What is Forces: Definition and 1000 Discussions

In physics, a force is any influence that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newton (N). Force is represented by the symbol F (formerly P).
The original form of Newton's second law states that the net force acting upon an object is equal to the rate at which its momentum changes with time. If the mass of the object is constant, this law implies that the acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.
Concepts related to force include: thrust, which increases the velocity of an object; drag, which decreases the velocity of an object; and torque, which produces changes in rotational speed of an object. In an extended body, each part usually applies forces on the adjacent parts; the distribution of such forces through the body is the internal mechanical stress. Such internal mechanical stresses cause no acceleration of that body as the forces balance one another. Pressure, the distribution of many small forces applied over an area of a body, is a simple type of stress that if unbalanced can cause the body to accelerate. Stress usually causes deformation of solid materials, or flow in fluids.

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  1. chris25

    Fictitious forces on a rigid body

    I was confused by how to work this problem in a rotating frame. The solution read that the centrifugal force on the mass should be of magnitude 𝑚𝑤𝑅^2. However, I thought it would be 𝑚𝑤𝐿^2 where L is the distance between the mass and the center of the circle (L = l + R). What am I missing here?
  2. L

    B Football padding against forces -- What is the appropriate amount?

    I don't know much about forces, speed, and what is needed to cushion a blow. But after seeing the serious injury of Damar Hamlin, is there a way to create equipment that can easily cushion any impact an NFL player can apply to another player? Or does the equipment get too heavy by that point...
  3. W

    Help with wheels, stopping forces and possibly Simpack

    Hi all, I am new to the forum and hoping for some help please. I work in the rail industry and we use chock blocks to stop trains or carriages rolling. I would like to know about the technical side of how and why the chock stops the wheel from moving. Obviously it wedges the wheel to stop...
  4. G

    Calculating forces on a rod with bearings leaning against a wall

    Hello, id appreciate your help for the following case: in a room, a zero weight rod has zero friction bearings at its extremities. One of his ends lies on the floor, the other is against a wall, forming with it an angle alpha. A verical force fv is applied to the 'wall end'. How to calculate the...
  5. A

    The Work of Friction: Explained in .32m

    The answer is .32m. I set the elastic potential energy as equal to the work, but at first I put the force in the work equation as (F elastic - F kinetic friction) times distance and rearranged. 1/2kx^2 = (kx-Ff) d (0.5) (22) (0.035)^2 = (22 x 0.035-0.042) d 0.013475= 0.728 d 0.013475/0.728 = d...
  6. F

    I Conservative forces and internal and external forces

    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...
  7. paulimerci

    Torques exerted on a vertical disk by multiple forces

    This was how the solution was arrived in the text, Net torque = F block x d block x sin ϴ0 + F rod x d rod x sin ϴ0 - T R sin 90 0 = 2mg x 2R x sin ϴ0 + m x R x sinϴ0 - T R T = 5 mg sinϴ0 I'm wondering do we have to resolve the forces for rod and block in to...
  8. Clockclocle

    I Can a Tower of Books Move with Lighter Forces?

    Suppose n book stack on each other. Since each book have the same weigh then the last book exert a force N=nmg on the surface so it has the biggest static friction. But if we treat the whole tower of books as one particle it also has N=nmg. This mean if we exert enough force in the last book...
  9. Hadiza

    Engineering Finding reaction forces of a 3D plate fixed at 3 points

  10. F

    I Distributed Forces and force density....

    Hello, Forces can be concentrated (when acting at a single point) or distributed (when acting over a surface or line). In the case of distributed forces, we can find the resultant concentrated force by calculating a surface or line integral of the force density ##f(x)## w.r.t. an area or length...
  11. Z

    Forces on a rope when catching a free falling weight

    If I am not wrong, F = 2*10*9.81*4/0.2 = 3924N (0.2m considering 5% stretch in 4m long rope)
  12. Ahmed1029

    I Virtual work and constraint forces

    Suppose I'm considering a system of N particles that are constrained in their possible motions and so there are less that 3N generalized coordinates. Suppose now I perform a virtual displacement on one particle, which due to some constraints might force some other particles to more virtually...
  13. C

    Is gravity a force or just a consequence of mass?

    Since gravity is caused by mass deforming spacetime, is gravity actually a force or just a consequence? Are there really 4 forces in nature or just 3?
  14. J

    I Particle exchange explaining attractive forces

    I understand, and have unwillingly come to terms with the fact that virtual particles can carry negative momentum. This explains how momentum can be conserved in attractive forces via particle exchange. I have a problem with this that I cannot reconcile...wouldn't this imply that as a particle...
  15. T

    A Principle of Virtual Work and the forces that DO NOT do work

    In this 2D figure below, I can place: a motor at O a motor at J gravity on each link I can use Hamilton's principle, modified to the principle of virtual work and I can compute the motion of the linkage system. I do not have to account for these force FOUR forces (in this planar problem)...
  16. S

