# What is Internal forces: Definition and 47 Discussions

Force lines method is used in Solid Mechanics for visualization of internal forces in a deformed body. A force line represents graphically the internal force acting within a body across imaginary internal surfaces. The force lines show the maximal internal forces and their directions.

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1. ### I Does a person standing up on a swing in oscillatory motion do any work?

Also, I wonder if a system can have work done to it due to its internal forces, as it is generally said that the work done due to internal forces is zero or at least I have learnt that(cannot trust my knowledge anymore, lol). I got this question in my mind because of a situation in another...
2. ### I Understanding Net Internal Torque and Kinetic Friction in a System

In a system, the net internal torque should be 0. If we have two fly wheels, one spinning with angular velocity w, and the other at rest and the flywheel at rest is dropped onto the other flywheel, the two fly wheels reach the same angular velocity due to friction between the two wheels. I am...
3. ### Should I consider internal forces for this hinged structure?

Summary:: A hinged structure applied with a load, should the structure totally been calculated in order to find the resultant force? Hello, Two situations, the first is a simple structure with a 20 N load at point C. The moment-equation is composed next to it. Second situation: A hinged...
4. ### Sign convention of internal forces in vertical bars for bending moment

Hello: I was looking for a widespread convention (akin to Hibbeler's, Beer's, etc) that deals with the sign convention of a vertical bar for bending moments. For example, without knowing in advance, how do I draw the bending moment at a cut passing through point E in the figure attached? Beam...
5. ### Internal forces converting kinetic into potential energy (vice versa)

Hello, Trivial question: a system is isolated and all its internal forces are conservative. Because of Newton's 3rd law, all internal forces are pairwise and the net internal force is always zero (regardless of the forces being conservative or not) hence the system's total momentum is conserved...
6. ### Internal forces acting on a dive system

Hello please be aware that my understanding of physics is basic, but I want to learn - and I will if explained simply enough please. I looked on here and found the cube in space query from last year, and I just about understood it, but the scalars and vectors did throw me, so hopefully it will...
7. ### Internal forces and the work done by them

My book states that, '∑Finternal=0∑Finternal=0. But the work done by them may or may not be zero. Why is this true? Edit: How can the work ≠ 0 sometimes?
8. ### Work exchanged by a system, and internal forces' work

Homework Statement Hi to everyone, I'm studying thermodynamics, especially I'm focused on the work exchanged by a system. Book's explanation says that when the system received a force from the environment [F(e-s)] (on its surface) it reacts with a force from the system to the environment...
9. C

### B Stupid question of forces from one who skipped Physics B

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...
10. ### Inelastic collision and the sum of internal forces

Homework Statement Suppose I have a system which contains two bodies m1 and m2 with initial velocities v1 and v2 , respecitvely. they hurl toward each other and make an inelastic collision. such that they are now one body of mass m1 + m2 I know that the difference in momentum is...
11. C

### Solving for the internal forces and reactions

Homework Statement I'm having a hard time finding the reactions at the supports and the force exerted by the spring. Given: 20 lbs 25 lbs 35 lbs k = 200 lb/ft lo = 2.5 ft members are 4 ft longHomework Equations ΣM ∑Fy = 0 ∑Fx = 0 The Attempt at a Solution I tried solving for the reactions...
12. ### Work done by internal forces on a system of particles

Dear Experts, Trying to analyze the work done by internal forces in a very simple two particle system which is attracted to each other with a constant force, i performed calculations based on two frame of references, 1.The center or mass frame of reference and 2. Frame of reference of one of the...
13. ### Internal Forces in Truss: Solving for AB at Point A and Point B

Homework Statement For this problem , i found that the internal force AB at point A and point B pointed in the same direction ( as shown) in my working , so , how they cancel off each other ?Since they can't cancel off each other , so they are not stable , right ? The structure is statically...
14. ### I Sum of internal forces equals zero

This is probably a very trivial question, but my brain isn't "playing ball" today so I'm hoping someone can help me with this. Suppose I have a system of ##N## mutually interacting particles, then the force on the ##i##-th particle due to the other ##N-1## particles is given by...
15. ### Internal Forces Diagram: how to do this one?

