Forces Definition and 1000 Threads

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.

View More On Wikipedia.org
Recent contents Top users
  1. devilish_wit

    A question about contact forces and friction

    Homework Statement Two blocks of masses m1 and m2 are placed on a table in contact with each other as shown in the figure below. The coefficient of kinetic friction between the block of mass m1 and the table is μ1, and that between the block of mass m2 and the table is μ2. A horizontal force of...
  2. T

    I Gravity Propagation: How Does Time Affect Orbital Modeling?

    I was thinking about the idea of looking up at a particular minuscule spot in the sky to see a particular planet at a very precise time, say, with a telescope. I was considering how light takes a matter of minutes to reach us. But then, remembering that gravitational forces must travel through...
  3. J

    Forces on a Block and Friction on a Surface

    Homework Statement A 2.5 kg block is initially at rest on a horizontal surface. A horizontal force of magnitude 5.8 N and a vertical force are then applied to the block. The coefficients of friction for the block and surface are μs = 0.43 and μk = 0.24. (a) Determine the magnitude of the...
  4. P

    Question about resolving forces

    Homework Statement On a model ship, the mast OC has length 50 cm and weight 20 Newtons. The mast is hinged to the deck at O, so that it can rotate in the vertical plane of the ship. Small smooth rings are fixed at points A and B on the deck in this plane such that AO=OB=50 cm. Threads from C...
  5. C

    Physics rifle problem -- Forces and acceleration of the bullet

    Homework Statement A rifle bullet of mass 10g is fired and takes 2.0 ms to travel down the rifle barrel of length 0.50m with constant acceleration Calculate the muzzle velocity (bullet velocity on leaving the barrel) Calculate the acceleration in the barrel After traveling a short distance...
  6. archaic

    Maxwell's wheel and the conservation of energy

    We experimented with the Maxwell's wheel today and at the end we were asked about why does this apparatus stop since there is conservation of energy. I did some research and apparently there is a type of friction called "rolling friction", wikipedia defines it as "the force resisting the motion...
  7. I

    Electric Fields and Electric forces help

    Homework Statement This is the question Structure opposite comprises 5 ions each of which has lost one electron. The sides of the square formed by the 4 outer ions are of length 0.2nm Ion E is at the midpoint 1.What is the force on A due to ion A?Homework Equations I'd imagine: F= kq / r^2...
  8. alexi_b

    Find when the acceleration is 0

    Homework Statement A block of mass 2.42kg is accelerated across a rough surface by a rope passing over a pulley, as shown in the figure below. The tension in the rope is 13.9N, and the pulley is 12.3cm above the top of the block. The coefficient of kinetic friction is 0.395. (a: 2.28m/s^2)...
  9. alexi_b

    How far apart are the astronaut and satellite after 8.67 minutes?

    Homework Statement A 63.8kg spacewalking astronaut pushes off a 678.0kg satellite, exerting a 156.0N force for the 0.870s it takes him to straighten his arms. How far apart are the astronaut and the satellite after 8.67min? Homework EquationsThe Attempt at a Solution I tried just finding the...
  10. alexi_b

    Finding the angle in which the resultant force points

    Homework Statement Forces of 11.8N north, 19.2N east, and 15.9N south are simultaneously applied to a 3.93kg mass as it rests on an air table. What is the magnitude of its acceleration? What is the direction of the acceleration in degrees? (Take east to be 0 degrees and counterclockwise to be...
  11. vincent

    I What is it that forces an object down curved space time?

    To explain the concept of curved space time, we often use analogy of rubber sheet. If we put a heavy ball at the centre of sheet then it creates a depression and now a smaller ball will fall towards that heavy ball because of depression. But in this analogy smaller ball is falling down the slope...
  12. Physicsterian

    Effect of density on downward rolling spheres

    Homework Statement If two balls, being identical in volume, but different in density (one ball is made of iron, the other of aluminum) roll down from an inclined plane, which will reach the bottom first and which will cover a larger distance after having reached the bottom? IMPORTANT NOTE...
  13. Lukeblackhill

    I Source of Energy in a Field of Forces

    good evening! My question is the following: it is well stated that energy is conserved in any isolated system, and we can in most circumstances expand our system to a many-steps conversion of energy, but always conserved. The classic example could be the energy of the sun stored in form of...
  14. Exath

    I Why is T+f=Ma in Cylinder Roll Without Gliding?

