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 ...

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4answers
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Action and reaction pair problem

If I sit on a chair, the first action and reaction pair force is my weight and the force acting on the earth by me. And the 2nd pair is the force acting on the chair by me and the opposite force. ...
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2answers
193 views

Force acting on center of mass

Pictured above is a biker (traveling in a curve) whose center of mass has two forces acting on it. These two forces - a horizontal centrifugal force F(centripetal)* and a vertical gravitational ...
2
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2answers
210 views

What happens to gravity in the middle of the earth? [duplicate]

So I was curious about, what if we make a tunnel from one side of the earth, to the other side of the earth? Gravity is ofcourse always negative, which makes you "fall" and not "float". If we take ...
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3answers
593 views

The sign of the normal force.

The notes state that $N=mg$. However, if I have a $4\text{kg}$ box on a table, taking upward as positive $$N=-mg=-4*-9.81$$ Is this correct? If it is correct, why do the notes state that $N=mg$. ...
3
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2answers
180 views

How to calculate the force acting on a magnet due to a solenoid

Can I assume both magnet and solenoid as magnetic dipole and use Coulombs law to find the force acting the magnet?
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4answers
4k views

How can we move an object with zero velocity?

Consider there is a box of mass $m$ at rest on the floor. Most books give an example that we need to do a work of $mgh$ to lift the box $h$ upward. If we analyze this work done, the external force ...
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1answer
118 views

Work Power & Energy Question [closed]

If a person starts from rest with an acceleration of 10 m/s^2 and applies force of 1000N then What will be its work done at the end of 20s . If the object moves in the horizontal surface? Can ...
3
votes
1answer
86 views

Mathematically impossible for a vortex line to have loose ends?

Could someone show the math behind it? Source : "A vortex is a bunch of air circulating around itself. The axis around which the air is rotating is called a vortex line. It is mathematically ...
0
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0answers
95 views

Why does air circulate on an airfoil — The Kutta Condition [duplicate]

Why does the air circulate on a flowing airfoil, thus giving rise to increased velocity (circulation + relative airspeed) above the wing and hence decreased pressure.
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0answers
258 views

Why friction force is force of constraint?

My understanding about constraint force is that it is a force which limits the geometry of particle's motion. For example, situations such as the particle trapped in a track or limited in domain can ...
0
votes
1answer
143 views

Do we stand on the earth because of Newton's third law?

This is the question I encountered and I'm not sure I understand. The Newton's third law say that, when a body 1 exerts a force on the body 2, then the body 2 will exert the same force on body 1, ...
0
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1answer
128 views

Airfoil theory — Why does air travel faster above? [duplicate]

I do not understand, why the air above the airfoil travels faster than the air below it? The air above has to travel faster than the air below so that both streams meet simultaneously at the ...
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2answers
370 views

What is the third law action pair of buoyancy?

Consider a block floating in a fluid. The force due to gravity balances the buoyant force exerted on the block. But there must be a third-law action pair to the buoyant force. If the block exerts a ...
0
votes
1answer
133 views

How do magnetic objects exhibit attraction/repulsion across empty space?

Magnets will attract or repel over a distance before physically touching each other. What makes this effect possible? My best guess is that the forces generated by the angular momentum of the ...
9
votes
2answers
267 views

Why don't black holes form from forces other than gravity?

Gravity is the weakest of the fundamental forces, so what is so special about gravity that it can form an inescapably strong field while a force like the EM force cannot? It seems to me that if there ...
1
vote
1answer
640 views

Why does shaking your head with short hair hurt? [closed]

When my hair gets long I start doing a little flick of my head occasionally to get my hair out of my eyes. This grows into a habit though, and for the first few days after cutting my hair I end up ...
14
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6answers
2k views

Are there forces which do not involve a change in momentum?

I am familiar with the equation $$\vec{F}=m \vec{a}$$ I am wondering as to whether it is possible for something to exert a force on another object without changing the momentum of said object. My ...
2
votes
3answers
29k views

Tension on a string between two objects on a friction less surface [closed]

I just need my work checked here. Please let me know if I am correct. I had this problem in one of my test in which I did not get full credits. I am re-doing this(hopefully correctly this time) just ...
0
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0answers
76 views

Questions on electron orbits

I have three questions to ask: Why do electrons (in an atom) specifically move in orbits and not some other type of motion? Where does the energy comes from, for the electron to move at much higher ...
0
votes
1answer
3k views

When to use Cosine or Sine? [closed]

Three blocks of masses $M_A=15.0 \; \text{kg}$, $M_B=12.0 \; \text{kg}$, and $M_C=8.0 \; \text{kg}$ sit next to each other on a frictionless surface. A force $F=54.0 \; \text{N}$ acts on Block $A$ ...
0
votes
1answer
103 views

Why is it easier to turn a heavy hanging object?

Assume if we had a 2 tonne weight placed on ground attached with a rope that we can use to turn the object around. It will immensely hard to turn that object around by pulling the rope. However, if ...
0
votes
0answers
32 views

How to measure power used to sustain something in a (gravitational) field? [duplicate]

I've read this question and answers, but neither of the posts does seem to address my concerns. The problem is as follows: Assuming a device that keeps something in a gravitational field on a ...
2
votes
2answers
451 views

Solving actual weight lifted

I was wondering if anyone knew the formula to calculate how much force is required to lift a tire or any object by the edge. For example if a tire weighs 500 lbs (or has a mass of 226.8 kg). I would ...
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1answer
1k views

The nature of the gravity force: Is it attractive or repulsive between positive and negative mass? [duplicate]

If negative mass is rotating around fixed positive mass, then what will be the nature of force and how?
0
votes
2answers
167 views

Equal and opposite reaction - forces coming in pairs - or just common sense?