I'm wondering how should I do analysis on those problems below: Figure 1: Figure 2: I mean, my professor, while doing an example very similar to Figure 2, he didn't do an imaginary cut in the region BD to find internal forces there, and the internal forces diagram that he drew only contained...
16. ### Inertia: Internal Forces & Object Motion

Homework Statement My textbook says that internal forces have no effect on an object's motion and that external forces are required to change the motion of an object. But, we looked at an expirement where a raw egg was spun and then stopped. After being stopped, it went back to spinning...
17. ### Effect of internal forces on the weight of a body.

+ A man is holding a closed cage on the palm of his hand with a bird in it. When the bird flies with an upward acceleration inside the cage, it feels heavier. But the force due to the bird in the cage is actually an internal force on the cage, and internal forces don't cause acceleration of...
18. ### How do I draw a free body diagram of a car on a slope?

Homework Statement A car attempts to accelerate up a hill at an angle θ to the horizontal. The coefficient of static friction between the tires and the hill is µ > tan θ. What is the maximum acceleration the car can achieve (in the direction upwards along the hill)? Neglect the rotational...
19. ### Internal Forces on a Truss (Again)

Homework Statement Hello, in the attached file shows a picture of the problem I need to resolve. The question asks to calculate the internal force in the member BC. There is a pin support located at joint A and D, which is not shown in my diagram, and it is also symmetrical, which makes things...
20. ### Internal Forces of a Truss

Homework Statement Hi there, I've been having problems with this kind of truss in the attached file. I'm used to resolving forces where the supports are located along the same x-axis, but this time I have the supports on the y-axis and a point load in the same direction. I haven't shown it...
21. ### External and internal forces

Can external forces be considered as the forces acting from a distance and internal forces are the contact forces(except frictional forces)? since we know that according to Newton 3rd law every object making a force on a second object the second object will make the same force on the first...
22. ### Internal Force Diagram for Rigid Body and Distributed Load

I'm working on a homework problem for Statics and I'm stuck. Could someone please help?Problem: Draw the internal force (N,V,M) diagrams and include all significant figures Here is all of my work: Resulting F from W1: W(x) = W1 ∴ F2=∫02b W1dx eq (1) ⇒ F2 = W1⋅2b x1 = (F1)-1 ∫02b W1dx =...
23. ### Internal Forces and External Forces

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...
24. ### Internal Forces in System of Particles

I am following along with Goldstien's Classical Mechanics Book and I am on page 11. The text is breaking down the total potential energy of a system into two parts: the external conservative forces and the internal conservative forces. My question pertains to the internal forces. Writing the...
25. ### Internal Forces In a System

My high school teacher told me that internal forces of a system do not change the Centre of Gravity of a system. I have doubt regarding this. If internal force is applied to the wall of a system, the CG must change right? If not, where'll be the result of applied force. I have some examples...
26. ### Discover How to Select Internal Forces for Free Body Diagrams

Homework Statement http://postimg.org/image/4lvunjeoz/ Solution: http://postimg.org/image/v1d7gilef/ When you have to calculate the internal forces at a point, how do you decide which forces are included in the free body diagram? For D, why isn't the rectangular bit of the distributed load...
27. ### Get Help Determining Internal Forces for Part A

Homework Statement http://postimg.org/image/i1r51jolz/ [Broken] The Attempt at a Solution can someone help me determine the internal forces I need to work out the stresses for part A? http://postimg.org/image/c2uv3fwmt/ [Broken] I can't really get any forces from the diagram I drew?
28. M

### Cuts when finding internal forces

Hello, I do not understand how to know where to ''cut'' the sections when finding internal forces in a structure, for example why they cut it into 3 sections at the locations the textbook did in figure 9.17. Thank you
29. ### Statics: Internal forces at a point along a beam

Homework Statement Homework Equations ƩFx=0 ƩFy=0 ƩM=0 The Attempt at a Solution FBD of the entire thing: ƩMB = 780(0.3) + (5/13)T*(0.72) - (12/13)T*(0.6) = 0 T = 845N ƩFx = Bx + (5/13)*845 = 0 Bx = 325N ← ƩFy = By + (12/13)*845 = 0 By = 780N ↓ Now here is where I...
30. ### Internal forces in members of structure

what are the internal force in the hinged member 1-4?
31. ### Truss analysis, determining internal forces

