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144 votes
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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 ...
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110 votes
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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 ...
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78 votes
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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 ...
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50 votes
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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. ...
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  • 117k
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 ...
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  • 835
32 votes
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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 ...
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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 ...
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  • 47.2k
23 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 ...
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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 ...
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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 ...
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  • 33.9k
22 votes
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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 ...
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20 votes
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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 ...
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  • 35.6k
20 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 ...
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  • 9,227
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 ...
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  • 9,854
19 votes
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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 ...
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  • 1,543
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 ...
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  • 47.2k
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. ...
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  • 43.1k
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 ...
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  • 33.9k
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 ...
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  • 34.7k
15 votes
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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 ...
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  • 74.1k
15 votes
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Why does rotation occur?

The scientific explanation is easy: if the sum of all forces is not directly in line with the center of mass of the object, it will rotate, and friction forces are applied at the point of contact. ...
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  • 43.1k
15 votes

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

Maybe because when the cup is the right way up, it’s centre-of-mass is above the point on your finger meaning that as your finger tries to balance the cup any small motion will generate a torque about ...
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  • 25.6k
14 votes
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Angular momentum paradox with 2 identical gears

Here's the key question: what is supporting the two gears' axles? What happens when you turn that handle depends on your answer. If the two axles are connected by some floating frame, then when you ...
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14 votes

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

The forces $F_1$ and $F_2$ are not applied to the wheels. They're applied by the wheels to the road surface, which is presumably attached to a very, very massive self-gravitating object that doesn't ...
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  • 875
14 votes

Why is the concept of torque necessary?

In principle you could always use force and not use torque. But, the use of torque greatly simplifies the analysis of the motion of a system of particles. For example, for rotation of a rigid body ...
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  • 6,921
13 votes

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

In these cases it always helps to draw a diagram: The green vectors represent the force of gravity $w=mg$ (dashed) and its components along the inclined plane and perpendicular to it. The red forces ...
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  • 117k
13 votes
Accepted

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

Those other sources were probably referring to kilogram-force instead of Newtons. Given the constant conversion between mass and weight on Earth (i.e., $g = 9.8\,\textrm{m/s}^2$), mass and weight ...
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  • 21.7k
13 votes
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How does a twisted piece of string/yarn wind back on itself? What kinds of forces are responsible for this?

It is much easier to explain with a diagram than it is with a chart or equations. Imagine threading tiny weightless disks along your length of blue wool. Twist it and hold it stretched out ...
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