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. H

    B Alpha particle in a force field depiction

    Hello, I was glazing through what I would consider an advanced physics textbook and I saw this image. It is a schematic picture of an alpha-particle in a field of an atom. Now, can someone get me started on what (and why and how) is going on in it? Especially with the fraction with pi.
  2. LCSphysicist

    System subject to a driving force

    The displacement of the end of a spring varies as , the block on the spring is subject to a viscous force proportional to the velocity. Spring stiffness k. (a)Find the displacement of the block: (b)When γ -> 0 First of all, i have a doubt i we could start saying the component of the force is...
  3. E

    Is the Total Radial Force on a Current-Carrying Hoop Zero?

    I was just reading through these lecture notes regarding the stresses in solenoids, and came across the following regarding a current-carrying ring orthogonal to a uniform magnetic field, I wondered if this is a piece of terminology that I haven't come across? To me the total radial force is...
  4. E

    Force derived from magnetic energy of a current carrying spring

    The magnetic energy of a current carrying spring, with ##N## turns, length ##x## and cross sectional area ##A##, is $$E_m = \frac{\mu_0 N^2 I^2 A}{2x}$$The (negated) spatial derivative of this yields a quantity with dimensions of force,$$F = - \frac{dE_m}{dx} = \frac{\mu_0 N^2 I^2 A}{2x^2}$$How...
  5. E

    Force on a capacitor plate as the derivative of the energy -- is it a fluke?

    Bit of a random question... capacitors can be a bit weird, in that if we connect one up to a source of EMF and do positive external work to separate the plates of the capacitor, the energy of the capacitor decreases (and instead the work you do plus the decrease in capacitor energy goes into the...
  6. LCSphysicist

    Simple pendulum subject to a driven force

    The problem is how to construct the right diagram of forces actually Unfortunately, the Fo*cos acting on the ball will not carry the g/l of the solution -mg*sin(´p) + (-bv) + (Fo*cos(wt)*cos(p)) = mx'' Fo*cos(wt) = mx'' + mg*x/l + bv
  7. Selfphysicslearner

    Why does a car apply a force to a wall?

    when we have 0 acceleration(constant speed) like a car traveling at a constant speed, ƒnet=ma ƒnet=(mobject+mfriction).(aobject+afriction) let m object be 1500 kg. m friction be 10 kg a object be 0 a friction be 20m/s^2 so when we calculate fnet=-20*1600=-32 000 Newtons. which means wall applies...
  8. P

    Minimum force required to rotate a lamina

    When the lamina rotates about A, FA must act on B (because it is the farthest away) perpendicular to AB (so that all of FA contributes to rotation). Same argument is valid for rotation of lamina about B as well. Having noted that, I tried two approaches: Approach 1- If I assume that the...
  9. andrewkirk

    1 pulley with mass, 1 mass on the cord, 1 external force, no gravity

    I remember, a few weeks ago, when looking for homework problems I could help with, seeing one as described in the title. I couldn't think of an easy solution and was busy at the time so i made a mental note to think about it later. I finally got around to doing that yesterday, and brushed up on...
  10. G

    Is Gravity the only force doing work on a roller coaster cart?

    Why I think gravity *is* the only force doing work on the rider: 1) The only forces acting on the rider are gravity and the normal force. Broken down into their component vectors, we have: -> The component of the force of gravity moving parallel to the rider's direction of motion -> The normal...
  11. K

    The Lorentz Force Law Can be Completely Inaccurate

    If you take a bar magnet and place a wire with current a short distance from the end, Lorentz's law can be used to accurately predict the location and magnitude of the resulting forces. The same is true if you use a large volume uniform magnetic field to create an induced field in a bar ferrite...
  12. J

    Linkage Analysis and force study

    Hi All, I am working on this linkage mechanism. Please see the attachment for images. There are 3 pivot points. Two of which are connected to the ground. The" Inside Leg 1" is telescopic to "Leg 2" and has a 120 LB compression gas spring connected at mount points as shown in the image. The leg...
  13. R

    Design of a cell phone stand - force balance question

    Hello all- I am trying to design a simple cell phone holder that will be printed using a 3D printer. For this phone holder I would like the base to be the minimum length possible to avoid the thing tipping over backwards. I drew a simplified slope and base which represents the phone on the...
  14. P

    How is force applied? Is resistance or load necessary for force to exist?

    I don't know if this is a dumb question, but I just can't seem to wrap my head around it. I can imagine and feel when I'm pressing against a wall, that there's a force. Especially when the wall is not moving, I can continually increase the strength of my push and feel a greater resistance from...
  15. ttesss

    Newton's 2nd Law: Force, Mass and Acceleration

    I´m currently studying Newtons 2nd law and I don't get how can force only depend on mass and acceleration and not on velocity.. I mean, if I punch something with my fist going at constant velocity ( acceleration=0) it stills has a force right?
  16. S

    Force acting on a current carrying conductor within a magnetic field

    I am new to this forum, and this is my first post. Please bear with me if my query has any inaccuracies. In the attached figure, a coil is wrapped around the central arm of a flat H-shaped thin metallic plate (such as iron). DC current flows through the coil and magnetizes the arm. At the...
  17. Kaushik