    So I'm looking at a problem that involves a situation that looks like this the cylinder rolls without gliding. And there are these following equations that apply to it (1) mg - T = ma (for the block hanging vertically) (2) T + f = Ma (for the cylinder f = friction force, T = String force) (3)...
  15. V

    Calculating Free Body Diagram Forces

    On EngineersEdge.com they have a free body diagram of a scissor lift https://www.engineersedge.com/mechanics_machines/scissor-lift.htm Can anyone explain how the Fy, Fx components are determined? What is specifically confusing to me is how Fy is opposite in direction given the system. To me...
  16. S

    I Forces of Constraint: Solving Euler-Lagrange Equation

    Hello! I have this Lagrangian: $$L=\frac{1}{2}m\dot{r}^2(1+f'(r)^2)+\frac{1}{2}m\dot{\phi}^2r^2-mgf(r)+\lambda(\phi-\omega t)$$ This represents the motion of a point-like object of mass m along a curved wire with shape $$z=f(r)$$ The wire rotates with constant angular velocity around the z axis...
  17. Physicsterian

    B Why does the force increase when holding a bag at an angle of 45 degrees?

    Hi, If 2 people are holding a bag at an angle of 45 degrees each, and then only person is going to hold it, it is being said that the force that will have to be applied by that one person will be 1.5 times greater than when he was applying it together with the other. Can anyone explain this or...
  18. J

    Which forces am I missing in the FBDs?

    Homework Statement Two blocks with pulleys attached lie on a frictionless surface and are connected by a massless rope strung over the pulleys as shown at right. A force of magnitude F is applied to the second block. Draw a FBD for a system consisting of Block 1 and its pulley, and also a...
  19. C

    I Two blocks connected with a fragile rope

    Let's say that we have two blocks - 2 kg and 1 kg. They are connected with a rope that can stand the maximal tension of 20 N. Does the maximal force we can apply to the block so that the rope does not break apart depend on the choice of the block?
  20. Beth N

    Newton’s Law of Motions: tension forces in a pulley

    Homework Statement Problem: 4.93[/B] The pulley is assumed massless and frictionless. The mass of the object attached to the pulley is given in terms of m, the force applied to the pulley is F (refer to diagram), and frictional force is f. Question: Find the acceleration of the mass m in...
  21. CMW328i

    Engineering Help with magnetic field forces in a motor

    Hi all, Not a question about completing homework here, but I'm a teacher looking to create a realistic engineering question for an assignment. I have an engineering scenario I've set for the assignment which is a junior engineer working for a marine engineering company so all of the questions...
  22. Y

    Calculating Forces & Stress of Vertical Cut Line on Beam

    Homework Statement 1.Explain how one with the help of the free body diagram can calculate forces and moments on the vertical cut line through the beam in a distance 1 m to the right from the point C. 2. Explain how one calculate size and distribution normal stress of the cut line. The beam...
  23. Y

    Determine the forces acting on the two rods - Free body diagram

    Homework Statement Determine the forces acting on the two rods in the figure. Explain which stresses that act on the rods and the bolts holding the rods to the brackets in point A and C. Homework Equations ∑Fx = 0 ∑Fy = 0 ∑Fz = 0...
  24. V

    Why is the normal reaction force zero in the absence of any external force?

