Questions tagged [torque]

The tendency of a force to rotate an object about an arbitrary axis.

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46
votes
10answers
43k 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 ...
37
votes
7answers
8k 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. ...
30
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6answers
12k 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,...
29
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2answers
6k views

Why are arc bridges stronger than flat bridges?

To keep the question brief: in bridge design, why is the arc structure favoured compared to a simple flat one? In other words, how does the curved platform alter the force decomposition of the load ...
28
votes
8answers
7k 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 ...
26
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3answers
5k 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?
19
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6answers
14k 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 ...
18
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4answers
8k 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 ...
16
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2answers
3k 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 ...
15
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6answers
22k 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 ...
14
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5answers
14k 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 ...
13
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3answers
1k 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 ...
13
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2answers
1k 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 ...
10
votes
3answers
6k 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 the terms are too complex for me to understand. I have only a rudimentary ...
10
votes
3answers
5k 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?
9
votes
4answers
25k 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 ...
8
votes
4answers
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 ...
8
votes
4answers
4k 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 ...
7
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5answers
3k views

How to choose origin in rotational problems to calculate torque?

We know that $\text{Torque} = r \times F$ and $r$ is the position vector. But the position vector depends on the choice of the coordinate system and in turn on the choice of origin. So, where should ...
7
votes
3answers
15k views

Which force makes a wheel roll down the hill? What causes friction?

A wheel rolling down a hill has two axis of rotation. One is where the center or mass is and the other is the point of contact with the surface which acts as a fulcrum. I was trying to understand how ...
7
votes
6answers
10k 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 ......
7
votes
2answers
798 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 ...
7
votes
3answers
5k 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 ...
7
votes
2answers
512 views

Is there an equivalent of a scalar potential for torques?

For a given scalar potential $V$, it is known that the corresponding force field $\mathbf{F}$ can be computed from $$ \mathbf{F} = -\nabla V $$ Suppose a rigid body is placed inside this potential....
7
votes
1answer
865 views

Wind turbine: Single large propeller or multiple small propellers?

Assuming smaller propellers would spin even in very low wind, whereas a large propeller would stall; Would it be efficient to use a bunch of propellers from axial fans geared to a single motor ...
7
votes
2answers
451 views

Why do airboats roll into turns?

I have read this question and the answers left me with almost the same question about airboats: Assuming that an airboat CoM is below the center of the horizontal thrust provided by the rudder. Along ...
6
votes
2answers
15k views

Unit of torque with radians?

Usually, the angular frequency $\omega$ is given in $\mathrm{1/s}$. I find it more consistent to give it in $\mathrm{rad/s}$. For the angular momentum $L$ is then given in $\mathrm{rad \cdot kg \cdot ...
6
votes
5answers
9k 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 ...
6
votes
3answers
7k 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 ...
6
votes
3answers
24k views

Why is less force required to open a door when we apply a force at a greater distance from the hinge?

I know that the magnitude of a torque increases with the distance from the axis of rotation. What is the reason of that : how can a force exert a greater torque when the force is applied at a ...
6
votes
3answers
1k views

Why do things spin?

Let's say I have 2 boxes, one of mass M and one of mass 2M. They are separated by a distance of 1 meter. I drop them from the same height and see that due to the earth's gravity, they accelerate at ...
6
votes
2answers
413 views

Balancing an orange on a spoon experiment

I did a little experiment with balancing an orange on a spoon. And i realized i don't really understand why i feel a stronger force under both fingers, the further my forefinger is from the orange (...
6
votes
1answer
32k 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 ...
6
votes
1answer
3k views

finding angular velocity and regular velocity when bouncing off a surface with friction

Take the game of pong as a simple example. When you hit the ball with a paddle that has a frictional surface, the ball will spin as well as change direction according to the coefficient of kinetic ...
6
votes
1answer
180 views

Why does this screwdriver roll in a curved trajectory?