    Intuition about electromotive force (EMF)

    My understanding of emf Let us consider 2 parallel plates with charges (opp. but equal in magnitude) stored on it. When we connect both the plates from the outer side, the electrons from the lower potential (i.e., negatively charged plate) moves to the higher potential (positively charged plate)...
  18. O

    I Governing force and Neutron elastic scattering

    Hi, Which force does govern the neutron elastic scattering and capture reactions? It seems to me it is the strong nuclear force, but I am not sure about that.Thanks.
  19. P

    Direction of Frictional Forces on Balanced Force Objects

    What is the direction of the frictional forces especially when there are 2 forces acting on an object? (see attached Image)
  20. Nomadyb

    Find EMF of ℰ on Wire: Speed v, Force, C, B, m, g, l & a

    a) the EMF of ℰ induced on The Wire, as a function of the speed 𝑣of the wire you can find it. b) flow induced in the wire𝑖, 𝐶, 𝐵, and are denominated in 𝑙 𝑎 (𝑎= the acceleration of the wire). c) find the magnetic force acting on the wire in𝐶,𝐵, 𝑙 and𝑎. d) 𝑎 acceleration, 𝑚, 𝑔, 𝑙, and are...
  21. A

    Is gravity an inertial force or interaction force?

    We feel that Earth pull as to the ground but is Earth accelaritng up "in some way" so gravity is also inertial force?? I read that we still don't understand what is gravity...
  22. Alfos001

    Seat-belt effects on passengers during a crash

    I have attempted solving this problem several times on an long since finished online quiz. However no answer I gave seemed to be correct despite what I could reasonably understand regarding the effect of seatbelts and their effect on reducing force by extending the distance of deceleration...
  23. Flaming Physicist

    Electric force between two equal parallel rings of charge

    The problem is symmetric around the z axis, thus the force must be in the z direction only. I tried dividing both rings into differential elements, then integrating through the upper ring to get the z component of the total force on the upper ring due to a differential element of the lower ring...
  24. LCSphysicist

    Finding the Force Applied by a Support on a Falling Chain

    A chain with length L and mass density σ kg/m is held in the position shown in Fig. 5.28, with one end attached to a support. Assume that only a negligible length of the chain starts out below the support. The chain is released. Find the force that the support applies to the chain, as a function...
  25. E

    Motion of a particle under a tangential force

    This is diverted from the Classical Physics forum. My first approach was this: the force ##\vec{F}## can be written in polar coordinates as ##\vec{F} = F \hat{\theta}##. It follows that ##F_{\theta} = ma_{\theta} \implies F = m(2\dot{r}\dot{\theta} + r\ddot{\theta})##. This result also agrees...
  26. B

    Engineering Calculate shear force and bending moments with a UDL and a point load

    Hello, I am looking for some help. I have completed the question below. I will show my answers. My answer: I believe this to be correct. If it is not would appreciate someone letting me know so i can correct. Now i am attempting this question: Could anyone help, or point me in the...
  27. Uberhulk

    Force Required To Destroy Three Planets

    Astrophysicist Scott Manley determined the energy required to destroy a planet (2.25 x 1032 joules). Is the energy required to destroy 3 planets simply a case of multiplying his result by three? If not, how much energy would it take to destroy 3 planets? This scenario relates to Incredible Hulks...
  28. K

    Buoyancy force when a ramp is under water

    So I have made force diagram And I think that I should find the acceleration by using these equations: ##\sum Fx=w\sin(15)-f_k-T_{x-buoyancy} ## ##\sum F_y=N+T_{y-buouancy}-w ## I know that the volume of water displaced must be ##V=\frac{1}{2400}m^3## and the mass of the water is then...
  29. A

    Difference between centrifugal force vs reactive centrifugal force?

    Difference between centrifugal force vs reactive centrifugal force? and practic example
  30. X

    Y-component of the force vector in turning flight

    Hello, I have a question: Why is the y-component of the force at turning flight equal to the weight force? Here, Fs is equal to Fg. But why? I tried to explain it myself but I didn't get it
  31. P

    Calculate the maximum vertical point force P that this beam can sustain

    A simply-supported steel beam with a vertical point force P is shown in Fig. 2(a). A cross-section of the beam, which is composed of two identical C-shaped members bolted back-to-back, is shown in Fig. 2(b). Both C-shaped members have a uniform thickness of 1 cm. Pairs of bolts are located at a...
  32. A

    How to determine the effect of counterforce on the angle of force

    Can anyone help me with a formula that would show the effect the counterforce of the neck will have on the direction the knife is moving, to be able to determine what a cut in a given direction what will be its direction of force? (I apologize for not following the guidelines for homework...
  33. LCSphysicist

    Motion under a force perpendicular to the velocity

    I think i could deal with this problem interpreting this force like a central force, what seems pretty nice to me, since in a circular orbit the force will always pass through the center, if it is perpendicular to the velocity. I thought, since the force is central and in this case, spherically...
  34. R

    Force field suitable for studying silica crystal

    I need some advice and help please -Is There a two body force field suitable to study silica crystal or alpha quartz crystal? it's okay to gives Approximate results. - Is the BKS force field suitable for silica crystal? - If there are other simple terms of force field for studying the silica...
  35. Glype11

    How to Estimate Tensile Force on a Pre-Tensioned Rope?