    Correct vector diagram of forces

    The answer is (B) but I don't understand why (C) is wrong. The force acting on the hinge has two components, horizontal and vertical. The horizontal component must be to the right to balance the horizontal component of tension but the vertical component can be either upwards or downwards. Wow to...
  17. P

    I Defining the Forces from Magnetic Fields and Electric Fields

    We define Electric Field Intensity vector at a point as the force experienced by a unit positive charge kept at a point. Is it correct to define B vector similarly that is, is B vector the magnetic force acting on an unit magnetic north pole and is it correct to call B vector Magnetic Field...
  18. C

    I Steering a Car: Investigating the Forces of Rotation

    In a car we turn the wheels to steer. The wheels however are spinning about their axis of rotation when the car is in motion. Does the revolving motion of the wheels cause a force that opposes trying to rotate the wheels around the other axis to steer? How much opposition is created? Here's...
  19. tbn032

    B Unable to understand how these two forces are equal

    In the solution given in the above image, I am unable to understand and prove why N1=f and N2=W. I have tried balancing the torque on different point but still unable to prove. Explain how N1=f and N2=W can be proved. The justification for N1=f and N2=W which I have so far read is that it is...
  20. Cody Livengood

    B Is there an experiment that shows strong force grows with distance?

    Is there an experiment that shows strong force grows with distance?
  21. L

    Engineering Hand tool boundary conditions - Forces determination

    Hello, I' m trying to make a linear static analysis (Finite Element Analysis) on the following hand tool. I want to determine the boundary conditions. In order to do that I have decided to use a force couple to represent the forces that a bolt exerts on the jaws of this spanner. Despite using...
  22. L

    B Do forces of attraction really exist or are they just an illusion?

    I wonder if forces of attraction really exist. Can two things separated by vacuum get attracted? It seems impossible.
  23. F

    I Solving the Block-on-Spring Problem Using Forces

    Hello, The classic problem of a block dropped on a vertical spring from a height ##h## above the scale: find the overall compression distance of a spring when a block is dropped on it and brought to rest. This problem is easy to solve using conservation of energy, potential gravitational...
  24. link223

    Forces acting on a Walking Beam (oil rig pump) from a pivot pin

    So.. question: - How do I know that only the pin is at work at E and not those 2 beams? my guess: It is because those 2 beams are connected to the pin whilst the pin is the one that exerts a force on that walking beam DEF?
  25. A

    I If we take two charges and hold one still, what are the forces?

    If we take two positive charges +q1 and +q2, and we hold q1 still, does q2 experience a repulsive force due to q1 repelling q2 AND a repulsive force due to q2 repelling q1? That is ##|F| = 2*\frac{k*q1*q2}{r^2}##
  26. A

    I My textbook is deriving wave speed on a string under tension

    My textbook is deriving wave speed on a string under tension with confusing thetas. It assumes ##\tan \theta_1 = \frac{-F_1}{F_T}## and ##\tan \theta_2 = \frac{F_2}{F_T}## which confuses me. I know for sure theta is the angle due to the position of y and x, ##\tan \theta = \frac{y}{x}##, but I...
  27. A

    Engineering Problem with the reaction forces in the gearbox

    Greetings, I have tried to find the reactions of the bearings on the Y direction in the intermediate shaft. but I couldn´t find the numerical results of the solutions here is my attempt: in order to find the forces I did the moment equilibruim around the point A and B With FT1 FT2 and...
  28. G

    What mistake did I make in finding the reaction at hinge A?

    A very simple (I thought!) question: I'm just looking at the first part, finding the reaction at the hinge A. Here is my annotated diagram, with the reaction and A resolved into it's X and Y components, the force at E labelled as Fe and the length of ED labelled as L. Considering the body...
  29. V

    Forces when car wheels "lay rubber"

    Suppose the car is moving to the right, so if the wheels roll without slipping, they are rolling clockwise. To get the wheel to slip, a counterclockwise torque would need to be applied to cause the wheel to have some angular acceleration. If the wheel was slipping, then the bottom of the wheel...
  30. G

    How to find the forces in a particular framework of light rods

    Could I please ask for help in how to do this question. Is it in fact well formed, can it be solved as it is or do I need more information? Q. Find the external forces and the force in each rod in the following framework of light rods which is supported and A and C: So, I need to find Fa...
  31. Finn072

    Shear lines from a FBD with distributed forces

    , this is the FBD in question
  32. A

    I Forces to move on the level versus up an inclined plane

    If on a flat ground, we exert a force F to move forward, then we go to an incline plane of theta degrees. Why wouldn't the force F2 to move up the incline plane with respect to ground be F2*cos(theta) = F --> F2 = F/cos(theta) disregarding the effects of gravity?
  33. Amphibia

    Forces on two different blocks with two pulleys

    I don't know what to do as I know the forces acting on it but not the scalar like I just don't understand the question with two different pulleys I know the tension same everywhere
  34. Y

    Forces applied to a block on a ramp

    hey I am really hope for some help becuase i don't know what I am doing wrong, the angle of the first part is 21.801. here is my solution i would like to know what I am doing wrong
  35. G

    To find the forces acting in a system in equilibrium

    Could I please ask where I have gone wrong with my reasoning in the following question: The answers in given in the book are: (1/2)W tan(Ө) W vertically (1/2)W tan(Ө) horizontally Here is my diagram: Considering the system as a whole: (In the text below "Ya" and "Xa" are the forces at...
  36. B

    B Can You Use a Rubber Bracelet as a Buoyancy Reference for Freediving?

