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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64 views

How do I use the direction of a cross product?

The question is: An electron is projected at a speed of $3.70×10^6\text{ m/s}$ in the $u=\frac{(i+j+k)}{\sqrt3}$ direction into a uniform magnetic field $\vec {B} = 6.43 i + 3.43 j −8.29 k$. ...
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1answer
96 views

Which areas of physics are related to the act of playing drums?

I'm musician (drummer) and I'm trying to figure out what can I study (related to Physics) for better understanding of the drumsticks and wrist movements, the force applied and the better way to apply ...
9
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3answers
417 views

What does Feynman mean when he says that $F=ma$ is not exact?

Chapter 12-2 in Feynman Lectures Vol. 1 states: In fact the law, $F=ma$ is not exactly true; if it were a definition we should have to say that it is always true; but it is not ... First, ...
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1answer
662 views

What determines the speed of waves in water?

While I was walking my dog this morning, I passed over a canal filled with boats, barges, and kayaks all of different masses and moving at different speeds. I noticed that all of these vessels left ...
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1answer
66 views

Masses and Pulley

I have a two mass and a pulley system as shown here: I have the following two questions: First, why does $\dfrac{d^2}{dt^2} y_p$ is not zero, i I mean I don't understand why does $ y_p$ is going to ...
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1answer
5k views

Calculating electric motor's weight lifting ability knowing power. What else do I need to know?

So, my company works with electric motors, and my boss wanted me to calculate how much weight could be lifted by a motor rated at 750W. I know that I can't just calculate the work done by it without ...
3
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3answers
224 views

Higher order derivatives - Equation of motion

One possible starting point to create a physical theory is the Lagrangian $L$. There we assume that the variation of the action $\delta S = \delta \int_{-\infty}^\infty dt \ L = 0$. In classical ...
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2answers
99 views

Force calculation with a fulcrum not working out [closed]

I hope this "applied" question can be asked here (and forgive my naivety in physics for the simple question). I've built the following murphy bunk bed: It rotates around the circles (the fulcrum) ...
3
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2answers
157 views

How to use an accelerometer to correctly quantify the magnitude of vibrations

Consider the following setup: An accelerometer is placed inside a helicopter, measuring $g$ values along three axes $(x,y,z)$ every $10~\text{ms}$ (100 times a second), during a helicopter flight. ...
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1answer
859 views

Tension when two connected blocks are pulled in opposite directions [duplicate]

Suppose there exist two blocks A and B connected by a massless, inextensible cord. Both A and B have equal masses and are acted upon by equal forces in opposite directions. So, is the tension in the ...
0
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1answer
58 views

Spacecraft momentum question [closed]

Assume you're sitting in your spacecraft. The mass of you and the spacecraft is 2000 kg. You're traveling at 200km/h through the vacuum of space. You have a thruster pointing in the direction you're ...
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0answers
555 views

Tension on a string pulled by unequal forces

Suppose two cases where masses are attached to massless, inextensible strings. In the first case a body which has a weight of 20 Newtons(downwards) is attached to a string which is acted upon ...
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1answer
74 views

How is gravitational lensing possible? [duplicate]

How is it possible that a force that affects mass (gravity) can affect massless particles like photons?
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1answer
300 views

Why do whips crack?

Whips have been discussed in a previous question last year. I wish to ask a further question on the subject. (I can not comment or reply to the previous question since I am a new member here and my ...
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1answer
170 views

Tension in an Atwoods machine conceptual?

Assuming $T_1$ is the force that acts on box $1$ and $T_2$ is the force that acts on box $2$. Exactly what causes the Tension? Why does $T_1 = T_2$? The problem is we are told to memorize that ...
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3answers
397 views

How do electrons actually move in a circuit?

Last year, we were taught about electricity, about how electrons move in a closed circuit. But as our teacher had not taught us about electric fields yet, she gave us a simplified model of motion of ...
3
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2answers
168 views

What does an applied force on an already accelerating object do?

I'm a big beginner having only taken Grade 11 high-school physics. Imagine this situation: an object is accelerating north, and while it's accelerating, it's 'hit' by a northward force. To me it ...
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1answer
88 views

Will we feel the gravity of a star 10 light years away for the next 10 years if, somehow, it vanishes today from its position? [duplicate]

I was watching a relativity video, and although I am not sure, I felt that it was trying to tell that the effect of gravitation of a body is instantaneous, in the sense that a sudden change in the ...
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3answers
505 views

Proof that a force applied to the center of mass is the same as force applied off-center

There is a similar question that gives a bit of an explanation, but little mathematical proof here: force applied not on the center of mass I would like mathematical proof that shows that the ...
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1answer
71 views

On conservative forces

We say if we calculate the work done by a force in going from 1 to 2 following a path say A is equal to -1 times the work done by the same force in getting from 2 to 1 following a path say B then work ...
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1answer
1k views

Block and Pulley Problem with Friction [closed]

1.) What is the acceleration of the hanging mass? (Indicate the direction with the sign of your answer.) 2.) Determine the tension in the cord. (Indicate the direction with the sign of your ...
0
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1answer
76 views

Curve of a rod bent by force on both sides

Suppose we have a flexible rod (i.e. it can be bent without breaking apart) and we excert a force on both sides, like this: If the force $F$ is not exactly horizontal, the rod will be bent and form ...
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2answers
488 views

What will happen if we put balloon in water tank in space

Imagine this. You have some object (let say barrel) and fill it with water. The water remains rather calm so it doesnt' spills out. Then we take a balloon filled with air and push it in that tank. ...
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4answers
2k views

Why is gas mileage typically better when traveling on the highway than on country roads or in the city?

The gas mileage of my vehicle tends to improve the more I have been driving on the interstate on that tank of gas - if I go through a tank of gas without at any point driving on the interstate, I will ...
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3answers
1k views

Is friction a product of Newton's third law?

Newton's third law is defined as $F_{12} = -F_{21}$ Is friction a product of this law? For example, if I take my hand and slide it across the floor in the $+x$ direction... My hand exerts a force ...
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1answer
178 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
2k 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
255 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?
5
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1answer
154 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 ...
0
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2answers
178 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
327 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
38 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
139 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
29 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
117 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
6k 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
votes
1answer
193 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 ...
3
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4answers
612 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
82 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} = ...
2
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2answers
76 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
86 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
103 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
0
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4answers
145 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 ...
14
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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 ...
0
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2answers
163 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 ...
0
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2answers
412 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?
2
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3answers
214 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 ...
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1answer
134 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
8k 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
3k 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 ...