External forces Definition and 41 Threads

I don't quite understand the "subtle point" at the end of the author's solution. Ok, let's imagine for a second that the external forces have an impact on the internal forces. How does that change the mathematical result that the two forces are equal and opposite to each other? Even if...

If the dipole is to be in equilibrium at all positions as it's moved so that it's always inclined at 60° to the horizontal, then the torque due to electric field needs to be balanced by torque due to external forces ##F_{ext}## as shown in above diagrams. But such external forces will not make...

Hello Everyone, This is not a completely new dilemma but I have been discussing it with several people and teachers and read different physics textbooks and continue to get different perspectives, sometimes overlapping sometimes not. Let me explain and summarize: Forces are interactions...

Summary:: Bernoulli’s Equation and External Forces Hello all, I am a bit confused on this question: Water flows through a spray nozzle (as shown in the figure below) at a rate of 50 cm3 h -1 . The internal inlet diameter of the nozzle entrance is 5 cm and the internal exit diameter of the...

So I have a trolley of mass m that moves on a straight line. A sphere of mass m, is attached on the trolley with a light string of length a and it is left to oscillate. Just to give some idea of their positions: r_trolley = xi r_sphere = (x-asinθ)i - acosθj (θ is the angle between the string...

I am still intrigued by the neutral buoyancy of a body and have come up with some other questions. As previously encouraged, I attempted to answer my own questions (this is not home work, it is just me trying to wake up my brain in my retirement). Fig 1 has no question but builds on what...

in the limit as Re→0 , we can neglect the material derivate of v ( Dv/Dt =0 ) but why in books they always make the gravity effects equal to 0? i can't understand and no one really explains this stuff

So Pearson is telling me that, basically, the ratio of internal to external forces and the briefness of the time interval is what determines whether the external forces on a system whose momentum we're studying will affect whether we can obtain a decent approximation of the momenta of the...

Homework Statement The work done by an external force to move a -8.0 uC charge from point a to point b is 25*10^-4 Joules. If the charge was started from rest and had 5.2 * 10^-4 Joules of kinetic energy when it reached point b, what must be the potential difference between a and b? Homework...

Sooooo yeah... My question is something very basic: Internal Forces vs External Forces. From my knowledge, Internal forces does not change the total mechanical energy while external does. But than why is gravity an internal force while normal force is external? Is there any other ways of...

Homework Statement 1) Are F and F' as shown in figure, the action-reaction forces between the two boxes? 2) What is the action and reaction force between the two boxes? Homework EquationsThe Attempt at a Solution I think F = 28N and F' = - 28 N Simply adding the action- reaction forces for...

I am studying momentum and I just want to check that I understand the idea correctly. Think there's a system.In this system there's two masses ##m_1## and ##m_2## moving with some velocity ##\vec v_1## and ##\vec v_2## and they exert a forces each other.Lets call the total force acting on...

Okay, so I've reached the Work Chapters in my textbook, and I've noticed some contradictions, especially in what consists (and what doesn't) an Isolated System, plus the external and/or applied forces. For example, in one of the "Speed Questions" it categorizes a single cube as a non-isolated...

Homework Statement Two metal spheres of equal radius ##R## are placed at big distance one from the other. Sphere 1 has total charge ##q## and sphere 2 has no charge. The two speheres are moved one towards the other until they touch, then they are moved again far away one from the other. What is...

If one fires a pistol while holding it in one's hand, the pistol will exert a backwards force above the grip of the hand, causing the gun to both move backwards as well as exerting a torque, as the hand is essentially behaving as a fulcrum. If a pistol is fired in a gravity-free environment with...

Homework Statement I couldn't understand why the forces act on section Bc of steel rod is 715-325 = 390kN ?Homework EquationsThe Attempt at a Solution Since there's also 215kN to the right in the rod, so forces act on section BC should be 715-325 +215 =605kN ?

Homework Statement While reading course lecture at ocw.mit i have stumpled upon such an equation $$\dot H_O = \sum_{i=1}^n (\dot r_i \times m_i v_i) + \sum_{i=1}^n (r_i \times m_i \dot v_i) = 0 + \sum_{i=1}^n (r_i \times (F_i + \sum_{j=1, j\ne i}f_{ij} )) = \sum_{i=1}^n (r_i \times Fi) +...

Homework Statement A question came to my miind: Wouldn't everything be conserved? I know that the law of momentum conservation only applies when the system doesn't have any external forces or the net of them is equal to 0. Lets just say that someone hits the ball with a bat, He applies a force...

Homework Statement apparently momentum is only sure to be conserved in a closed system - no external forces. So that means if a toy car collides with a wall then the total momentum efter won't be the same as before. That is because the wall would be joined to something else and ultimately to...

1. Question. A 15 kg child slides from rest, down a playground slide that is 4.0m long. The slide makes a 40degree angle with the horizontal. The child's speed at the bottom is 3.2 m/s. What was the force that the slide was exerting on the child? Answer: 75 N [up the slide] 2. My Attempt. Ek...

