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. M

    Why does a ball move in your hand when net forces are 0?

    This is a more general and probably silly question, but if you have a ball in your hand and you move it upwards, the acceleration will be positive then after you move it at constant velocity, acceleration will be 0. However, how is net force is 0 when the ball is still moving? You are clearly...
  2. P

    Fictitious forces and artificial gravity

    I'd be extremely grateful if anyone could help me with this ... its mainly part iv) that I'm stuck on but the other parts build up to it! Thanks very much! 1. Homework Statement I have a wheel shaped space station of radius 100m rotating about its symmetry axis (defined to be the z axis) at...
  3. S

    Work energy theorem and forces at equilibrium -- Conceptual doubt

    A pendulum of mass m and length l is suspended from the ceiling of a trolley which has a constant acceleration a. Find the maximum deflection θ of the pendulum from the vertical. When I used work energy theorem, I got θ = 2 arctan(a/g). But when I took the equilibrium position and equated the...
  4. 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...
  5. H

    With which forces and why spring acts on two bodies

    Homework Statement Suppose we have spring which is between two bodies of different mass, with which force spring will act on every of them? Homework Equations Spring acts on a body with force kx The Attempt at a Solution A-/\/\/\/\/\/\/\-B Suppose body A has mass 2kg and body B has mass...
  6. D

    What forces contribute to a runner's forward propulsion?

    Homework Statement Homework Equations None. Only free-body diagrams I think, but I'm unsure how to exactyl draw it. The Attempt at a Solution So, my reasoning is that it is D, but I'm not exactly sure. When drawing a free body diagram, I only draw one force of friction, but I think...
  7. C

    How Does Biaxial Force Affect Stress and Strain in a Rectangular Bar?

    Homework Statement consider the rectangular bar undeformed a=b=2cm and c=20cm . The elastic modulus of the bar material is E=100 Gpa and its poissons ratio is u=0.3. The bar is subjected to biaxial forces in the x and y directions such that Fx=Fy=0.4x106 N and that Fx is tensile while Fy is...
  8. Pushoam

    Forces on Dielectrics: Understanding Griffith's Equations

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  9. C

    What are the new dimensions of a cube under biaxial forces?

    Homework Statement Consider the cube with sides a=b=c=10cm . This block is tested under biaxial forces that are applied in the X and y directions. Assume that the forces applied have equal magnitudes of Fx=Fy=2x10^6 and that the modulus and poissons ratio of the block material is E=2x10^11 Pa...
  10. M

    Forces of 3 charges in an equilateral triangle?

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  11. kmm

    I Understanding Conservative and Velocity Dependent Forces in Physics

    I actually have a few things I'm thinking about here. I'm curious as to whether a velocity dependent force field absolutely cannot be a conservative force field, in principle. I have at times come across statements in physics that I found out had mathematical exceptions for, but we don't...
  12. J

    Find the tension in the cable and reactive forces at ground

    Homework Statement A 160-kg utility pole extends 12m above the ground. A horizontal force of 250 N acts at its top and the pole is held in the vertical position by a cable, as shown in the figure (I have attached the photo). 1) draw a free-body diagram for the pole 2) Determine the tension...
  13. Alexander350

    Finding all the forces in this system

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  14. A

    I Can a 63mm Bore Pneumatic Ram handle the required weight at an angled position?

    Hey So I am still a beginner in Physics and work asked me today to calculate if a 63mm Bore Pneumatic Ram will do the required job. Anyway, so I know how to calculate the weight capacity the ram can handle if it is at a 90 deg angle however if the ram is on an angle then I am stuck. I need to...
  15. deuce123

    Pulleys with friction , FORCES

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  16. A

    Forces acting on a thrown object

    Homework Statement A ball with a mass of 200 g is thrown vertically upwards with an initial velocity of 16.0 m/s. It is allowed to fall back to its original position. Ignore air resistance. Calculate the force acting on the ball during its upward movement. Homework Equations w=mg The Attempt...
  17. A

    Alternative approach -- Bicycle on a curve

    Homework Statement A bicycle of mass m is traveling at constant speed v around a curve of radius r without slipping. You can take the acceleration due to gravity as g. Calculate the angle of tilt, θ, that will enable it to balance. Homework Equations R=mg Rsintheeta (Length)=Fcostheeta...
  18. W

    What Forces Affect a Motorcyclist's Cornering Performance?