    Homework Statement My question is not a homework problem. I am having difficulty in understanding the concept oof contact forces. Homework Equations What I don't understand is that when two bodies A and B are in contact and left on a frictionless surface, they apply normal reactions on each...
  25. P

    Motion of particle changing forces

    I've got a question and can't find any answer on google. So the centripetal force for a particle in uniform circular motion is mv^2/r. This also means that if F > mv^2/r than the particle will get closer to the center and if F < mv^2/r than it will travel further from the center. Say a 5 kg...
  26. jacobthejones

    Statics - Compression and tension forces in a truss

    Homework Statement I am trying to find the forces in KJ, CJ, and CD. Homework Equations ΣFx=0 The Attempt at a Solution I found the magnitude of KJ to be 26.8kN, which is correct (according to answers in the back of the book). I believe it is a compressive force. The book lists all 3 forces...
  27. jk22

    B Equivalent gravitational forces in other galaxies

    Could it be imagined that due to a particular stars' distribution in a galaxy the gravitational force felt would be like $$f(\vec{e}_r)/r^{\alpha}$$ where $$\alpha\neq 2$$ but near 2 and f a non spherically symmetric function (like a comet around a flat galaxy) ?
  28. Krushnaraj Pandya

    What Force Keeps a Bar at a Constant Angle on a Smooth Plane?

    Homework Statement A uniform bar of mass m is being moved on a smooth horizontal plane by applying a constant horizontal force F acting at the lowest point. If the rod translates making a constant angle 'theta' with vertical, the value of F must be? Homework Equations 1) F=ma 2) Torque=Moment...
  29. E

    Basic forces question with a pulley problem

    Homework Statement A boy holds the rope so it can't move. What is the tension in point A? Homework Equations ma=F The Attempt at a Solution The answer in the book is 4N. I understand how to get it, but, I believe that the tension of S1 would be 9N (due to m1), and then the forces on m2 would...
  30. E

    Acceleration of Body in Elevator & Earth with 0.5g Rise

    Homework Statement A slope is inside an elevator. A body with the mass of m is on the slope. θ=30°. What would be the acceleration of the body in relative to the elevator if the elevator rises with an acceleration of 0.5g? What would be the acceleration of the body in relative to the Earth if...
  31. S

    I Exploring Radial Forces Generated by Wrapping Strings

    Hello all, I have been racking my brain with this one all day and I can't seem to figure it out. I have been looking into the forces that would arise from wrapping a string, wire, etc. around your finger. The scenario is illustrated in the attachment. Basically the question is, if we start to...
  32. ephphatha16

    Find Value of tan φ of Reaction Force | Isaac Physics

    1. Find the value of tan φ to 2 sf where φ is angle made by line of action of reaction force at hinge above horizontal. https://isaacphysics.org/questions/trapdoor_num Part B Angle of reaction forceHomework Equations Mass of uniform trapdoor: m=4.0kgm=4.0kgThe trapdoor is held open making an...
  33. alijan kk

    Finding the Magnitude of 3 Forces on a Square

    Homework Statement Three forces each of magnitude 1N act from one corner towards the other corner of a square there sum has a magnitude nearest to : Homework EquationsThe Attempt at a Solution diagonal of a square is equal to √2 , the answer should be 3√2
  34. T

    B Find Perpendicular Forces Given 20 N at 60 Degrees

    I have a question. If you're given a force of 20 N and its 60 degrees to the horizontal, how could you find two perpendicular forces?
  35. Andres Padilla

    Find the mass M so that the net torque is zero

    Homework Statement Hello, I have a problem I don't want how to approach, it is a little weird. It is a problem of equilibrium (torque) The situation and my approach is in the picture I uploaded. A beam is supported by the blue structure. On the beam it is a person(100N) at a distance of 3m in...
  36. B

    How should forces be drawn on a ladder? Considering the cross-members

    < Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown > In the attached file we can see simplified ladder. I am interested on which side (right or left) the forces are correctly drawn. I would draw forces as the right side shows. So which one is right?
  37. A

    B How do helmets help prevent injury to the head?

