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. 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 =...
  2. 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...
  3. Miguel Velasquez

    Forces due to atmospheric pressure won't cancel in an open tank

    Homework Statement [/B] I am trying to understand why books always point as a fact that hydrostatic force on the bottom of a open liquid filled tank doesn't depend on the force due atmospheric pressure because they these forces cancels each other. Homework Equations [/B] P=[P][/o]+ρgh F=P*A...
  4. B

    B Understanding Forces and Fields in Classical and Quantum Interactions

    Classically... interactions are referred to as forces, then after it is quantum(ized) or quantized, it becomes fields... so strong force becomes strong field, weak force becomes weak field, and electromagnetic force becomes electromagnetic field, gravitational force becomes gravitational field...
  5. R

    Finding Forces in Member AE & AB: Exam Follow-up

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  6. Alok

    How pulley diameter affects forces?

    Whether the diameter/radius of pulley affects the force required in lifting a weight?
  7. T

    Analyzing Forces in Circular Motion: Ramp and Ball System

    Homework Statement A ball rolls down a ramp which forms a quarter circle of radius 0.5m. The ball weighs 25g. The bottom of the ramp is 1.5m above the floor. Assume no friction between the ball and the ramp. Assume no air resistance. what is the force exerted by the ramp on the ball? whatis...
  8. M

    Hydrostatic forces on a hinged gate

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  9. D

    I I need to find out the forces acting on an anchor point

    Hello all. i am after some help and hopefully you are the right people for this! i have a problem will a kitchen wall cabinet. i can only get fixings which rate at 40kg, but i work out my cabinet fully loaded (including self weight) could be 95kg. i have attached a drawing which hopefully...
  10. L

    3-D vectors forces exerted upon a mast

    Homework Statement The thick arrows represent forces exerrted upon the mast Let ## \vec{r_2} = ~~the~~ longer~force~ vector~~## ## \vec{r_1} = ~the ~shorter ~force ~ vector## correspondinly ## \vec{R_2} = cable ~~2## ## \vec{R_1} = cable~~1 ## r_1 is in the same direction as R_1 r_2 is in...
  11. gelfand

    Gravitational forces, speeds and masses

    Homework Statement A comet moves around a stat in ##xy## plane along elliptical orbit, described by $$ 0.16 x^2 + y^2 = 4 $$ where ##x, y## are in ##AU## 1) Sketch the comet in the ##x,y## coordinate system denoting all orbit parameters 2) Find the semi major and minor axes of the orbit...
  12. santeria133

    Forces acting on motorcycle swingarm

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  13. DevonZA

    Determine the magnitudes and nature of forces in space truss

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  14. Qwerty42

    I Forces applied to a leaning levitating bicycle

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  15. JEngineer

    Forces transferred to beam and to supports from box

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  16. C

    B What Forces are Missing in a Balanced Block on a Floor?

    I've drawn the diagram of the forces exerted on and by a block resting on a floor. For now, it seems that the forces exerted on the block are balanced - thus it doesn't accelerate. The forces exerted on the floor are, however, not balanced. The floor is not accelerating downwards, I must then...
  17. Eclair_de_XII

    How do I find the forces acting on a ladder?

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  18. NoahCygnus

    Direction of motion if there are two unequal opposite forces

    If a particle is in the state of rest on the ground , and I apply a force such that it produces an acceleration 4.9 ##\frac{m}{s^2}## upward , will it move upward , considering there's gravitational acceleration 9.8 ##\frac{m}{s^2}## in the downward direction, or will it remain at rest?
  19. Y

    Need some help with forces on a dangling prism Internal Assessment

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  20. R

    Use of forces to escape the Earth

    If gravity is as we know a weak force and magnetism is a strong force, would it be possible to use electromagnetism to propel an object fast enough away from the Earth's surface to escape the pull of gravity and at least put an object in orbit. I assume you would need a long horizontal then...
  21. B

    Calculate forces on a double scissor jack

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  22. MathematicalPhysicist

    Cartoon/comic fictitous forces in superheros or villains.

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  23. BrendanKerrisk

    B RC Helicopter in a Train (Forces)

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  24. Anatalbo

    Sliding blocks and a string problem

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  25. Anatalbo

    Normal forces for small car performing a vertical loop

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  26. Aryan Andaleeb

    Action-Reaction Force for external forces

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  27. kach22i

    Aerodynamic Forces on 6' x 3' plate at 45 degrees - 70 MPH?

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  28. physea

    Piston Force Graphs: Manipulating Parameters in Combustion Pistons

    Hello guys! Can I view graphs of Piston Force versus Piston Displacement or time, in a combustion piston? I would be interested to see how the graph varies by manipulating parameters such as fuel amount, air amount, piston geometry, rpm, etc Thanks!
  29. Nikhil Rajagopalan

    Work done by internal forces on a system of particles

    Dear Experts, Trying to analyze the work done by internal forces in a very simple two particle system which is attracted to each other with a constant force, i performed calculations based on two frame of references, 1.The center or mass frame of reference and 2. Frame of reference of one of the...
  30. Albi Mema

    Forces between the dipole and a point charge

    Homework Statement We have an electric dipole with moment P=2*[10][/-5] pointing in x direction. What is the force experieced by dipole at origin when a point charge Q=3*[10][/-4] is located at (0.014 m ,0,0) Homework Equations 1. [E][/dipole]=(1/4π[ε][/0])(2p/r3) 2. p=qs 3. F=qE 4. τ=pEsinϑ...
  31. 1