    I have never studied any form of mathematics other than through 'getting through life' and working stuff out and trying to understand the mechanics/ physics in terms I understand. Non scientific / mathematical terms but words in English. I have a few questions I would like to ask a physicist to...
  19. V

    I Comparison of tidal forces acting on the Moon vs Enceladus

    I have heard about a moon Enceladus. Which is powered by tidal force. I suppose this force press back and forth on the moon and friction in the core causes heat. I hope I'm right :-) Now tidal force ##F_t=\frac{2GMmr}{R^3}##, where ##G## is Gravitation constant, ##M## is mass of planet causes...
  20. C

    Finding forces in members of frames

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  21. A

    How can two equal and opposite forces produce motion? [EM]

    I am having hard time understanding the definition of work done to move a charge in an electrostatic field book says that if charge A and B sitting at some position and now if I move the charge say A towards B from arbitrary direction that force required is exactly equal and opposite to the the...
  22. R

    Electrical forces, application of Colomb's law

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  23. ibkev

    B Length Contraction: Real Compressive Forces?

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  24. B

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  25. deuce123

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  26. G

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  27. deuce123

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  28. M

    B How is the Schrodinger Equation used in calculating forces of nature?

    First, if you want to refer to the Schrodinger Equation only.. is it only called Quantum Mechanics? Or does the word Quantum Mechanics also include Quantum Field Theory? If true. Then what words should you refer to the non-relativistic Schrodinger Equations only? About the forces of nature. The...
  29. deuce123

    Understanding Tension Forces in a Slowly Rising System

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  30. K

    Fluid Dynamics: Dimensional Analysis and important forces

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  31. Astronaut

    Magnetic Forces on objects in nonuniform fields

    I have this question which I need to solve . But I am unable to. Suppose I have a current ring of radius R and current I (constant). at a distance x from the centre of the ring, I have a iron disk (radius r and thickness t) present in the plane of the ring. This disk will be attracted to the...
  32. deuce123

    Question about gravity and forces acted upon

    Homework Statement My professor said that since gravity pushes down any object on earth, the objects themselves push up against earth. Intuitively this is difficult too grasp, can anyone give an example or anything to help me out. Thank you
  33. G

    Problem with torque, angular momentum and forces

    Homework Statement I have the following problem to solve: A 1.8m board is placed in a truck with one end resting against a block secured to the floor and the other one leaning against a vertical partition. The angle the Determine the maximum allowable acceleration of die truck if the board...
  34. B

    Frictional Forces on a Wooden Crate with External Forces

    having major issue with this question. Homework Statement a wooden crate of mass m= 60 kg sits on the horizontal surface. the coefficient of static friction between the block and the surface is u=0.5 . an external force P = 220 N, is also applied to the block in a direction parallel to the...
  35. A

    Forces, 3 masses, 2 by Tension, constant A

    Hello, This is an advanced forces problem from introductory Calculus with Physics. I have mostly solved the problem I am posting here but am just missing a force on my diagram somewhere. M. The jpeg below "MassCartTheProblem_1.jpg" shows a picture of the situation and the goal is to find the...
  36. B

    Solving Forces on a Ring & Bead: A Discriminant Analysis

    Homework Statement Homework EquationsThe Attempt at a Solution For ##0 \ge \theta \ge \pi/2## Forces on the ring, ##Mg + 2N\cos \theta = F\qquad 1## Forces on the beads ##mv^2/R = - N + mg \cos \theta## By conservation of energy when the bead has fallen through some angle ##\theta##...
  37. U

    Need explanation about magnetic forces

    Hi, I saw this video and I can't explain why the magnet is propelled to the end of the coil with such a huge force. Can someone explain it in clear terms. I also tried to use Flemings left hand (or right hand) rule but it did not give me a satisfactory answer. Thanks, SM
  38. M

    B What's the energy source for the Van der Waals' forces?

