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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1answer
288 views

Derive force from the pair-wise potential equation [closed]

How can we calculate the force exerted on particle $i$ by particle $j$ given a general potential function $V(d)$? Let's discuss this genenal question with a concrete example below. To simulate the ...
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3answers
3k views

Why is using Newton's First Law failing me?

A factory worker pushes a 29.7kg crate a distance of 5.0m along a level floor at constant velocity by pushing downward at an angle of 32$^\circ$ below the horizontal. The coefficient of kinetic ...
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2answers
501 views

force of hail falling straight

There is a hail storm and it's falling straight down (0 wind factor) will the hail meet a moving car with greater force than a car standing still?
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1answer
224 views

How can a reel of thread move in opposite direction?

There is a nice puzzle about reel of thread. I wonder why the solution works. The puzzle: You take a reel with threads on it. Inner radius r, outer radius R. You put it on a table. If you pull ...
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2answers
690 views

Is friction always constant?

Can the norm of the friction force change with time? Notice I said the norm which is in Newtons, can it change with time? Sorry if the question seems a bit confusing but I've done my best to explain. ...
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2answers
1k views

How do we convert between the maximum value of the adhesive force (in kilograms or in Newton) of a magnet and its magnetic field strength (in tesla)?

On the following website they indicate how powerful is a magnet by giving the theoretical maximum value of its adhesive force in kilograms or in Newton (its adhesive force at a distance of zero). ...
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1answer
41 views

Clarification what exactly this question wants regarding force [closed]

I would like to ask you if the question asked What is the minimum force parallel to the plane, which can be prevent the block from slipping down the plane? I just did the calculation and and ...
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3answers
295 views

Solving for Normal Force

I'm working through The Nature of Code, which is an awesome book, lots of fun. I've come across Exercise 3.13 and I'm not sure how to solve it. I'm assuming that the force of gravity and the force of ...
2
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1answer
30 views

am i finding the force correctly?

I am trying to see if I am doing a problem correctly. Problem Suppose you carry a 50kg sack of potatoes up two flights of stairs, a total height of 10m. How much work did you do? If it took you ...
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1answer
279 views

How do internal forces affect motion?

As per newton's first law, only an external force can bring any change in the acceleration of a body, internal forces cannot. So, when we apply brakes to an accelerating car, aren't those ...
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15answers
11k views

Is Newton's third law always correct?

Newton's third law states that every force has an equal and opposite reaction. But this doesn't seem like the case in the following scenario: For example, a person punches a wall and the wall breaks. ...
2
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1answer
351 views

Relative strength of a dome [closed]

Is there a rough way to determine how the height of a dome affects the load that dome could support? For instance, assuming the bases of two domes are 24" in diameter, and one dome is 2" high while ...
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4answers
872 views

Definition of force, kinetic energy and momentum

I've edited the post. Q1 and Q4 are the important ones but I didn't delete Q2 and Q3 since some older answers would not make sense anymore. To begin with, the formula of the kinetic energy $T$ is ...
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1answer
119 views

Help me understand constant speed on a slope

An object is pulled up a slope at constant speed. I'm trying to find the Tension on the rope. $$m = 65kg \Rightarrow W=-637$$ $$\mbox{constant speed} \Rightarrow a = 0$$ $$\mbox{angle of slope} = ...
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2answers
112 views

In this particular problem: Is the mass of the system the mass of the person?

I am doing this problem: The upward normal force exerted by the floor is 620 N on an elevator passenger who weighs 650 N. What is the magnitude of the acceleration? This is how I solved ...
2
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1answer
113 views

Pulse Reflection

I wanted to ask why a pulse that reaches to a free end will reflect off and return with the same direction of displacement that it had before reflection. I am looking for explanation with forces.
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1answer
119 views

Why $\vec F=m\vec a$ instead of $\vec F=m\vec v$? [duplicate]

$\vec F=m\vec a$ ,for moving object with 10 $km/s$ in a constant manner ,where acceleration is zero. ie No force on object ? http://www.physicsforums.com/showthread.php?t=622711
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4answers
277 views

The work done by running in a rectangle

This may be quite off-topic but please help me. Is there any work done when I run in a rectangle? I thought that the answer should be no. But my teacher says that we should calculate each side ...
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4answers
2k views

Force as change in momentum vs. change in velocity

Is there ever a situation where the distinction between $F = m \frac{dv}{dt}$ and $F = \frac{dp}{dt}$ is important? I can't think of a situation where one is true and not the other (assuming only ...
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2answers
254 views

Conservative Forces & Conservation of Energy?