Consider a scenario where a car accelerates along a path, with no air resistance. If we model the car as a particle, we have the following equation: ##ΣW_{ext} = ΔK + ΔU## By question is with regard to the LHS of the equation. It i my understanding that since the engine is a part of the...

As the title says, can there be any rotation if only one force is applied to an object at rest. For instance, if I had a rod laying flat on a frictionless surface, and I pushed one end, am I correct to say that Newton's second law says that all of that force goes into acceleration the center of...

I'm having trouble with this line from Goldstein-"the center of mass moves as if the total external force were acting on the entire mass of the system concentrated at the center of mass." If a bar is floating in space (at rest in the frame) and a point mass strikes the bar perpendicular to...

Homework Statement I am providing a solution up to the point when I'm having a little issue with defining the generalized force. An eccentrically hollow cylinder of radius r rolls down a plane of inclination angle \alpha. Inside the cylinder, there is a cylinder-shaped hole of radius...

Homework Statement Two identical permanent dipoles, each consisting of charges ##+q## and ##-q## separated by a distance ##s##, are aligned along the ##x## axis, a distance ##r## from each other, where ##r >> s.## a. How many external forces act on each dipole? Homework Equations...

If we have a box with mass m placed on a horizontal tabletop. Then we push the box a distance of d. THe coefficient of kinetic friction between the box and tabletop is .35. When we're asked to find the external work done on the block-table system, it's not the work done by the friction...

Hi all, Doing some self-study on Lagrangian uses on the internet and I'm getting it pretty well thus far, but I'm just not sure how external forces fit in exactly. Up until now I've only tackled problems with gravity and constraints involved but intuitively I know that kinetic and potential...

I am a bit confused about a statement I read in my textbook saying the sum of external forces on a body is equal to the body's mass times the acceleration of it's center of mass. It seems to me if a long rod is suspended in the air by a string at its center of mass, and I tap the rod near one of...

Suppose I have a ring of mass 'M' rotating about an axis passing through its center with constant angular velocity 'w', now if its mass changes by dm due to some unknown reason during rotation, will the angular velocity change, assuming no external force acts on it during any point of time?

Hi I got a question regarding conservation of mechanical energy. The mechanical energy of a system is conserved, as long as there are no transfer of energy (like heat, work, mechanical waves etc.), right? So, why is the mechanical energy in the system of the pivoting rod and the earth, as...

Hi, I had a question about Torque, Internal forces & external forces. Here are 2 examples helping me to state my question: 1) The Earth is rounding the sun with respect to the Sun. The forces which are acting on the system are 2 gravitational forces that are action & re-action. The Force...

Homework Statement I'm trying to crudely approximately the distance individual masonry units fly when a blast load impacts a masonry wall. I'm a structures guy so I can calculate when the wall will failure, but I am having trouble with the calculus associated with the equation of motion. My...

I've got a pretty simple question, and it may sound dumb, but I'm going to ask it anyway: First, I'll preface by saying that from what I understand (and please verify or correct me), planetary orbit in a star system decays over time a la net force. I also have come to understand that the...

The question I am having trouble with shows an image of 2 different conductors, one is at +300 volts, while the other is -600 volts with a few lines going in between which circle around. The points on the equipotential surface are on lines labeled as a (which lies on a -200V line) and the other...

I remember seeing a commercial advertisement in which a hamster in a cage moved itself towards a desirable item (food or something) that its owner had on the counter by throwing itself against the side of its own cage. Would this really be possible? On the one hand it seems like there are only...

We're learning about a systems center of mass and have: F_{net} = Ma_{com} But I having a problem understanding why the net force is only from external forces? Is it because internal forces follow Newtons third law so the net force from internal forces is 0? If that's the case then take...

I have this equation: dP = P(t+dt) - P(t) = mdv +dmv(ex) (ex=external) I have to show that that will equal ma = -ma + F(ex) (Although in the book, acceleration is shown as the derivative of v, (dotted v)). In the example I was reading, there are no external forces, so my first...

Two lifeguards pull on ropes attached to a raft. If they pull in teh same direction, the raft experiences a net external force of 357 N to the right. If they pull in opposite directions the raft experiences a net external force of 127N to the left. A what is the magnitude of the larger of the...

Homework Statement A crate is puled to the right with a force of 75.3N and to the left with a force of 126.2N it is pulled upward with a force of 652.2N and downward with a force of 231.9N A - what is the net external force in the x direction B- what is the net external force in the...

I'm having trouble understanding when to include work done by external forces. I think that's probably hard to understand so I'm going to ask about some examples. Let's say, for example, you have a cylinder with a piston. When the pressure of the inside equals the (pressure of the outside...

I'm sitting in my car on a flat road while it is stationary. The only forces acting on my car for this exercise are gravity and the normal force, which add up to give no net external force on my vehicle and me. My center of mass is at a fixed point somewhere in the vehicle, and according to the...