    I figure that there is a connection with how much a given forces displaces the rope in the perpendicularly direction. Obviously the more tension on the rope then there will be less displacement with a given force. I'm not sure how to progress from that point. Any other ideas on how to estimate...
  36. P

    Automotive Vehicle Parking Brake Test on a Slope [How Convert to a Flat Ground test?]

    Vehicle Mass : 3 tonnes Manufacturer's instruction is to test vehicle's parking brake effectiveness is to engage parking brake on 30.96 degrees slope. If it holds, it is effective. I do not have 30.96 degrees slope & I want to replicate the same test on flat ground. I was thinking of...
  37. Luke2642

    Force & energy in cutting and stretching magnetic field lines?

    I have an ordinary switchable magnet for holding tools to a lathe. It's like a magnetic force gearbox, but I can't quite understand the force multiplication. When placed on a steel surface the switch force is approximately ~5N on both finger and thumb at 1.5cm radius acting over a 3cm arc...
  38. C

    Calculating Force and Torque for Linear motion (image provided)

    This is how I understand the torque (T1) created at the axis of rotation due to the spring would be calculated. This is considering the spring is starting from a fully extended state and is not already compressed. This is considering that (A) has a distance of 2 in. and the force from the spring...
  39. K

    How Do You Calculate Forces when The Lorentz Force Doesn't Seem to Apply?

    If the mu of the ferrite is high, as suggested, the B field on that section of wire is zero, and therefore there is no force on the wire. Instead there is a comparable force on the ferrite itself. But suppose you allow the ferrite to have different values of mu. If mu=1 the force is just...
  40. apathetic_operator

    What does a force of zero indicate about the total force acting on a particle?

    "The force is zero" means the total force act on the particle is zero? Then there must be an external force. If not, then Fx=0 ? since Fx = d(U(x))/dx, the answer of (b) should be (i), but how about (a)?
  41. S

    Force to make 2 blocks move relative to each other at constant speed

    Let: Smaller block = m1 = 1 kg Large block = m2 = 2 kg Coefficient of friction between the two blocks = μ1 = 0.2 Coefficient of friction between larger block and floor = μ2 = 0.3 Tension connecting two blocks through two pulleys = T Angle between tension and horizontal = θ = 37o Friction between...
  42. Hajarmq

    I Conservative force pairs

    Let F_ki be the force applied by a point mass i on a point mass k. This force depends on the variables x_k and x_i which are the position vectors of respectively k and i (to simplify let´'s consider this in 1 dimension). Suppose this force is conservative. Then, according to my course and...
  43. jaumzaum

    How Does Magnetic Force Affect a Falling Coil's Velocity and Kinetic Energy?

    Consider a coil perpendicular to the ground falling with gravity. Under it, there is a magnetic field also perpendicular to the coil. When the coil starts penetrating the magnetic field there will be an induced current and therefore a magnetic force upwards. This magnetic force will reduce...
  44. mcastillo356

    Electromotive force when current is alternating

    Hello, let's imagine a generator as shown in image. I just don't understand alternative current. -\epsilon shouldn't be always positive? Otherwise, how can it provide anything? Thanks
  45. mcastillo356

    B Electromotive force and trigonometry

    Faraday's law: \epsilon=-N\dfrac{\Delta{\phi}}{\Delta{t}}=-N\dfrac{\Delta{(BA\cos{\theta})}}{\Delta{t}}=-N\dfrac{\Delta{(BA\cos{(\omega t)})}}{\Delta{t}} Applying calculus \epsilon=NBA\omega\cos{(\omega t)} Shouldn't it be \epsilon=NBA\omega\sin{(\omega t)}, just if I apply limits? Thanks
  46. J

    MHB Find the average force required to hold the gun in position

    A gun fires 6 bullets per second into a target. The mass of each bullet is 3g & the speed of 500 m/s. Find the average force required to hold the gun in position.
  47. J

    Net Gravitational force on an object

    First, start off with x and y directional forces F (Test object 1) - F x(Test object 2) I need help primarily with finding the horizontal component of the force from object 2. How do I find it and express it? Thanks
  48. J

    B Is Gravity Really a Force? A Look at General Relativity's Perspective

    According to general relativity, gravity is not a force, however it is referred as one of the four fundamental forces. This seems like a contradiction.
  49. rakailee

    Truck with attached inclined plane and a block on the incline

    I tried solving this by assuming the acceleration of the truck and block to be the same so the block would stay on the incline. Also, I would assume truck ma = static friction, block ma = mgsintheta... then I solved for a to plug into 1st equation to get 12990 N. Is this correct? I wasn't sure...
  50. J

    Calculation of the forces and the work of these forces

    Like AT asked I post a new thread because it is a different example, here there is no friction and I use a needle and an elastic to have the forces. The circle moves in horizontal translation, the circle doesn't rotate around itself, the red wall rotates around A0, and A0 is fixed to the...
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