I'm trying to relate them, I'm trying to find the key relation that would show how the conservative forces serve conservation of energy. How would they relate? Also, how are non-conservative forces ...
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2answers
786 views

Is the Lorentz force conservative? [duplicate]

Is the Lorentz Force acting on a wire, that has current $I$ in a magnetic field $B$ conservative? Or non-conservative? I understand that all the fundamental forces are conservative, am I correct?
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3answers
404 views

Time reversal symmetry in the presence of friction

I was reading a paper on time reversal symmetry, and came across an example of a pendulum swinging in the presence of friction: When we consider the more realistic physical situation of a swinging ...
0
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1answer
217 views

The force on an infinite line of charge perpendicular to a on a rod of charge

Essentially the question is to calculate the force exerted by a charged rod perpendicular to an infinite line of charge: $\vec{F}_{\text{rod}\rightarrow \text{line}} =\dots$ My question is it okay to ...
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2answers
20k views

Relationship between amplitude and frequency of a wave

What is the relationship between amplitude and frequency of a wave? Some say there isn't any relationship, some say that there is, but from their answers the relationship is still unclear.
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1answer
4k views

Acceleration of masses hanging from a system of two pulleys

"Masses $M_1$ and $M_2$ are connected to a system of strings and pulleys as shown. The strings are massless and inextensible, and the pulleys are massless and frictionless. Find the acceleration of ...
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1answer
113 views

Question about pendulum

I came up with this problem by myself: How much force do I need to make a pendulum revolve? Now I imagined that the force $\vec{F}$ must be enough to make the pendulum swing until half of the ...
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1answer
111 views

Question on Newton's law

I have heard my friends referring to quantity ma as the "force of acceleration" whereas my teacher told us it can't be referred as so. Is it correct to refer that quantity a force? If not, how can we ...
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0answers
605 views

Sum of energy in a free rotation with Algodoo software

I try to understand why Algodoo software (freeware) give an efficiency greater than unity. I tested several scenes and at each time I can look the software give energy, I don't say it's all the time ...
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0answers
169 views

Creep of materials at atomic/molecular level under stresses

Do the viscoelastic materials creep at the same rate under three types of fundamental stresses, viz.- TENSION, COMPRESSION and SHEAR??? My intuition tells me that the answer is no. But, I can't get ...
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1answer
67 views

Help for orbits of planets

Suppose that an asteroid) that moves at a velocity $v$ passes near to a planet: now I know that the gravity force of the planet will curve the trajectory of the asteroid so I tried to make a draw of ...
1
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1answer
120 views

Motion of a car rounding a bank

When a car is traveling round a banked track as fast as possible, it has a tendency to slip up the slope. Opposite in the case when the car travels slowly and has a tendency to slip down. Can ...
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2answers
320 views

Newton's third law, action vs reaction

When I push a wall with my hand, the wall does the same and because both forces cancel out the sum of the forces applied by me on the wall and the wall on me is zero, plus the friction on the ground ...
1
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1answer
245 views

Naming vectors in free body diagrams

Is there a convention for naming the vectors? Suppose there is a box on a table. I'm going to draw the forces acting on the box. So I focus on the box and ignore forces acting on the table, the ...
2
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5answers
634 views

Why Newton's law of universal gravitation is a valid law? What causes any two bodies in the universe attract each other with a force?