Homework Statement I've been given the above truss and have to analyze it's internal forces as part of the problem, the other part of the problem is to analyze for which lengths members will fail, in what order and for which force F, however this is not the part that I find difficult. In...
32. ### Internal forces for massless rods in rotation

Lets say we have a mass attached to a rigid, massless rod that is fixed at the other end, so we have a system that can only undergo rotation. My question is, if we apply a force to the rigid rod (and not to the actual mass), how can we derive (using only F=Ma) the shear force distribution in the...
33. ### Trouble Working Out Internal Forces in Truss - Need Help!

hi all, im having trouble working out the internal forces in this question, i tried before and got stuck working out the forces at joint C. i have been to see my tutor however he is on holiday till after the deadline for this assignment. handy... i was working out the forces by the...
34. ### Reaching any point mass configuration by internal forces

If we have N unique point masses, is it possible to use only internal forces (i.e. forces between each pair of masses) to reach any configuration of the points (modulo rotations and translations)? I assume this is well known, but don't know where to find a proof. Perhaps by induction from N=2...
35. ### Finding Internal Forces With Joints & Sections

I have the truss shown, I'm supposed to find the internal force for each of the vertical members...I understand method of joints and method of sections, but can you use both method of joints and sections in the same problem...for example I found Ly=52.5 and Ry=32.5 using moments and summing in...
36. ### Torque, Internal forces & external forces.

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...
37. ### Motion in Spacetime (using internal forces), swimming/gliding.

I'm here to discuss motion in spacetime and how it works to hopefully get a better understanding of it. Specifically "spacetime swimming", and the motion of a "relativistic glider", which is talked about in this article "Surprises from General Relativity: "Swimming" in Spacetime" By Eduardo...
38. ### Internal forces in a truss

the queen posted truss in the picture consists of the beam ABCD, compression members BE CF amd the cable AEFD, a stress analysis under the uniform load q is required. outline an analysis procedure, state the effects you would ignore and why. i am not sure what i am meant to be doing here...
39. ### Work done by internal forces in gravitation

Homework Statement Two bodies of mass M and m separated by a distance 'R' are released and they move towards each other under the influence of gravity . What is the work done by internal forces (considering the 2 bodies as a system) when they are at a distance 'x' apart ? The Attempt at a...
40. ### Collision problem /what internal forces?

Homework Statement A moves with momentum p into B at rest. The objects collide and move off with momenta p_1, p_2 at an angle (irrelevant but anyway). What are the internal forces in the system? Homework Equations F=p dot The Attempt at a Solution What's an internal force? Isn't it...
41. ### Internal forces on a system of particles

I am very much confused with this statements "The forces of action and reaction never cancels out each other , but the internal forces on a system of particles cancel out in pairs " I think i know the reason for the first statement.i.e,The forces of action and reaction are acting on different...

43. ### External vs. internal forces

I am having trouble explaining the difference between internal and external forces, and in fact, even *defining* internal forces. Does anyone understand the difference very well? My current example is: Hold an elastic band loosely between both your index fingers. Now, move your fingers...
44. ### Internal Forces (Engineering Mechanics)

problem says knowing radius of each pulley to be 7.2 in, neglect friction, find internal forces at point J of the frame. 1st question that I have is whether the tension in section C-E of the cable also 90 lb? how about section C-D. Also, in my diagram of each of the members, I don't know how to...
45. ### Internal forces and centre of mass

heres another conceptual one . there is a standard yet very elegant problem about a rocket bllowing up mid air and finding the final postions of the parts of the projectile using the principle of centre of mass remaining the same since the forces a re internal when the rocket blows up . now...
46. ### Internal Forces in Coaxial Bars of Different Materials

i have attached a doc file. it shows two coaxial circular bars of different material and a force(30kn) is applied in the top. I think the internal forces in both the bars will be 30kn. Am i correct. bar1 is made of steel and bar 2 of aluminium. the total length of the structure is 10metres.
47. ### Internal Forces in Statics

Please Help! Internal Forces in Statics I have been attempting to answer the problems on my Statics assignment for about a week now but I have not been so success full. I have only be able to answer on of the problems the other three below I get stuck on. Can someone please help me get past...