I would like to see mathematical definitions for both quantities. I also do not prefer definitions like "It is the tendancy..../It is a measure of ......
Amr's user avatar
  • 562
8 votes
3 answers
2k views

Why do things spin?

Let's say I have 2 boxes, one of mass M and one of mass 2M. They are connected by a rigid rod. I drop them from the same height and see that due to the earth's gravity, they accelerate at the same ...
joshuaronis's user avatar
  • 3,045
8 votes
3 answers
319 views

Apparent violation of the law of conservation of angular momentum in the torques experienced by two interacting electric dipoles

Question Consider two dipoles $({p_1}\hat{i}$ and ${-p_2}\hat{j})$ kept in the $x-y$ plane at $(0,0)$ and $(d,0)$ respectively. Calculate the torque about the COM. Approach 1 Suppose we select the COM ...
satan 29's user avatar
  • 1,295

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