This tag is for the classical concept of forces, i.e. the quantities causing an acceleration of a body. It expands to the strong/electroweak force only insofar as they act comparable to ‘classical’ forces. Use [tag:particle-physics] for decay channels due to forces and [tag:newtonian-mechanics] or ...
46
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7answers
3k views
Does juggling balls reduce the total weight of the juggler and balls?
A friend offered me a brain teaser to which the solution involves a $195$ pound man juggling two $3$-pound balls to traverse a bridge having a maximum capacity of only $200$ pounds. He explained that ...
19
votes
3answers
518 views
Is gecko-like friction Coulombic? What is the highest known Coulombic $\mu_s$ for any combination of surfaces?
Materials with large coefficients of static friction would be cool and useful. Rubber on rough surfaces typically has $\mu_s\sim1-2$. When people talk about examples with very high friction, often ...
16
votes
4answers
531 views
Particles for all forces: how do they know where to go, and what to avoid?
Here's an intuitive problem which I can't get around, can someone please explain it?
Consider a proton P and an electron E moving through the electromagnetic field (or other particles for other ...
16
votes
2answers
742 views
Why can't Humans run any faster?
If you wanted to at least semi-realistically model the key components of Human running, what are the factors that determine the top running speed of an individual? The primary things to consider would ...
15
votes
8answers
5k views
How exactly does curved space-time describe the force of gravity?
I understand that people explain (in layman's terms at least) that the presence of mass "warps" space-time geometry, and this causes gravity. I have also of course heard the analogy of a blanket or ...
14
votes
3answers
603 views
Why doesn't a fly fall off the wall?
Pretty simple question, but not an obvious answer at least not to me. I mean you can't just place a dead fly on the wall and expect it to stay there, he will fall off due to gravity. At first I ...
13
votes
5answers
4k views
Is there an equation for the strong nuclear force?
The equation describing the force due to gravity is $$F = G \frac{m_1 m_2}{r^2}.$$ Similarly the force due to the electrostatic force is $$F = k \frac{q_1 q_2}{r^2}.$$
Is there a similar equation ...
13
votes
2answers
1k views
Did the researchers at Fermilab find a fifth force?
Please consider the publication
Invariant Mass Distribution of Jet Pairs Produced in Association with a W boson in $p\bar{p}$ Collisions at $\sqrt{s} = 1.96$ TeV
by the CDF-Collaboration, ...
13
votes
7answers
2k views
What does it mean to say “Gravity is the weakest of the forces”?
I can understand that on small scales (within an atom/molecule), the other forces are much stronger, but on larger scales, it seems that gravity is a far stronger force; e.g. planets are held to the ...
12
votes
1answer
376 views
Do modern Formula One cars produce enough down-force to drive upside-down?
For example, if they were driving at top speed through a long tunnel, could they transition to and stay on the ceiling?
12
votes
2answers
1k views
Why isn't Higgs coupling considered a fifth fundamental force?
When I first learned about the four fundamental forces of nature, I assumed that they were just the only four kind of interactions there were. But after learning a little field theory, there are many ...
12
votes
2answers
821 views
Why do ships lean to the outside, but boats lean to the inside of a turn?
Small vessels generally lean into a turn, whereas big vessels lean out.
Why do ships lean to the outside, but boats lean to the inside of a turn?
12
votes
4answers
874 views
How to sail downwind faster than the wind?
Recently a group set a record for sailing a wind-powered land vehicle directly down wind, and a speed faster than wind speed. Wikipedia has a page talking about it, but it doesn't explain exactly how ...
10
votes
5answers
528 views
Why do we still need to think of gravity as a force?
Firstly I think shades of this question have appeared elsewhere (like here, or here). Hopefully mine is a slightly different take on it. If I'm just being thick please correct me.
We always hear ...
10
votes
4answers
800 views
Helicopter in an Elevator
You buy one of those remote control toy helicopters. You bring it into an elevator. The elevator goes up. Does the helicopter hit the floor or does the floor of the elevator push the air up into the ...
9
votes
1answer
209 views
Can a fly pierce itself onto a cactus needle?
Somebody on reddit posted a ridiculous picture today of a fly pierced onto a needle of a cactus: http://www.reddit.com/r/pics/comments/xarue/what_are_the_odds_of_this_accident/
Whilst the OP claims ...
8
votes
9answers
6k views
What's the core difference between the electric and magnetic forces?
