145 votes
Accepted

Toilet paper dilemma

I'll propose a theory, and I'll describe an experiment I did to test it. Both suggest that the "over" configuration is better, at least if the goal is to make the squares easier to rip off ...
Chiral Anomaly's user avatar
114 votes
Accepted

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
  • 11.9k
113 votes
Accepted

Is Nm the same unit of torque as mN?

Just like $2\times3=3\times2$, There is no difference between newton-meters and meter-newtons. They're two different ways of saying the same thing. Probably your book is trying to avoid confusion when ...
The Photon's user avatar
  • 27.6k
78 votes
Accepted

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

Take a look at this picture of a cup slightly out-of-balance : In case (A), generated torque is directed out of your reference axis and in case (B) - towards your reference axis. So in case A), you ...
Agnius Vasiliauskas's user avatar
50 votes
Accepted

Why are arched bridges stronger than flat bridges?

Fracture happens under tension - that is, when you pull on something hard enough, it rips. The key to understanding the arc design hinges on understanding that it lowers the maximum tensile force. ...
Floris's user avatar
  • 119k
47 votes

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

Is it inertia that a rotating object will rotate forever without external force? Someone told me that this is not inertia [...] Well, sort of - it’s somewhat correct to say it is inertia, and ...
GenlyAi's user avatar
  • 850
43 votes
Accepted

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
  • 100k
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
  • 506
33 votes
Accepted

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

The ruler is actually being supported by the handle of the hammer to provide two points of support so the downward force from the string lies between the two and the system balances. Moment on the ...
Lio Elbammalf's user avatar
32 votes

Why is the concept of torque necessary?

The concept of torque is not necessary as such. But it is useful. When things are useful, someone might eventually start to use them. Then, sadly if you will, everyone else will have to understand ...
Steeven's user avatar
  • 50.9k
25 votes
Accepted

Does the earth’s rotational angular velocity change?

The Earth is not a single rigid body, but consists of at least five separate regions which can move relative to one another. These are the crust (which is the region that we use to measure day length),...
gandalf61's user avatar
  • 52.9k
24 votes

Why does torque point perpendicular to direction of the motion?

To add to Steeven's answer and in particular his very pertinent statement: You can't define a vector direction as something that turns around. It may help you to understand that torque as a ...
Selene Routley's user avatar
24 votes
Accepted

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
Accepted

Why does torque point perpendicular to direction of the motion?

How can the bolt turn clockwise if the force is concentrated perpendicular to where it needs to turn? Because that force is perpendicular to the direction towards the rotation-centre. Not to the ...
Steeven's user avatar
  • 50.9k
23 votes

Are force vectors members of a vector space?

The answer is that force is a vector, you just have to be careful about what the vector space is. As a starting point, I think it would be useful to review what exactly a force is. Imagine we've ...
el duderino'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
22 votes

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

You can make two free body diagrams. One for the plank (with pink arrows), and one for the hammer (with blue arrows). Then examine if the forces can balance out. The reaction force on the plank from ...
John Alexiou's user avatar
  • 38.4k
20 votes
Accepted

In its free body diagram, there are only horizontal resistive forces. How can a car accelerate forward?

The friction forces on the bottom of the tyres should point in the direction of motion, not the opposite direction. If there were no friction the wheels would spin and the bottom of the tyres would ...
gandalf61's user avatar
  • 52.9k
20 votes
Accepted

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
19 votes

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

At its most basic, an object will rotate forever for the simple reason that there is no preferred direction in space. Emmy Noether's theorem of 1918 explains how various conservation laws arise from ...
hdhondt's user avatar
  • 10.9k
19 votes
Accepted

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

Take a stationary merry-go-round. Walk to the edge of the platform and jump radially away from the center. Does the merry-go-round start spinning? No, because you jumped at an angle that's parallel to ...
MichaelS's user avatar
  • 2,203
19 votes

Motivation for the definition of angular momentum?

Angular momentum is a conserved quantity$^1$. This is an experimentally confirmed reality, and the equations you talk about naturally follow from this fact since one of them defines this conserved ...
joseph h's user avatar
  • 29.4k
19 votes

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

No. You've discovered the Class 2 Lever, which places the load between the input force and the fulcrum. You have correctly calculated that the friction force is 4 times the input force. The reason for ...
g s's user avatar
  • 13.6k
18 votes

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

You are confusing the ideal case with the non-ideal case. Ideal scenario: No elastic deformation (so no internal work done), no air resistance, no uneven normal forces due to a flexible or soft ...
Steeven's user avatar
  • 50.9k
17 votes

Why is torque a cross product?

It doesn't have to be thought of as cross product. It's just very convenient to think of it that way, so we teach it first. Indeed, even when I apply it in my job, I think of it as a cross product. ...
Cort Ammon's user avatar
  • 48.5k
16 votes

Is torque as fundamental a concept as force?

In mechanics no. Torque is not a fundamental quantity. it's only job is to describe where in space a force is acting through (the line of action). Torque just describes a force at a distance. Given a ...
John Alexiou's user avatar
  • 38.4k
16 votes

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

I think the problem is using sloppy language and is implying that the person jumps by creating a force in the radial direction. So the "jump" is radial, but the path after leaving the ...
BowlOfRed's user avatar
  • 40.1k
16 votes

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

It's not like someone said "Ah ha, torque! What should the definition of torque be?" That doesn't make sense; you don't think up a term and then try to assign a definition. Instead, it was ...
BioPhysicist's user avatar
  • 56.4k
15 votes
Accepted

Why is a clockwise moment negative by convention?

We conventionally use a right-handed coordinate system in 3D physics, and in a right-handed system, positive moments are counterclockwise. You can show this starting from the definition of the cross ...
David Z's user avatar
  • 76.4k

Only top scored, non community-wiki answers of a minimum length are eligible