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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If the earth has gravity, why don't we all collapse to the center?

I'm sorry if the answer is obvious for you guys, but why don't we all (including buildings, road, people, the ground) collapse to the center of the earth because of gravity? Is it because we have ...
7
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3answers
4k views

What is the cause of the normal force? [duplicate]

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 ...
2
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2answers
4k views

Normal force: up or down?

The normal force obviously always has direction perpendicular to the surface of contact, but I'm a bit confused about its sense: is it going 'up' or 'down'? I've seen articles on the web that describe ...
3
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2answers
314 views

Silly bet about the best way to walk up a hill

I have a silly bet with a friend of mine. One of us argues that walking up a hill whilst leaning forward requires less exertion, due to your centre of mass being further forward, and therefore ...
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5answers
5k views

The Time That 2 Masses Will Collide Due To Newtonian Gravity

My friend and I have been wracking our heads with this one for the past 3 hours... We have 2 point masses, $m$ and $M$ in a perfect world separated by radius r. Starting from rest, they both begin to ...
3
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4answers
6k views

How does a pressure difference cause a force?

One more thing that's bothering me, could somebody clarify a little bit? I think it has something to do with being in the state of lowest energy state, what instigates that change? Perhaps even ...
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3answers
610 views

What was meant by the 'ponderomotive force' as understood by Minkowski?

Skimming through Minkowski's famous 1907 paper, he uses the term ponderomotive force. What does he mean by this?
2
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2answers
5k views

Why exactly does current carrying two current wires attract/repel?

When to parallel wires carrying currents in same direction I1 & I2. http://www.youtube.com/watch?v=43AeuDvWc0k this video demonstrates that effect. My question is, why exactly does this happen? ...
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2answers
379 views

Pollen Particle Attracted to TV due to Which Force?

A pollen particle has no charge so I cannot understand how the Lorentz force $\bar{F} = q \bar{E} + q(\bar{v} \times \bar{B} )$ could explain the event. I speculated that it is because of the electric ...
2
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1answer
1k views

How do I determine the amount of lift needed to launch an object?

So as part of a video I'm working on, I'm trying to launch a 10kg table about 8m into the air. A machinist friend of mine has provided a solution that involves pneumatic cylinders and a pump to ...
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2answers
5k views

Unit for Impact Force

Forgive me if this is a newb question but I am not a trained scientist, much less a physicist. I'm just curious and would like to know if there's a unit to measure impact force. I know the newton is ...
1
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1answer
389 views

Where are the centers of mass for a baseball bat vs. a cricket bat?

My friend and I were comparing the rightness of a cricket bat versus a baseball bat for general violence purposes. (Eg, rioting.) Where are the centers of mass for these two instruments? Which would ...
0
votes
1answer
199 views

determine temperature at which pita bread will expand in a toaster

suppose you have a pita pocket of radius 5cm or .05m. We will model the pita pocket as consisting of disk with a layer of air of volume V which is contained by the surface of the bread. We will ...
3
votes
1answer
135 views

3 current loops with non-conservation of momentum, when one specifically considers SIGNs

Thought experiment I. Consider two loops of wire, 2 small dipoles B and C , with a common axis z (facing each other) and (say) 30 cm apart B to C. At the speed of light, information (including a ...
1
vote
2answers
890 views

How to get statics out of a dynamic force concept?

If one defines force as the time derivative of momentum, i.e. by $$ \vec{F}= \frac{d}{dt} \vec{p} $$ how can this include static forces? Is there a generally accepted way to argue in detail how ...
25
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1answer
1k 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 ...
3
votes
2answers
434 views

Force exerted by rotating bar

Say we have a bar immersed in liquid, and that the bar in being rotated at a given constant angular speed. What force is exerted on the liquid by the faces of the bar moving through the liquid (so the ...
0
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2answers
2k views

Calculate the weight a simple plank can support [closed]

I'd like to build a simple desk; just a single plank of wood (or a few side-by-side) with solid supports on each end of the desk. What I'm trying to figure out is how thick a plank I want to use for ...
3
votes
2answers
1k views

Solve spring system

I am not physics in training so if anything is unclear please request clarification. I have a set of objects to be placed on a line, linked with springs of known lengths (L) and stiffnesses (K). I ...
10
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4answers
3k views

Deriving Newton's Third Law from homogeneity of Space

I am following the first volume of the course of theoretical physics by Landau. So, whatever I say below mainly talks regarding the first 2 chapters of Landau and the approach of deriving Newton's ...
2
votes
2answers
350 views

Small scale water power, how does water volume and hight convert into electric energy?

I was playing a little bit with the basic physics behind water power production but I can't get the numbers right. Let's say that I put a windmill that pumps water into a watertank on the top of my ...
1
vote
2answers
391 views

Problem with forces acting upon a sloped plane

We have a structure that is sloped at 20 Degrees to the horizontal and weights 160N. We then have a force (wind) which is acting along the horizontal plane which has been calculated to be of about ...
6
votes
1answer
2k views

finding angular velocity and regular velocity when bouncing off a surface with friction

Take the game of pong as a simple example. When you hit the ball with a paddle that has a frictional surface, the ball will spin as well as change direction according to the coefficient of kinetic ...
1
vote
2answers
935 views

What, if anything, makes forces the “cause” and acceleration the “effect”? [closed]

We typically say that forces cause acceleration inversely proportionate to mass. Would it be any less correct to say that acceleration causes forces proportionate to mass? Why? (Note that the ...
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vote
3answers
195 views