I've never understood why the conservation of momentum law is taught in schools as "every force has an equal opposite reaction". To me a gun's recoil is self explanatory; the explosion sits ...
1
vote
1answer
364 views

Where do these formulas for an object being pushed up an incline come from? [closed]

An object on an incline with an angle $\theta$ is being pushed at constant speed. Constant speed implies $a = 0$. Because $F=ma$ and the object must have mass greater than 0. If I want to find the ...
39
votes
6answers
7k views

Pouring water in an aircraft while upside down?

What is the physics behind the following photo? Someone had told me that this is because the aircraft might be moving towards a lower altitude, but I am still not sure.
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2answers
238 views

Force applied to two blocks [closed]

Due to application of force tow blocks of mass 1Kg and 0.5Kg move together. Each block exerts a force of 6N on each other. What is the acceleration by which both the blocks move?
4
votes
1answer
60 views

Taking pivot about an accelerating point

Given this question: A small ball of mass $m$ and radius $r$ rolls without slipping on the inside surface of a fixed hemispherical bowl of radius $R>r$. What is the frequency of small ...
0
votes
1answer
155 views

What happens to bodies that lift each other up? [closed]

Person A and Person B are same person (theortically). They both weigh 800N. They both can vertically lift 1000N. They want to ...
4
votes
1answer
198 views

Chain with mass gradient problem

This is the problem, basically. Now my question is about the approach. Mine, and my teacher's. I would like some help as to which one is correct... Mine: As the whole chain is in equilibrium, the ...
1
vote
0answers
48 views

Breaking force and translational force [closed]

Let’s say I have a horizontal rod whose total length = ΔXtotal = 5 inches. However, the rod is made up of two pieces (ΔXshaft = 4 inches and ΔXbase = 1 inch) that are welded together, where ΔXtotal = ...
1
vote
0answers
81 views

Impulse & Momentum

please could someone check this MIT video (http://www.youtube.com/watch?v=Lkuo6nZ6nZM) at 26mins 31secs. He says that if you threw a tomato on a bathroom scale then you would get a certain force ...
0
votes
1answer
106 views

Propulsion force [closed]

My physics instructor gave us a general problem A box is sitting motionless on a rough, horizontal patch of ground. You give it a good kick, so that it slides to the right with an initial speed of ...
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2answers
1k views

Different directions of frictional force when objects are rolling

My textbook has two instances of rolling bodies (smooth rolling). In the first, the body is rolling on the horizontal floor with some acceleration of its centre of mass. In this case, the book says ...
4
votes
1answer
192 views

Pendulum in a Turning Airplane

My friend and I are having an argument over what happens with a pendulum hanging inside an airplane during a turn. Assuming that the airplane is making a standard 1 G coordinated turn, how does the ...
1
vote
2answers
70 views

Final position of an object after aplying several horizontal forces [closed]

I'm given an object with a starting position. Several horizontal forces are applied to this object one after the other for different amounts of time. How can I determine the final position $x$?
0
votes
2answers
519 views

In Which of the following situations would force be exerted on on object and no work be done on the object?

Consider the following question: In which of the following situations would a force be exerted on an object and no work be done on the object? I. a centripetal force is exerted on a moving ...
0
votes
1answer
617 views

Help understanding a potential energy problem [closed]

Apologies if the following problem does not seem to involve potential energy, it is a homework problem that is labeled under the potential energy chapter. Question While a roofer is working on a ...
1
vote
1answer
121 views

Conceptual question on superposition of forces holding a specific mass in equilibrium

Consider a point mass $x$ (like for example the earth in space) and let $A$ and $B$ be two sets of point masses which each hold the point mass $x$ in equilibrium, meaning the acceleration induced by ...
1
vote
1answer
68 views

The superposition of force (or acceleration) configurations

My question is quite specific as it refers to this article but I hope that someone here could help me. I cite the relevant part of the article: ... The second example consists of gravitational ...
3
votes
1answer
88 views

Is force the unique possible relation between particles? [closed]

Is force the unique possible relation between particles? It was requested to further specify my question. The followings paragraphs contain some of my thoughts and confusions. Historical ...
15
votes
2answers
503 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 ...
0
votes
4answers
329 views

Two tensions in massless string

There should only be one tension in a massless string. But imagine this situation. Let a massless rope hang from a ceiling and two monkeys of different weights are holding on to the rope at two ...
1
vote
0answers
48 views

Melting ice in a tray in space shuttle

We have an ice cube in a tray in a space shuttle. As the ice cube melts, the centre of mass shifts downward, so the tray should move downward to maintain the position of the centre of mass. Now, some ...
0
votes
1answer
2k views

Does the tension of a rope/cable change if acceleration/velocity changes?

For example, imagine an object of a certain mass which is attached to a rope and is dangling in the air. The rope needs to have a certain tension to be able to hold the object up, that is, negate the ...
3
votes
3answers
298 views

Why meet at the center of mass?

If two objects of different masses are held at a distance $d$ and then I let them go, they will meet at the center of mass of the particle system due to mutual gravitational attraction My question ...
1
vote
2answers
2k views

Impulse and Change In Momentum — Are they really different?

My entire time learning physics, I have simply assumed that Impulse and Change in momentum are the same thing. It makes sense -- Force changes momentum, and impulse finds the total of force. ...
3
votes
1answer
59 views

On a horizontal plane, why does $F_N=W$?

I keep seeing this definition everywhere, but I don't understand. The forces of the weight and the normal force are going in opposite directions, so shouldn't $F_N=-W$?
2
votes
1answer
58 views

Understanding Normal Force Change

I know this has been asked around, my question is a bit more specialized towards my personal problem. If I have an object the magnitude of the weight is usually equal to the normal force, right? I ...