Why Newton's law of universal gravitation is a valid law? What causes any two bodies in the universe attract each other with a force? What causes a larger body in space to have a gravitational pull on ...
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2answers
577 views

Why neglect friction when computing torque?

There is a block of wood on a plank, with friction, and the block is moving down with uniform speed. They are asking what is the torque acting through the center of the block. My doubt is that ...
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1answer
143 views

Electric Field Homework Question

Can a charged particle move through an electric field that exerts no force on it?
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1answer
221 views

Question about resolving forces

Say, in the example below, the weight $mg$ of the object is $800N$. To find $R$, the conventional method is to use $R\sin(28^\circ) = 800$. But why isn't it possible to use instead the component of ...
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2answers
640 views

Tension in string and gravitational force on bob relative strength

When calculating time period of simple pendulum (an approximation of SHM at small amplitudes) we take gravitational force greater than tension in string and resolve gravitational force in two ...
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1answer
39 views

Modulus of action-reaction forces

I read somewhere that action-reaction forces satisfy the form $$ \mathbf{F}_{12}=-\mathbf{F}_{21}=f(|P_1-P_2|)(P_1-P_2) $$ meaning that the modulus can only depend on the distance between the two ...
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3answers
25k views

If something that is moving at constant velocity has no net force acting on it, how come it is able to move other objects?

Let's say 10 kg block is sliding on a frictionless surface at a constant velocity, thus its acceleration is 0. According to Newton's second law of motion, the force acting on the block is 0: $a = ...
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2answers
1k views

Upthrust vs Drag

24: A ball is falling at terminal speed in still air. The forces acting on it are upthrust, viscous drag and weight. What is the order of increasing magnitude of these 3 forces? ...
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2answers
2k views

Why mountain roads are winded up rather than keeping them straight? [closed]

In my opinion, to prevent the line passing from the center of gravity of a body (e.g bike or car) trying to climb the mountain or hill to fall outside the base of support so to maintain stable ...
2
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1answer
197 views

Confusion with curl of Lorentz magnetic force

Since the magnetic force is a no work force, $dW=\vec F\cdot d\vec r=0$ for $\vec F(\vec r)=q(\vec v(\vec r) \times \vec B(\vec r))$, therefore $\oint \vec F \cdot d\vec r=0$ by Stoke's theorem. ...
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1answer
1k views

Truck moving uphill, object suspended from ceiling, determine acceleration [closed]

A truck is moving with constant acceleration $a$ up a hill that makes and angle $\phi$ with the horizontal. An object of mass $m$ is suspended from the ceiling of the truck by a light cord. If the ...
8
votes
4answers
6k views

When we do pull-ups , does the bar takes more weight than when we hang down on the bar?

When I do pull-ups, I feel I push down to the bar. But does the bar really take more weight than just hang down? For people who don't know pull-ups and hang down, here is an illustration. Left: Hang ...
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1answer
4k views

Find the tension and angle from a vertical of a rope hooked up to an object

Below is the homework problem I am working on: A 18.3-kg object hangs in equilibrium at the end of a rope (taken as massless) while a wind pushes the object with a 63.9-N horizontal force. Find ...
2
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3answers
472 views

How come water in a cup won't shoot out of a straw placed inside it?

I've been learning about pressure and fluid dynamics, and I've stumbled onto a problem. Say you have a straw in a cup. The surface area of the water in the cup is much greater than that of the straw. ...
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1answer
80 views

Falling metal cylinders more devastating than a nuke?

I just saw G.I JOE - Retaliation. In the movie the bad guys (spoiler alert) Could this device cause more destruction than a nuclear weapon? If so, how?
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0answers
177 views

Balloon tethering cable tension model (catenary tether)

If a moving balloon (let's say it's a blimp system with some rotors for movement) is connected to the ground by a cable that describes a catenary curve, how can we model the tension being applied on ...
2
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0answers
105 views

what forces are present when slicing an object.

I found here:What happens when we cut objects? that it is due to fracture wood splits open when using a wedge. Does it only apply when the wedge is already in the wood? How about the moment before a ...