    Hi, I know that if you put a dipole and a neutral molecule at a certain distance, there is the Debye's force that attract them. If you put the same molecules in the first position, they will attract another time. I know this can be a stupid answers, but what's the font of that energy? The...
  39. Kaneki123

    B Will a Rigid Ball Bounce off a Wall?

    Okay...If we consider two bodies which are perfectly rigid (meaning they can't deform under any conditions), let's say a ball and a wall...If we throw the ball at the wall, will the ball bounce back?Any answer in terms of action/reaction pair will be appreciated...
  40. Schwarzenberger

    Student Questions on Forces for Forensic Anthropology Research | MSc. Project

    Hi everyone! I have few questions which I think might actually be simple, but I would like some confirmation if possible. Here's the situation: I am doing a research project for my MSc. in Forensic Anthropology. The goal of my project is to study the marks left on bones by bladed weapons (in...
  41. J

    B What Causes Strong and Weak Forces in Our Universe?

    What causes Strong and Weak Forces? I know that they bind certain things around in our universe but what causes them?
  42. N

    B The Arrow Of Time As Defined By The Forces

    I was wondering why the arrow of time can't be defined by the direction in which the forces hold their observed values. It seems to me that if the arrow were to be reversed this would necessitate a reversal of the forces' ability to attract or repel. For example: wouldn't gravity have to start...
  43. C

    Action-Reaction Forces on an Elevator

    Homework Statement An elevator containing three passengers (with a mass of 72 kg, 84, and 35 kg, respectively) has a combined mass of 1030 kg. The cable attached to the elevator exerts an upward force of 1.20 x ##10^4## N but friction opposing the motion of the elevator is 1.40 x ##10^3## N...
  44. Bunny-chan

    Dynamics problem about two-dimensional tug-of-war

    Homework Statement In a two-dimensional tug-of-war, Alex, Betty, and Charles pull horizontally on an automobile tire at the angles shown in the picture. The tire remains stationary in spite of the three pulls. Alex pulls with force \vec{F_A} of magnitude 220~N, and Charles pulls with force...
  45. F

    I Show me how terms with Q disappear

    The general Ampere force law equation given by Maxwell is: According to Maxwell, all terms containing function ##Q(r)## will vanish after closed integration w.r.t. ##s'## because they will get reduced to functions of ##r## and the upper and lower integration limits will be same since the...
  46. W

    By how much is each post compressed?

    Homework Statement A large 30000 L aquarium is supported by four wood posts at the corners. Each post has a square 6.00 cm x 6.00 cm cross section and is 90.0 cm tall. Take Y=10^10 for wood Homework Equations ΔL = FL/AY F = ma ρ = m/V The Attempt at a Solution So the total area is 0.06 * 0.06...
  47. Bunny-chan

    Understanding an equation in a dynamics spring problem

    Homework Statement A block of mass M = 0.5kg, attached to a spring of elastic constant k = 3N/m on a vertical wall, slides without friction through an horizontal air table. A disk of mass m = 0.05kg is placed on the block, whose surface has a coefficient of static friction \mu _e = 0.8. What...
  48. G

    Dynamics exam prep -- forces explanation in FBD please

    I am doing a review for a Dynamics exam, but I am having trouble with this one question (below). Please note the top of the picture contains the answers. What I have done is make a free body diagram that includes the tension of the cable and the weight of the rider/swing. So according to my...
  49. Bunny-chan

    Basic trigonometry in dynamics force problem

    Homework Statement Two spheres, with 0.5g each, are hanging by 30cm threads, tied on the same spot. The same electric charge is communicated to each sphere; in consequence, the threads move apart until they are about 60^\circ from each other. What is the value of the charge? \theta =...
  50. T

    B Basic question - balance of forces....?

    I'm trying to find some basic physics information. Suppose you have a (massless) beam, supported at two points, 10 feet apart. You have a force (m*g) of 100 Newtons resting on that beam, say 4 meters from the left point (6 meters from the right)... How much force does that exert on each point...
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