Force 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.

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  1. M

    "KICK" force exerted from a hydraulic cylinder

    Hello,I am building a press that will squeeze hot steel ,back off .5"push hot steel in further and repeat.The cylinder is 4" bore , 1.375"rod and travel is 8" (the majority of squeeze will be at 5"of extension)at a max of 2500PSI. my question is when putting @15 tons straight down what will the...
  2. E

    Does the electrostatic force create current in the circuit?

    As I understand potential difference is the reason of current. Does it mean that the electrostatic force creates current?
  3. goodOrBad

    Block A Reaches Equilibrium: 92N Force

    I tried to solve it via equilibrium equations and I got 92N for block A
  4. PeterDonis

    Insights Intro to Centrifugal Force Reversal in Kerr Spacetime

    Continue reading...
  5. L

    Momentum and force multiple choice

    i. Rearranging the equation for kinetic energy in terms of momentum; 1/2mv^2=(mv)^2/2m=p^2/2m Inputting the values given KE=1^2/2*4kg = 1/8 J Kinetic energy of a body with twice the momentum; KE=2^2/2*2kg =1 J The ratio of the kinetic energy of the first body to the second body is therefore...
  6. C

    What is the resulting force between the beam CB and E?

    I assumed that the vertical force at point B would be the same as the force between beam CB and E because of Newtons law. Did I assume wrong? Look below for my calculations and answer which I got to 250N. I don’t know the correct answer but assume someone here can tell me if I solved the problem...
  7. G

    Equation for Fourth Force and its Line of Action?

    Could anyone please help with the following? Three forces of magnitudes 10, 3 * sqrt(5) and 25 Newtons act along lines whose vector equations are respectively: r1 = i - 2j + k(4i + 3j) , r2 = -2i + 4j + k(2i - j) and r3 = 4i + k(7i - 24j) A fourth force F4 is introduced which reduces the...
  8. P

    What causes an electric force in one situation but not in another?

    Hi, I'm dealing with a more or less trivial question. Let's have a look on two situations. Consider a (negative) charged metal plate. If the plate is infinit in size it will produce a perfekt homogeneous electric field. If we now place a second plate parallel to the first one the electric...
  9. E

    Drag force on a descending sphere

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  10. Leo Liu

    I Why is a time varying force nonconservative?

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  11. zstraught

    Question about work and finding the force of friction

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  12. Like Tony Stark

    Tension and reaction force in circular motion

    Hi I'm having trouble to understand the centripetal force in a rotating rod with a mass in its end. When ##90°<\theta<270°##, the centripetal acceleration is produced by the tension, which counteracts the radial component of the weight. But what happens when ##\theta<90°## or ##\theta>270°##...
  13. P

    I Stability of circular orbits in an arbitrary central force field

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  14. cwill53

    Force on a charge at the tip of a hollow, charged cone

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  15. Jason234578

    Engineering Moment of a force about point O

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  16. madafo3435

    How Is the Electric Field Difference Across a Charged Surface Determined?

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  17. cwill53

    Zero Force from 3 charges placed at the vertices of a Triangle

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  18. D

    How to calculate magnetic braking force?

    I want to know how to calculate the braking force acting on a magnet falling through a copper tube. The setup can be seen in this video (YouTube, @ 1:49 - 3:12): Copper's Surprising Reaction to Strong Magnets. Note that it's not a copper tube in the video but a plastic tube surrounded by a...
  19. Hamiltonian

    Work done by a conservative force using change in potential energy

    we know ##W_g = -\Delta U## but here to find ##\Delta U## we will need another equation won't it be wrong to write $$-\Delta U = -\int_1^{0.8}mgdy$$ as this equation is derived from ##W_g = -\Delta U## and as we have 2 unknowns we will need two equations. this is a rather easy problem but I am...
  20. AN630078

