# Questions tagged [forces]

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 the [particle-physics] tag for decay channels due to forces and [newtonian-mechanics] or one of the other subtopics of [classical-mechanics] for the dynamics of classical systems.

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### Why does a plastic bag rip when it is raised to a higher position when being held but not when it is stationary?

Let me preface this as I am not a physics expert but this is something I was wondering as I would love a technical explination. For ex.: say a person is walking with an filled plastic grocery bag and ...
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### Falling Blocks and Forces [closed]

After discussing this problem with multiple others, I've heard tons of different answers. I understand the explanations for both A and D. I've found differing sources on the web for this question as ...
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### Direction of undetermined contact force & undetermined Normal force [closed]

I am not getting the following sattements related to contact force and normal force which are: If the direction of contact force cannot be determined, it should be shown as two components. If ...
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### Bar suspended by three vertical ropes [closed]

A two dimensional homogeneous bar of mass m is suspended by three vertical inelastic ropes. The bar is horizontal and in equilibrium. One rope is fixed in the middle of the length of the bar (at point ...
39 views

### A force acts in a 30g particle in a way so that displacement acts as function of time $s = t³-4t²+3t$ in SI units. The work done in first 4 sec is: [closed]

The formula for work is $\vec F\cdot\vec s=Fs\cos\theta$ or you can use integral operators for the same But by doing so, I am getting two different answers. One is 5.4J and one is 5.28J. The question ...
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1 vote
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### Newton's third law in thermodynamics

In my physics textbook, the foundation for work is derived using newton's third law, where F_surr = - F_gas, where surrounding represents a piston-cylinder device and gas is pushing against the inner ...
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### Mean and higher moments of final position of particle subject to time dependent central force along x

I would like to find the expected value and higher moments of the x and y components of the final position of a particle moving in the xy plane, subject to a central force, centered on a positive x ...
1 vote
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### Does a linearly accelerating spherically symmetric body emit gravitational waves

According to Birkhoff's Theorem, any spherically symmetric body will not emit gravitational waves. I can understand this for an object that is contracting and expanding because from far away the ...
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### Friction vs drag/resistance force

we know that friction is a kind of resistance force, so friction should some how related to resistance force, but i just curious why they have different formula indeed. The friction formula is given ...
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### How to calculate normal force acting on part attached with a screw? Can the force on screws be decreased by lowering the height of the normal force? [closed]

I have an L-shaped beam (equal height and width lengths) hanging from a screw on the wall, and I model the screw as a pin joint (hopefully okay approximation even though it does apply a constant force ...
2k views

### Why does force perpendicular to the velocity change only its direction; not the speed?

While analyzing the case of a force and consequently an acceleration acting perpendicular to the velocity of a given body, I do understand that force's component along the velocity will be 0 causing ...
1 vote
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### Meniscus formed by water is a weighted catenary? [duplicate]

When I modeled the meniscus with a weighted catenary, B cosh(x/A) + C, it perfectly fit the curve of the meniscus. I get why this curve might be a catenary, but why can the meniscus curve be modeled ...
51 views

### A man moves on a straight horizontal road with a block of mass 2 kg in his hand. He covers a distance 40m with an acceleration of 0.5m/s^2 [closed]

We have to find the work done by the man on the block. Can we consider the situation above as a non-inertial frame? The man is accelerating forward, therefore, a pseudo force will act on the box in ...
133 views

### Computing Cauchy stress tensor in a static cube of uniform isotropic material

As the title says, I'm interested in explicitly calculating the value of the Cauchy stress tensor in a static (non-moving) cube of some material that has uniform density an is isotropic (e.g. concrete)...
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### How can we prove that tension on both sides of string will be equal?

In my text book the following system was given A massless inextensible string wearing a bead of mass $m$ is suspended from point A and B. At the given instant they said that the tension on both sides ...
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### Elevator: pushed vs. pulled

Suppose an elevator is accelerating upward, and the acceleration has one of two causes. Either: the elevator is being pushed from below, or the elevator is being pulled from above. From inside the ...
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### Friction force on a cuboidal block for different orientations - how does $\mu$ change with orientation? [duplicate]

It's been a long time since I did problems on friction but here's one... Consider a cuboidal block of dimensions 123 meters with a total mass of 1 kg. There are 3 cases that we need to look at: Its ...
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### Sign of force for assumed $mgy$ gravity [closed]

If gravity force of earth is $mg$: if positive y is pointing upwards, then: $m\vec a = -mg\hat y$ and $\ddot y = -g$ if positive y is pointing down, then: $m\vec a = mg\hat y$ and $\ddot y = g$ If ...
1 vote
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### Why do we only consider tension pointing toward one direction when deriving the power of wave on a wire?

How about the tension pointing in the opposite direction? Does it not contribute to the work done by wave to make the wire move up and down too?
1 vote
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### Work done by a moving normal force [duplicate]

Say someone climbs up a downward moving escalator. He climbs up with 1.5 the speed relative to the escalator. How would you go about calculating the work done by the someone? If it was a still ...
127 views

### A Conceptual Doubt in a Question on Work-Kinetic-Energy Theorem

I have a doubt in this question, Diagram given below In the Question it is asked that "calculate the final velocity of the block in the figure" and in the solutions it is given that the ...
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### Discrepancy between theory and real life: spool and mass system connected via pulley

I was given this problem in physics class. My teacher said that they reached the ground at the same time, but when I tested it out (video), I got drastically different results. I showed the professor ...
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### Is sum of forces always zero along the axis that momentum is conserved?

Lately I've been approaching problems with conservation of momentum of systems, I was wondering, if I draw the free body diagram, then write the equations and then add them togheter; if momentum is ...
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### Conservative forces and Variation

I am currently studying "Classical mechanics by Goldstein" and have just started. The book introduced something simple. For a conservative force, the work done in taking a mass from one ...
1 vote
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### Why a person when jumps out of boat applies a force at the corner of boat?

I marked the red region why the person pushes the boat at that point? What I think is they push the boat at that point to avoid friction between his leg and boat surface. If I am wrong then please ...
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### Gravity is not a Force? [duplicate]

I don't know much about this topic, but I read something saying that gravity is not a force using an example of inertial observation. I started thinking about the topic again when I was researching ...
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### Where would a string with multiple points of equal weakness break?

In a purely theoretical scenario (no gravity, etc.), consider a string with n evenly spaced points of equal weakness along the length of it. If equal tension is applied from both ends, where will it ...
2k views

### Cause-effect definition of fictitious forces

I'm currently teaching a general-physics-for-engineers class, and we approached fictitious forces. As I was explaining them, students asked me how to discriminate "real" forces from ...
Take a look at the picture of a standard wheel where $f_m$ is the force due to the torque induced by the a motor $\tau_m = rf_m$, the static force $f_s$ and $f_r$ is the driving force causing the ...
Consider a system of two blocks having masses $m_1$ and $m_2$ lying on smooth floor, where $m_2>m_1.$ They are attached by an ideal inextensible string. There are two different situations $1$ and \$...