    First of all, I'm new to the forum, and rather uneducated, so I apologize this is likely in the wrong section. I've recently (like last night lol) started looking into freediving. I haven't taken any classes or anything yet, so all of this might be common and answered there. However, one of...
  37. J

    A Generalized Forces and QED/QCD

    In Lagrangian mechanics we learn about generalized forces. However, I haven't seen these explicitly mentioned in books on QFT. Can the Lagrangians of QED or QCD be expressed in terms of generalized forces or is there some connection there, in particular to the Nielsen form.
  38. S

    Body forces in static equilibrium (FEM issue)

    Hi, I have an issue with understanding the body forces in the context of FEM simulations. I am using freefem to do the simulations. So I took a simple unit cube, fixed the displacements at side ##x=0## and prescribed a displacement ##u_x = 0.025## at ##x=1##. If required, the entire script is...
  39. SpectraPhy09

    Plotting the graph of Friction Force vs. time (Laws of motion)

    F.B.D Of first block (I have shown only the horizontal Forces) f1(max) = μ (1kg)(g) = 0.5 * 10 = 5N F.B.D Of the second Block f2(max) = μ (3kg)(g) = 15N Now the string will become taut and the tension will start acting when f = t = 5N But for 0<f<5N there will be no motion between the 1 kg...
  40. runningphysics

    Two forces acting on an object given in vectors - SOLVED

    I tried splitting the forces up into F1 and F2 making Newtons second law equation into F1+F2=ma. Then I added over the the first force given. multiply the mass to the acceleration terms to get F2= (m*ai + m*aj) - F1
  41. M

    I Unbalanced forces acting on a ball at maximum altitude

    When a ball projected vertically upright reaches its maximum height is it instantaneously stationery (calculus provides for instantaneous events) before it starts its downwards journey? is gravity acting on the object at that instant? Newton provides that an object at rest must be subject to...
  42. U

    Comparing hydrostatic forces at the bottom of these 3 containers

    hello all , 1st and 2 nd water containers sit on ground and 3rd type of water container sit on fixed support along X axis and bottom parts are suspended in air , pls see picture , the drawing is half cross sections of cylinders All water level heights H and tubes D diameter are same ...
  43. B

    B Forces on a passenger in a car?

    What would the forces acting only on a passenger in a car going at constant speed on a level surface be? Of course, there would be weight and the reactive force acting on any passenger, but is there a horizontal net force? Or are there just the forces acting on the car and there are no...
  44. Z

    Torque and forces (Beginner level)

    Summary:: I am trying to figure out how much force that either end of a tilted rod are subjected to as a consequence of gravity. One end being attached to a hinge, the other to a string from above. Hello, it's my first time on this forum. I have recently started learning about torque (Nm) and...
  45. Yossi33

    Forces that act on a physical pendulum

    hello, i have a question about the forces that act on a rod at it's pivot point. the rod is free to rotate about the pivot point at the edge and it starts from rest parallel to the ground.the question is : when it reach to angle theta find the a] the angular velocity b] angular acceleration c]...
  46. G

    To find direction and magnitude of forces acting in a system

    Please could I ask for help with the following: Here is my diagram, I show the rod displaced from the sphere so as to label the internal forces acting on each of the rod and the sphere: In the diagram below I have added the line through DE at angle ꞷ the the horizontal, and a few other...
  47. R

    I am calculating the forces on String cheese with net force = 0

    Here is the link to the question. https://www.khanacademy.org/science/physics/forces-Newtons-laws/Newtons-laws-of-motion/a/what-is-Newtons-second-law What is the magnitude of the force F1 ? What is the magnitude of the force F2 ? Here is my drawing I made At this step I am lost. ## ay...
  48. J

    A Problem solving forces with pressure integration

    Angle theta is different for every place at airfoil surface, so it can't be one theta from leading edge to trailing edge. Can please someone explain pressure integration in depth, step by step?
  49. K

    I Potentials of conservative forces

    Goldstein writes "only if ##V## is not an explicit function of time is the system conservative"That means ##V(r,\dot{r})## is a conservative potential, however I think that only potentials of the form ##V(r)## are conservative potentials. Could you please tell me where I'm going wrong. Thank you.
  50. rudransh verma

    What are the forces at play in an elevator cab?

    I mentioned answers above in the question.
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