    Force and change in momentum in an inelastic/elastic collision

    Question 1: Since Force=Change in momentum = ∆P / ∆t = mv / t Momentum of the water coming out=mv mv=ρVv=ρAv Force d/dt (ρAv2t)=ρAv^2 Force = 1000*40*30ms^2 Force = 3.6*10^7 N Question 2: This is where I am confused because I understand in an elastic collision total kinetic energy and...
  21. F

    Finding angular Momentum from a force

    Is it correct to say that that τ=0 since r has the same directacion as F?? and for \vec{L} que need to find \vec{p} So I thought solving this dif equation ## \int dp/dt =−kq/r^2 +β^4/r^5## Do you agree in the path I am going?
  22. Big Tommy C

    Force required to accelerate upward against gravity

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  23. Hamiltonian

    A rotating rod acted upon by a perpendicular force

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  24. oondi

    Electromagnetism: Force between two charged plates

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  25. MichaelTam

    How Can You Solve a Differential Equation Involving Exponential Drag Force?

    There ‘s an equation, I think it should like a “drag factor” because the -(b) now has a negative symbol, which means it should be affecting the particle going parallel of the x-axis to the right, I suppose F=ma need to be apply into this equation, but before this, I need to get it into a...
  26. GreenOlive

    Change in acceleration with change of angle of applied force

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  27. G

    I Lorentz Force Equation: Coercion & Maxwell Stress Tensor

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  28. F

    Torque and angular momentum with a central force

    HI τ= r ˆr x - ##k / r ^ 2## ˆr= 0 right? since ˆr x ˆr is zero What about L?
  29. P

    Finding the Magnetic force on the coil

    Surely a tough one, I am doing it from the basics. This is the diagram i tried to draw showing the Force and current I The Length L is the tangent to the circle. The Force F is pointing upwards at ##90 Deg## to the ##\vec B## and also perpendicular to ##\vec L##. I am considering a small...
  30. E

    Pressure force on curved surfaces: vertical component

    hi guys, i can't understand why they calculate F yin this way, the part of floor that is the vertical proyection has less water than the floor in the left so i tought Fy would be less, please can someone explain this concept to me?
  31. P

    How does the velocity of a ball change without buoyant force acting on it?

    So my doubt is at the beginning of the problems hey are saying that the ball obeys stokes law and on the latter part of the question they are saying that no buoyant force is acting then how does the velocity of the ball change in the end? Also what is the use of specifying 'the ball never...
  32. LCSphysicist

    Hanging Spring with non-negligible mass is subjected to a driving force

    First i will use a equation resulted in considering a spring as a continuum limit of massive mass: ω = √(KL/ρ)*kn ρ is the linear density ρ ω = √(KL/ρ)*kn X = A*cos(kn*x)*cos(ω*t) ξ = A (first consequence) X = ξ*cos(kn*x)*cos(ω*t) ∂y/∂x need to be zero in x=L (for strain be zero) kn*L =...
  33. aspodkfpo

    Where does the force of tension act on a box?

    Given a force of tension with a string attached to the front of the box, do we take the force as acting with an angle to the centre of the mass of phi or theta? Explain please?
  34. Like Tony Stark

    Direction of the normal force when acting on a tilted object

    My question is: given a rigid body which interacts with a surface, what's the direction of the normal force? Because, as the word says, it has to be normal to the surface. But when treating problems of a vertical rod which is slightly pushed and forms an angle ##\theta## with the surface, some...
  35. P

    What is the relationship between pressure and force inside a spherical object?

    So I already have a solution available to this problem and the link for the solution is: I have understood everything in the video except the part where they are equating the force dF=GM/r²*dm According to my reasoning the inner part of the sphere (the part below the dm element we have taken)...
  36. aspodkfpo

    Spring Force vs Height on a Trampoline

    There is a trampoline drawn here and a graph of the spring force vs height. I don't see why the spring force is decreasing at a decreasing rate with respect to height above trampoline. F= kx = k * h/sin(theta), letting theta be between the horizontal and the spring.
  37. Lucasterre

    Weak Force Weapon Effects on Wall Elements

    Hello, I am a creative scifi writer and i only have basic quantum physics knowledge, if someone could answer my question please. I was wondering some type of weapon that uses weak force, therefore it would promote distorcions at weaf force field causing interaction with atoms My question is, if...
  38. T

    Is this a valid way to calculate capillary force / pressure?

