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

Since contact forces act on the axis of rotation, they don't exert any torque. mg would exert a torque unless the book's center of gravity is directly above or below the "hinge". You are ...
AlphaLife's user avatar
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43 votes
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If the tidal bulge on the earth speeds the moon up, how does the moon move to a higher orbit?

To understand this, let’s start with a simpler example of orbital mechanics. Suppose we have a rocket in a circular orbit that wishes to transfer to a higher circular orbit. This proceeds in the ...
Dale's user avatar
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37 votes

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

Adding to Chemomechanics' answer, the resulting forces is dependent also on the plank's geometry and material. Here's the behaviour of a particular material and geometry of a plank. Fine, it's ...
Passer By's user avatar
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24 votes
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Apparent contradiction between moment of inertia and Archimedes's law of the lever

You are making an interesting subtle version of the confusion between mass and weight. Archimedes's law of the lever is equivalently a statement about torques. It is not about $$(2\text{ kg})(1\text m)...
naturallyInconsistent's user avatar
23 votes

How does a lever arm increase force?

The "extra" force is generated by the fulcrum. Consider the following setup: We load a seesaw with a weight of 1 and 3. The fulcrum has to supply a force of 4 in the opposite direction, ...
AccidentalTaylorExpansion's user avatar
20 votes
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Plank overhanging off a building — where does the normal force go?

Of the existing answers, one suggests that the reaction force can be represented by two forces, whereas the other says that the force is a distributed force. These aren't incorrect, but they aren't ...
Chemomechanics's user avatar
15 votes

How does a lever arm increase force?

You can think through the mechanics of this phenomenon with a bendy lever. Suppose you press down at one end that is 3 m from the pivot and the other end is 1 m from the pivot in the other direction. ...
user21820's user avatar
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13 votes

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

It is really a distribution of small force pieces across the entire plank-roof surface. But it can be simplified as two forces, one at the edge of the building, and the other at the other end of the ...
naturallyInconsistent's user avatar
13 votes

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

If you consider the static friction involved it is possible to achieve balance. By contrast, trying to do this with a block of ice would be almost impossible. The net squeezing forces from the top $...
John Alexiou's user avatar
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10 votes

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

For vertical equilibrium the normal reaction force must equal the weight of the plank. But that force is distributed over the length $L_2$ varying linearly from the ledge of the building to the other ...
Bob D's user avatar
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7 votes

Like force, does the rotational effect of force/torque require some finite time to travel through a real body?

When you start to make a rigid body more "real," the usual approach is to break it up into a bunch of points which convey forces with one another. Atoms are pretty darn close to a bunch of ...
Cort Ammon's user avatar
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7 votes

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

Interesting question, I actually had to take a book to empirically check my answer. The axis of rotation is defined by where the finger on the bottom of the book (the side nearer to the ground) ...
Koschi's user avatar
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7 votes

Confusion In rotational dynamics

The axis of rotation is arbitrary. But generally speaking, if you pick a random point, the result will be a combination of rotation and translation. Let's consider a linearly moving particle to show ...
AccidentalTaylorExpansion's user avatar
6 votes

Weight and rate of movement

Short conceptual answer (which seems to be the desired level): You are correct that objects generally accelerate at the same rate due to gravity as the mass that causes their weight famously cancels ...
JohnA.'s user avatar
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6 votes

Like force, does the rotational effect of force/torque require some finite time to travel through a real body?

What you are describing is the propagation of stress waves inside an elastic body. The equations that govern a body's internal strains and stresses are agnostic to how the stresses arise from ...
John Alexiou's user avatar
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5 votes
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Is the position vector an axial vector?

Now, if one of the requirements to be defined as a pseudovector is to be origin dependent This is false. For the magnetic field $\vec B$, which is a pseudovector, we have the Biot-Savart law That ...
naturallyInconsistent's user avatar
5 votes

What does the geometric product between displacement (or maybe position) and force vectors mean?

You want to read David Hestenes paper New Tools for Computational Geometry and Rejuvenation of Screw Theory. In that paper, Hestenes uses geometric algebra to unify linear and angular quantities in ...
Nullius in Verba's user avatar
5 votes

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

The key to this situation is the fact that the finger does not apply a force only at the axis of rotation. It actually provides force over an area, specifically the area of the pad of your finger. ...
Cort Ammon's user avatar
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4 votes

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

As other answers have said, the normal force is distributed along the length of the plank, and it is greater near the edge of the building than at the fan end of the plank. Without making additional ...
Peter's user avatar
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4 votes

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

In essence there are three forces acting on a book as shown below. There are various ways of analysing the situation but one way is to regard the thumb as the pivot and then you have a balance "...
Farcher's user avatar
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4 votes

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

Why is it possible to keep a (reasonably sized) book horizontal while holding from its corner? Friction. The key point is to realise that you are typically holding the book between a finger and a ...
gandalf61's user avatar
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4 votes

Confusion In rotational dynamics

The key issue is that rate of change of angular momentum of a body about a point P is equal to the torque about P only if one of the following is true: the point P is at rest the point P is the ...
mike stone's user avatar
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3 votes

Convention or reality?

The direction of torque (which, incidentally, is not really a vector but a pseudovector) is a matter of convention. It originates from the fact that we use the right-hand rule to define the direction ...
gandalf61's user avatar
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3 votes

How many components does the net force have on a rotating object?

The idea is that, any vector in space (e.g. Force) has 3 degrees of freedom to it, and you can choose an arbitrary coordinate to break it down. If the object is rotating around an axis, breaking down ...
Lynnx's user avatar
  • 168
3 votes

Weight and rate of movement

In Newtonian physics (which is very accurate for this sort of set-up) the mass of a body has two roles: doubling the mass (a) doubles the pull of the Earth on the object (gravitational role of mass), ...
Philip Wood's user avatar
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3 votes
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Confusion In rotational dynamics

The relationship between torque and rotational acceleration is depended on the moment of inertia. If you calculate about a different axis, you will get a different moment, and thus a different torque....
Cort Ammon's user avatar
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2 votes

Combined torque of torques at different points

Torques are not "with respect to a point". The moment about any point is independent of the point. The resultant torque is just the vector sum of all the torques.
mike stone's user avatar
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2 votes

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

Let's set aside lifting for now, and only consider static systems where the force applied to the lever is exactly enough to counteract gravity. Your confusion is that the torque is the same in each ...
Vaelus's user avatar
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2 votes

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

This really seems to be a fake, although there are not enough details given to be 100% certain. The simple intuition they are playing with is strong...if the stick is moving with the fluid, there is ...
ganv's user avatar
  • 21
2 votes
Accepted

What forces are responsible for the circular motion of the center of mass of a gyroscope-like setup in precession?

You've argued that a centripetal force is required to maintain the motion of the center of mass (CoM) about the pivot. The most straightforward solution is that this would need to be applied by the ...
FTT's user avatar
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