I require only a simple answer. One sentence is enough... (It's for high school physics)
8
votes
3answers
364 views
Whether $m$ in $E=mc^{2}$ and $F=ma$ are both relativistic mass?
I know that $m$ in $E=mc^{2}$ is the relativistic mass, but can $m$ in $F=ma$ can also be relativistic? If the answer is yes, then can you tell me whether this equation is valid $E=\frac{F}{a}c^{2}$? ...
8
votes
2answers
245 views
What IS Color Charge?
This question has been asked twice already, with very detailed answers. After reading those answers, I am left with one more question: what is color charge?
It has nothing to do with colored light, ...
7
votes
1answer
800 views
Are all central forces conservative? Wikipedia must be wrong
It might be just a simple definition problem but I learned in class that a central force does not necessarily need to be conservative and the German Wikipedia says so too. However, the English ...
7
votes
4answers
664 views
How many fundamental forces could there be?
We’re told that ‘all forces are gauge forces’. The process seems to start with the Lagrangian corresponding to a particle-type, then the application of a local gauge symmetry leading to the emergence ...
7
votes
4answers
5k views
Can the coefficient of static friction be less than that of kinetic friction?
I was recently wondering what would happen if the force sliding two surfaces against each other were somehow weaker than kinetic friction but stronger than static friction. Since the sliding force is ...
7
votes
4answers
780 views
Is the EmDrive, or “Relativity Drive” possible?
In 2006, New Scientist magazine published an article titled Relativity drive: The end of wings and wheels1 [1] about the EmDrive [Wikipedia] which stirred up a fair degree of controversy and some ...
7
votes
1answer
254 views
Is acceleration an average?
Background
I'm new to physics and math. I stopped studying both of them in high-school, and I wish I hadn't. I'm pursuing study in both topics for personal interest. Today, I'm learning about ...
7
votes
3answers
310 views
Force through quantum mechanics
In classical physics force is:
$$F=\frac {dp}{dt}$$
How about quantum mechanics?
In Old Quantum Mechanics momentum is: $p=\hbar \cdot k$ so force will be:
$$F=\hbar \frac {dk}{dt}$$
what does $\frac ...
7
votes
4answers
459 views
Which direction will the yoyo move?
This question has been around the net for a while, and I haven't seen a good explanation for it:
A yo-yo is initially at rest on a horizontal surface. A string is pulled in the direction shown in ...
7
votes
2answers
1k views
Tension in a curved charged wire (electrostatic force) - does wire thickness matter?
Consider a conducting wire bent in a circle (alternatively, a perfectly smooth metal ring) with a positive (or negative) electric charge on it. Technically, this shape constitutes a torus. Assume ...
7
votes
3answers
491 views
How can we make an order-of-magnitude estimate of the strength of Earth's magnetic field?
The source of Earth's magnetic field is a dynamo driven by convection current in the molten core. Using some basic physics principles (Maxwell's equations, fluid mechanics equations), properties of ...
7
votes
1answer
173 views
How would nucleosynthesis be different if the neutron were stable?
If the strong nuclear force were just 2% stronger, the neutron would be a stable particle instead of having a half life of about 13 minutes. What difference would that have made to Big Bang ...
7
votes
4answers
801 views
How far does a trampoline vertically deform based on the mass of the object?
If a baseball is dropped on a trampoline, the point under the object will move a certain distance downward before starting to travel upward again. If a bowling ball is dropped, it will deform further ...
7
votes
3answers
551 views
Is the normal force a conservative force?
Most of the time the normal force doesn't do any work because it's perpendicular to the direction of motion but if it does do work, would it be conservative or non-conservative?
For example, consider ...
7
votes
5answers
274 views
Why don't we use the concept of force in quantum mechanics?
I'm a quarter of the way towards finishing a basic quantum mechanics course, and I see no mention of force, after having done the 1-D Schrodinger equation for a free particle, particle in an ...
7
votes
2answers
2k views
What is the force on the arms in a pushup?
What force do the arms have to generate to do a pushup? Let us look a this simplified model:
The body can be represented by the green plank of mass B. Its angle to the ground is $\theta$.
This ...
7
votes
4answers
2k views
What is the cause of the normal force?
I've been wondering, what causes the normal force to exist?
In class the teacher never actually explains it, he just says "It has to be there because something has to counter gravity." While I ...