    When figuring out the capillary pressure on a liquid in a tube of a certain cross-section, the typical approach is to consider the Young-Laplace pressure and the curvature etc. I was looking through some of my old notes and I had an equation for the capillary force: fc = γ cosθ dS/dx where γ...
  39. B

    Pressure generated by a force on a pump with a lever

  40. burian

    How to solve questions in pseudo force frames?

    $$ \frac{62 - .4mg + (.2) (m_{net} g)}{m} = a(t)$$ And, then I put ths into $$S=\frac{1}{2} at^2$$ But, my answer is not quite coming with what's in the options? So, my main confusions are, SHould the bottom most surface exert a force on the top most block, also where am I making mistakes...
  41. Adeel Ahmad

    Modeling Suction Force: Tips & Solutions

    At a total loss here
  42. P

    Is My Tractor Stable Enough to Support a Lift at Maximum Height?

    I am seeking some professional help regarding a recent single man lift I purchased. My question is regarding tipping force calculations. I will try to explain all of the pertenent details as best as possible. The lift I bought weighs approximately 600 lbs. It moves around on caster wheels but...
  43. P

    Calculating the Force and Torque on a current loop

    ##\vec \tau = \vec \mu \times \vec B -eq1## ## F = I \vec l \times \vec B -eq2 ## The forces i have indicated The magnetic moment is coming out of the page. I am only attempting only the part (a) as of now a. The force calculations are F1 = 1.4*0.22*1.5 = 0.462 +X direction F2 = 1.4*0.22*1.5...
  44. V

    Misc. Pneumatic cylinder selection - Help calculating the force needed

    This is my first post in this great community. English is not my native language, so i am sorry for any mistakes. I need to build a pill crusher using a pneumatic cylinder with 30mm stroke (effective travel). i have e metal container with 25mm base diameter where i have to make powder from 6-8...
  45. B

    Direction of acceleration of a projectile with drag force

    First and foremost, thank you so much for your help! I'm new to Physics Forums, and this is my first post. I thought I understood the problem, but as I moved on to the subsequent parts of it became apparent that I was missing something. I set up my ##F = ma## equation as below, and solved that...
  46. P

    Force on a current carrying conductor

    I am studying the theory of force on a current carrying conductor in a uniform magnetic field, the Force is ## F=ILB##. I am slightly confused here let us say the conductor is placed in the uniform magnetic field ##B##, then if the current is passing through the conductor then that current would...
  47. aspodkfpo

    Exploring Force Pulleys: The Surprising Truth Behind Non-Net Forces on Objects

    If a free body diagram is constructed for 1kg, one might be inclined to draw an arrow downwards with only the weight of 10N. However, the downward, non-net, force on 1 kg is (1+5) x 9.8. Why is this the case? It seems very counterintuitive.
  48. Leo Liu

    Normal force acting on a block on an accelerating wedge

    We have a wedge whose surface is ##\theta## from the horizontal surface. After a block is placed on its frictionless slant surface, the wedge starts to accelerate due to a force F. What is the normal force acting upon the block? I have been trying to solve it but I got no clue. Could someone...
  49. G

    The Vertical Force of a Spring: A Logical Argument

    Because this problem is easier to understand with a picture, I'll just copy paste the original problem. There is no question about the validity of the solution. My question is about the statement in the solution "Consider the instant when the mass is moving vertically upward. In this instant the...
  50. Jiman

    What is the Relationship Between Time and Force in Particle Motion?

    I feel that force is also function of time.
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