Constrained particles under distance dependent force

This question is from the 1975 Canadian Association of Physicists Exam. No solutions are posted and I am quite lost on how to proceed with it. A particle is constrained to move along the x-axis of a ...
3
votes
4answers
349 views

existence of other forces obeying inv square law

Is there any restriction in what we know of physics to the existence of other type of forces that obey the inverse square law in 3 dimensions. I mean other than electromagnetic and gravitational. ...
2
votes
1answer
4k views

calculating thermal energy [exam prep]

I'm preparing for a grade 11 physics exam but having trouble with a question: a 19.9kg mass slides in a straight 28.8m line experiencing friction with a coefficient of 0.105, how much thermal energy ...
3
votes
4answers
917 views

What do we consider “Perpetual Motion”

I know this is a bad question to most serious Physics but I have a question about what is considered “Perpetual motion.” The Foucault pendulum in the UN consists of sphere that passes directly over a ...
1
vote
1answer
289 views

Stresses put through joints on a steel climbing wall

Being a fan of stackoverflow, I thought I could try this question here, hope its not too off topic. I'm wanting to build a free standing climbing wall in my garden and dont know where to start ...
0
votes
2answers
2k views

Can someone explain the physics of a standing jump?

I would like to use something like a kinematic equation to know where the person will be in the jump at a given time. However since I think acceleration is not constant because of the jump force it ...
2
votes
1answer
4k views

Will a lighter car have a higher top speed than a heavier car with an equal power engine?

If I have a car (with a particular engine) optimized (shape & weight distribution-wise) for attaining the top speeds possible, and I put that engine into a car which is heavier (but otherwise the ...
2
votes
2answers
3k views

Work done by gravitational field

To calculate the work done by a gravitational field, the equation is written as $$W=GMm(r_\text{initial}^{-2} - r_\text{final}^{-2})$$ Suppose a small mass has distance $R$ from a big mass. So here ...
7
votes
2answers
15k 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 ...
0
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3answers
304 views

Action - Reaction pair, through photons

Here's an example to describe the issue Supossed a high power laser (eg a 100 kW laser, ie, electromagnetic weapons) is fired to a target, then it will receive energy and move. (and likely to burn or ...
2
votes
1answer
623 views

What are the forces of constraint if there are multiple equivalent constraints?

Suppose a large (rigid) block is sitting on top of two smaller blocks of equal height $1$, both of which rest on the ground. We wish to find the position of the block (easy) and the forces of ...
0
votes
2answers
502 views

Would a car tow rope withhold an adult person?

I am not exactly sure how the physics work here. If we take a car tow rope, which is manufactured for towing a 3500 kg car and we actually start towing such a car on this rope, what force do we really ...
0
votes
1answer
401 views

if a force is 1 newton metre, what is it at 2 meters?

If I have a force, say 24 kg/cm what would that equate to at 2cm? I would like to know the formulae for calculating this. For example. If a motor can hold an object of 24kg at 1cm from its pivot ...
2
votes
6answers
1k views

Which person can handle falls from big heights better: lighter or heavier?

This probably includes a little of biology, but I believe it's mainly physics, so I hope it's ok to ask here: Imagine 2 persons: Person A weights 120 pounds and person B weights 180 pounds. Imagine ...
4
votes
3answers
517 views

Why trimming the mainsail gives a larger torque to head up

I am learning sailing on a 5m catamaran (Nacra 5). I am familiar with basic aerodynamics and the physics of the sail and keel. We learned that when sailing closed hauled, too tight a mainsail tends ...
-1
votes
1answer
529 views

Acceleration Force [closed]

A 6500-kg helicopter accelerates upward at 0.60 m/s² while lifting a 1200-kg car. (a) What is the lift force exerted by the air on the rotors? (b) What is the tension in the cable (ignore its mass) ...
0
votes
3answers
4k views

Does an Increase of Force affect the Speed of an object if the acceleration stays constant?

If I had a mass of $100\:\rm{kg}$ accelerating due to gravity, using $F=ma$: $F = 100\:\rm{kg} \times 9.8\:\rm{m/s^2}$ $F = 980 \:\rm N$... If I increased the mass to 200kg, the force would be 1960 ...
0
votes
1answer
527 views

How is a magnetic field translated into physical force?

Related to this question Where do magnets get the energy to repel? If I have a magnet repelling another, eg one in my hand, the other being pushed along the desk, how do the each of the magnet's ...
4
votes
3answers
5k views

Which will stop first a heavier car or a lighter car?

If the friction from brakes, wind resistance and all such factors remain constat, which will stop first? A heavier car or a lighter car? How will the momentum of the car and graviational pull on a ...
10
votes
2answers
2k 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 ...
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votes
1answer
20k views

What is the extent of Earth's gravitational pull? [closed]

Where the gravitational pull of Earth exist up to? What distance from Earth it will be zero? How do the skydivers fly at a same altitude? Won't they feel gravitational pull? What is the Earth's ...
6
votes
2answers
584 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 ...
13
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4answers
1k 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
2answers
7k 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 ...
2
votes
4answers
272 views

Rocket engine, perpendicular to the end of stick, in space

Rocket engine, perpendicular to the end of stick, located in cosmos, without atmosphere and gravitation. As in picture below: In what direction will the stick move? In V or another direction?
6
votes
4answers
554 views

What exactly is $F$ in $W = \int_{a}^{b} F dx$?

I am trying to teach myself some basic physics, here is something I do not really understand about the definition of work: When moving from $a$ to $b$ (in one dimension), the work done is defined to ...