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. 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?
  2. 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...
  3. 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...
  4. 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...
  5. 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...
  6. 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...
  7. 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...
  8. 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...
  9. 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) ?
  10. 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...
  11. 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...
  12. 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...
  13. 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...
  14. 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...
  15. 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
  16. 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?
  17. 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...
  18. 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?
  19. 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...
  20. 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...
  21. 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
  22. K

    Nuclear Weak Force & Strong Force: Intro Recommendations

    Any recommedations for intro into the weak force? Also ones for the strong force?
  23. 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...
  24. 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...
  25. 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...
  26. 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...
  27. 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...
  28. 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...
  29. 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...
  30. 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...
  31. 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...
  32. 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.
  33. Maria

    Simple closed force system -- Mechanics

    Homework Statement There is a system of 4 points located along a circle of radius R. Points are connected by undeformable ropes (pink on the picture). There is a force applied to each point. Scheme: https://we.tl/MsCEViCQdB I need to find resulting force in the system. The Attempt at a...
  34. Amitayas Banerjee

    Problem in getting correct coefficients of frictional forces

    I am getting correct equations on using the Lagrangian method in Systems with no non conservative forces, but when I use it in Systems with friction, sometimes I get correct equations, and sometimes I do not. Most of the equations have some problem with the coefficients of the frictional forces...
  35. M_Abubakr

    How Are Reaction Forces Calculated in a Hinged Beam and Strut System?

    Homework Statement Consider beam “A-B” and strut “C-D” from Figure QA1. The beam “A-B” is hinged at point “A” and hinged with strut “C-D” at point “C”. It has an applied force of 8 kN applied at point B as shown in Figure QA1. Strut “C-D” is hinged at point “D” and it is also hinged with beam...
  36. harambe

    How Does Friction Affect Acceleration on an Incline?

    Homework Statement Figure shows two blocks in contact sliding down an inclined surface of inclination 30.The friction coeffecient between the block of mass 2kg and the incline is μ=0.20 and that between block of mass 4kg and the incline is μ=0.30.Find the acceleration of 4 kg block (g=10m/s2)...
  37. lc99

    MATLAB MATLAB: forces acting on an object

    i have a assignment for my MATLAB (first year) class, and I am slightly confused om what exactly i should be coding. im not struggling with the coding part, but what i am suppose to code. im used to coding based on what it is needed. So, for my project, I am suppose to represent the forces...
  38. D

    I Adhesive Forces: Bigger vs. Smaller Straws

    Is the adhesive force greater in a smaller tube then a bigger tube at the top? So would it take more energy to take out water at the top of the straw from the thinner straw? Or would, due to gravity cancelling eventually the capillary rising, the adhesive force at the very top of the straw is...
  39. isukatphysics69

    Calculating Rank Pulley Forces with Newton's Equations

    1. Homework Statement In picture Homework Equations f=ma The Attempt at a Solution i can use Newtons equations here, but i am confused about the frictional force.. so it doesn't specify whether it is a frictional or unfrictional surface.. is there something I'm missing here? does friction not...
  40. isukatphysics69

    Ranking Forces in Picture: Net Force and Friction Equations

    1. Homework Statement in picture Homework Equations My equations are netforceyA = -MG + Friction between A and Hand + Friction between A and B netforceyB = -MG + Friction between B and A The Attempt at a Solution Friction between A and B are equivelant due to Newtons third law since they...
  41. harambe

    B Constant Force: Energy Conservation & Path Independence

    I had an argument with my friend regarding constant forces.He said that work done by constant forces are conservative as we can take the force out of the integral and it doesn't depend on the path .But the fact that this information is not given in my textbook as well as by teachers makes me...
  42. isukatphysics69

    What is the ranking of forces in a problem on ranking forces?

    Homework Statement Homework Equations f=ma The Attempt at a Solution ( i have put numerous different forces into the answer box and nothing worked. i was just taking a screenshot of problem here) Edit: never mind about kinetic friction with wall i just saw that there is no friction with...
  43. harambe

    Tension in the wire connecting two blocks

    Homework Statement Two blocks each having masses of 3.3 kg are connected by a wire CD and the system is suspended from the ceiling by another wire AB.The linear mass density of the wire AB is 10X10^-3kgm^-1 and that of CD is 8X10^-3.The speeds of the transverse waves produced in AB and CD are...
  44. I

    Forces acting on a spool of mass M

    Homework Statement Two forces ##F_1## and ##F_2## are applied on a spool of mass ##M##, moment of inertia, ##I##, and radius ##R## as shown in the figure. If the spool is rolling on the surface, find the ratio of forces, ##F_1/F_2## such that friction between spool and the surface is zero...
  45. M

    I Forces applied to trailer hitch

    If I am towing a 10,000 lb. trailer downhill on a 6% grade at 65 mph and I hit the brakes coming to a complete stop in about 5 seconds, how much force is being applied to the trailer hitch? I am trying to build my own hitch and the forces applied will help me determine plate thickness. I will...
  46. R

    MHB What is the resultant force and angle for two forces acting on an object?

    A 100 pound force and a 150 pound force act on an object. If the angle between the force 40 degress, find the magnitude of the resultant force and the angle the resultant force makes with the 150 pound force.
  47. EF17xx

    Understanding Static Friction on a Ramp: Why is the Object Stationary?

    Hi , The Static Friction force of an object on a ramp(inclined plane) making an angle with the horizontal is greater than the weight component down the ramp. My textbook says that this means that the object on the ramp is stationary as the frictional force is greater than the weight component...
  48. Liam C

    Coefficient of Friction when Normal Force is Reduced [HS]

    Homework Statement A 5.0kg block of wood sits on a table with a string running over a pulley suspending a mass. The largest mass that can be suspended without moving the block of wood is 0.75kg. a) Calculate the coefficient of friction in this case. b) What type of coefficient of friction has...
  49. thomassmith235

    Finding Unknown Forces on a Beam Structure

    Homework Statement I've been trying to find the unknown forces for this beam so I can complete an FEA analysis. Homework Equations ∑mz=0, ∑fx=0,∑fy=0 The Attempt at a Solution Attempt is attached
  50. A

    How does the transfer of electric forces between two objects work

    Questions 1.) Here, the negative charge from the wool is transferred to the plastic rod. I'm curious as to what determines which object is responsible of transferring their charge to the other object. Like why didn't the plastic rod give its negative charge instead? 2.) If we rub 19 more...
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