    Hi. So I've got an essay I have to write on sports equipment and how they increase safety. I know that in football, equipment to increase safety includes helmets, shoulder pads or paddings and cleats. Unfortunately, I have no idea how each of those equipment work to increase safety. Okay, let's...
  38. A

    Dynamics: Forces in mechanism problem

    Homework Statement Viewing a solution on the internet to this question, the free body diagram is drawn as: The forces here are then used to solve for ##F_D## and this is given as the answer to "the force which the bar exerts on the link at D". I'm just confused as to why there is no...
  39. thevinciz

    Finding Equilibrium of Forces: A Shortcut Method

    Homework Statement The questions are in the comment box. Sorry! https://www.img.in.th/image/V9K8Rg Homework EquationsThe Attempt at a Solution 1. I think α = 90 because BD is normal force. and I don't know anymore PS. sorry for bad english
  40. K

    Nuclear Weak Force & Strong Force: Intro Recommendations

    Any recommedations for intro into the weak force? Also ones for the strong force?
  41. B

    Forces acting on a truck on a slope

    Homework Statement The truck is on the slope 45 degrees and on it is acting a horizontal external force. How big is the force of the ground if the mass of the truck is 5000 kg and the truck is not sliding down the slope? Homework Equations First of all I drew myself a sketch. Then I asked...
  42. J

    Magnitude of the force of static friction

    Homework Statement A block of mass M is at rest on an incline that makes angle theta with the horizontal. Given that the static coefficient of friction is mu_s1 and the kinetic coefficient friction mu_k, what is the magnitude of the force of static friction exerted on the block by the incline...
  43. H

    Need help with shredder calculations -- Plastic bottle shredder forces....

    this is my design i want to calculate its various stress and forces. since the bottle is being cut, i think there is perpendicular force upwards, how do i calculate this cutting shear force? 1) force and stress applied by cutting blade on the plastic bottle 2) force moment and stress...
  44. Ghost117

    A Consequences of unifying Gravity with the other forces?

    When humanity unified electricity and magnetism, that ushered in a massive technological evolution for our species, perhaps only equalled by the advent of QM and modern electronics. Is it even possible to imagine the consequences of unifying Gravity with EM? Is it possible that we will...
  45. E

    What are the forces on the center of mass?

    Homework Statement [/B] A light rope passes over a light frictionless pulley attached to the ceiling. An object with a large mass is tied to one end and an object with a smaller mass is tied to the other end. Starting from rest the heavier object moves downward and the lighter object moves...
  46. I

    Calculating the forces on a crankshaft assembly

    Hi everyone, So I'm working on this project where I have to determine the total force that is being applied to an assembly that is attached to a crank shaft which is in turn attached to a motor that's driving it. I looked up the equations for the acceleration and velocity of a crank shaft (or a...
  47. K

    Calculating the frictional forces on a sliding box on a ramp

    Homework Statement We kick a box upwards a ramp. The box slides upwards, then slides backwards again. We ignore the air drag and other factors. We only look at the gravity, normal forces and friction forces. Forces on the box when it's moving upwards: Weight of the box is 0.123 KGs. Angle...
  48. P

    A Compute Normal Forces on Box Sides via Stress-Energy Tensor

    Suppose one has a box moving through flat space-time with a stress energy tensor ##T^{ab}## that's non-zero inside the box and zero outside the box. How does one compute the normal forces on the faces of the box associated with it's motion? I am assuming that the normal forces are measured...
  49. C

    Forces in inertial and non-inertial frame of reference

    Imagine such situation: A bus is moving at constant speed, a man is standing on the bus and is not holding to anything - he is simply standing. Now, the bus starts breaking with constant negative acceleration and so the man will change his relative position due to the force of inertia. Now...
  50. Khatti

    Technological mastery of fundamental forces

    This may be an overly-broad question, but is there any reason to think we will ever have the technical mastery over gravity, the strong force, and the weak force, that we have over electromagnetism? Example: turn on a switch, gravity; turn off a switch, no gravity.
Back
Top