7
votes
0answers
284 views
Measurement of Tangential Momentum Accomodation?
(this question is a crosspost from theoretical physics.)
I am using atomic force microscopy (AFM) for characterizing
pores of the size of nanometers for application in gas flow. For
this, knowing ...
6
votes
3answers
419 views
How does $F = \frac{ \Delta (mv)}{ \Delta t}$ equal $( m \frac { \Delta v}{ \Delta t} ) + ( v \frac { \Delta m}{ \Delta t} )$?
That's how it's framed in my Physics school-book.
The question (or rather, the explanation) is that of the thrust of rockets and how the impulse is equal (with opposite signs) on the thrust-gases and ...
6
votes
6answers
530 views
Derive $\frac{\mathrm{d}}{\mathrm{d}t}(\gamma m\mathbf{v}) = e\mathbf{E}$ from elementary principles?
It is experimentally known that the equation of motion for a charge $e$ moving in a static electric field $\mathbf{E}$ is given by:
$$\frac{\mathrm{d}}{\mathrm{d}t} (\gamma m\mathbf{v}) = ...
6
votes
2answers
9k views
How much force is in a keystroke? (estimated, of course)?
I'm a software developer, and I need to calculate the estimated amount of force expended typing stored text. Preferrably in some interesting way. (i.e. the force exerted on keys thus far is enough to ...
6
votes
3answers
1k views
Physics of a skateboard ollie
Does anyone have a good explanation of the physics and vectors of force involved in the skateboarding trick the ollie (where the skater jumps and causes the skateboard to rise off the ground with ...
6
votes
3answers
4k views
How does the 'water jet pack' work?
So I was cruising around at YouTube and saw this sweet vid, and as I was watching started to wonder: "How is this possible?".
For a little bit of background, in case you decide to not watch the ...
6
votes
2answers
548 views
What physical forces pull/press water upwards in vegetation?
Each spring enormous amounts of water rise up in trees and other vegetation. What causes this stream upwards?
Edit: I was under the impression that capillary action is a key factor: the original ...
6
votes
2answers
301 views
Why don't we consider centrifugal force on a mass placed on earth?
Let us say a block of mass is placed on the surface of earth. Then while drawing the forces on that body, we say:
Force F = mg acting towards the center of earth
Normal reaction N offered by the ...
6
votes
3answers
230 views
Tidal force on far side
I have a question about tidal forces on the far side of a body experiencing gravitational attraction from another body.
Let's assume we have two spherical bodies $A$ and $B$ whose centers are $D$ ...
6
votes
2answers
150 views
Is it possible to use a balloon to float so high in the atmosphere that you can be gravitationally pulled towards a satellite?
A recent joke on the comedy panel show 8 out of 10 cats prompted this question. I'm pretty sure the answer's no, but hopefully someone can surprise me.
If you put a person in a balloon, such that ...
6
votes
1answer
358 views
Degeneracy Pressure, What is it?
There has been numerous question, some violent even in physics@SE regarding PEP and EM forces.
But what baffles me is what is degeneracy pressure? I know there are 4 fundamental forces- EM, gravity, ...
6
votes
1answer
630 views
How does placing objects in liquids affect the mass?
I was dazing off in my physics class when I came up with this question and I was wondering about it all day. I could not provide myself with an adequate solution, so here I am asking the forum about ...
6
votes
1answer
284 views
Is Pauli-repulsion a “force” that is completely separate from the 4 fundamental forces?
You can have two electrons that experience each other's force by the exchange of photons (i.e. the electromagnetic force). Yet if you compress them really strongly, the electromagnetic interaction ...
6
votes
2answers
586 views
Can a force in an explicitly time dependent classical system be conservative?
If I consider equations of motion derived from the pinciple of least action for an explicilty time dependend Lagrangian
$$\delta S[L[q(\text{t}),q'(\text{t}),{\bf t}]]=0,$$
under what ...
6
votes
1answer
464 views
Ping-pong ball pontoon
Imagine a vertical pipe (both ends opened) in the water. Drop several ping-pong balls into pipe and cover them with a cylinder. When you have enough balls, the cylinder will float. Now start adding ...
5
votes
4answers
4k views
Rope tension question
If two ends of a rope are pulled with forces of equal magnitude and opposite direction, the tension at the center of the rope must be zero. True or false?
The answer is false. I chose true though and ...