I am trying my level best to give this screwdriver a horizontal push only, but it is going in a circular path. I thought of uneven friction due to non-uniform normal reaction...what do you think? I ...
6
votes
1answer
419 views

Does it require a torque to spin a rectangular plate around its diagonal?

If a rectangular plate is rotating aroud its diagonal, one can find its inertia tensor and see that the angular momentum vector is changing in time. Thus it should require a torque to keep spinning, ...
6
votes
1answer
5k views

What is rotational potential energy?

I don't understand the concept of rotational potential energy. During linear motion, when a force is applied, the work it does gets converted to kinetic energy and there is no change in the ...
6
votes
1answer
831 views

How does a lever magnify force? [duplicate]

I understand that energy is conserved when a force is applied to the end of a lever and magnified closer to the pivot point. However, I would like to know how it is the force is transferred and ...
6
votes
2answers
2k views

Conservation of angular momentum experiment

I've learned in that in this experiment: ...the skater will start rotating faster when she brings her arms in and there is no net torque acting on her. But what would happen to her angular momentum ...
5
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3answers
17k views

Proving angular momentum is conserved for a particle moving in a central force field $\vec F =\phi(r) \vec r$

A problem I am trying to work out is as follows: A particle moves in a force field given by $\vec F =\phi(r) \vec r$. Prove that the angular momentum of the particle about the origin is constant. ...
5
votes
3answers
2k views

Why is torque sometimes reported in kg m, instead of the usual N m?

On various websites I see torque expressed as $\rm kg\: m$, but I was always thought that torque is $\rm N\:m$ or $\rm kg\: m^2/s^2$. These are clearly not the same, so why are they called the same, ...
5
votes
4answers
1k views

What is a moment?

As a 8th grader I am still a newbie at science and I have problems understanding the forces that I can not imagine and one of these "forces" is the moment. We are told that, $$\text{moment}= Nm$$ ...
5
votes
2answers
12k views

Why is a clockwise moment negative by convention?

My physics book states that by convention a clockwise moment is said to be negative and an anti-clockwise moment is said to be positive. I'm curious about the origin of this convention. My thoughts: ...
5
votes
4answers
1k views

Meaning of the direction of the cross product

I was doing calculations with torque and then I came across something very confusing: I understand that the magnitude of the torque is given by product of the displacement(from the center of rotation)...
5
votes
1answer
411 views

Considering the theory of special relativity: Is torque still a vector?

Considering the theory of special relativity: Is torque still a vector? In classical mechanics it is easy: You have 3 axes and thus 3 planes. Every plane has its own torque so torque has 3 ...
5
votes
3answers
857 views

Electric engine transmission

If we compare a petrol engine and an electric engine car, it is sufficient for the electric engine car (e.g Tesla Roadster) just to have 1 gear (forward), whereas petrol engine can have up to 7. Why ...
5
votes
2answers
842 views

How to find the axis with minimum moment of inertia?

If a system of particles is given, in a 2D plane, with particles having masses $M_1$, $M_2$, $M_3, \ldots M_n$ and coordinates $(x_1,y_1)$, $(x_2,y_2)$, $(x_3,y_3), \ldots (x_n y_n)$, then how can one ...
5
votes
3answers
2k views

How can I relate linear and angular motion using a single formula? [closed]

I want to relate linear and angular motion using a single formula. Assume I have a 10m rod, and I apply a force of 5N on it, 2.5m away from the axis of rotation for 1s. How can I determine the ...
5
votes
3answers
2k views

Motion of space ship when thrust is off-center

I have this dilemma: Suppose you have a space ship somewhere in deep space, where there is no drag force or substantial gravity. If the ship has a single engine situated in such a way that the center ...
5
votes
3answers
143 views

Why does it get harder with time, to rotate a screw with a screwdriver?

A couple of forces act on the machine screw and create the turning effect. But after sometime it gets harder. So, net torque is decreasing, but why is it decreasing? Frictional force comes into action,...