    A Solenoid (single or multi turn) and forces

    Hi all, I need help in understand what happen when a coil, let's say a solenoid with N turn (N = 1 or whatever) produces a very high magnetic field. I know that the solenoid could explode, this means that there are forces on it, and I don't know how to calculate. Can somebode helps me? Can you...
  32. M

    Displacement of the node, superposition of forces

    Homework Statement Hi, I was wondering if anybody has an idea how to divide the forces on this structure to get the dispacement of the node 4. 2. Formulas I would like to solve this task by superpositioning forces working on different elements of the construction. (note different EI) using...
  33. Dorian Black

    Magnetic forces within a closed ferrous core

    If a toroidal ferrous core has a number of windings round it and a DC current going through them, would this magnetic core be subjected to a 'shrinking force'? To phrase it more clearly, if the toroidal core was compressible, would it actually get compressed? On the one hand, since the...
  34. S

    Leverage and Forces on a Pivot System

    The diagram drawn below shows a lever system fixed to a point on the left with a pivot point (circle) on the line which attaches to vertical line. I have dimensions for Y and Z and the forces pulling the lever down on the right, and the force acting upwards from below. I am trying to work out...
  35. O

    Torque needed to accelerate rail-mounted vehicle

    <Moved from a technical forum. Therefore no template.>I am currently analysing the forces acting on a moving vehicle. The vehicle is mounted on a rail with wheels on both upper and lower side of the rail with a spring increasing the reaction force, and thereby the friction. The vehicle should be...
  36. Brittany W

    (forces) Find the force problem

    Homework Statement http://imgur.com/a/Bz6PR Homework EquationsThe Attempt at a Solution I included my attempt in the picture
  37. FallenApple

    I Tidal forces on a spring-mass system

    Say I have two masses connected by an unstretched massless spring at some height above a planet with a strong gravitational field. Once I let go, both masses would follow geodesics in spacetime towards the center of the planet. Because the masses travel radially, the spring would compress and...
  38. S

    Calculating Forces and Kinetic Friction for a System of Connected Blocks

    Hi everyone, I'm having a difficult time figuring out this problem. Could someone give me some pointers? I set up the equations and the free body diagrams (hopefully they're correct) ; all I have to do now is solve for F.1. Homework Statement A block of mass m is on top of a block of mass M =...
  39. F

    Why is the friction force at A not considered in forces in equilibrium?

    Homework Statement [moderator note: Post edited to make problem statement images visible] https://www.physicsforums.com/attachments/question-jpg.113056/ My question is when forces are resolved horizontally, why is the friction force at A not taken into account? The Attempt at a Solution...
  40. M

    B Rethinking Gravity: The Reality of General Relativity and Spacetime Curvature

    Orbits of electrons around nucleus can never exist because it can lose energy as charges accelerate so there is never orbits of electrons. Why can't we treat gravity as forces in similar fashion.. as something that doesn't really exist. My point is. General Relativity and spacetime curvature...
  41. anony123

    Why does adding more pulleys increase the force on a block?

    Hello, First of all, I'm new to this forum and to not know whether this thread is in the right place or not. If not, feel free to replace it. I'm in my first year of university, studying industrial engineering. On a test I got the following image: The red square is a block with a specific...
  42. L

    I Forces between two straight conductors

    Hi, I am having trouble figuring out how the forces work between two straight conductors with currents going through them. I know that when the currents go the same way, the forces are attractive, and when the currents go opposite ways, the forces are repelling. I know one has to use the right...
  43. S

    Forces acting on a box-pulley system in an elevator?

    Homework Statement [/B] For this problem, use g = 10 N/kg. As shown in the figure above, a pulley is mounted to the ceiling of an elevator. The elevator is initially traveling with a constant velocity of 1.40 m/s, directed up. The rope (which we assume to have no mass) passing over the...
  44. EthanVandals

    Two identical forces on two different masses....

    Here is all the work I have done so far (hopefully it is right). My brain somewhat shuts down when it comes to ratios.
  45. fuvest

    Mechanical Energy Problem/non-conservative forces

    Homework Statement A body of mass "m" is let go from on top of a surface A, where it slides down to B(without friction). From that point on, it displaces itself on an horizontal surface 5 meters away from B, where it stops at C. Being "m" a mass in kg "h" in meters and g = 10 m/s^2 The value...
  46. Arman777

    Momentum-Determining External Forces

    I am studying momentum and I just want to check that I understand the idea correctly. Think there's a system.In this system there's two masses ##m_1## and ##m_2## moving with some velocity ##\vec v_1## and ##\vec v_2## and they exert a forces each other.Lets call the total force acting on...
  47. jlmccart03

    Electric Forces and net electric force?

    Homework Statement Problem # 4:[/B] A thin semicircular rod has a total charge +Q uniformly distributed along it. A negative point charge -Q is placed as shown. A test charge +q is placed at point C. (Point C is equidistant from -Q and from all points on the rod.) http://imgur.com/AeVfddb Let...
  48. F

    I Can nonconservative forces change either KE or PE?

    Hello everyone, conservative forces only depend on position (cannot depend on time), i.e. ##F(x)## and are equal to the spatial derivative of the potential energy function ##U(x)##: $$F(x)= - \partial U(x)/\partial x$$ Conservative forces always have to change the kinetic energy KE and...
  49. C

    How Do You Calculate the Weight of a Block on an Inclined Plane?

    Homework Statement A block is placed on a inclined plane. The incline of the plane is 21° .A 24 N force is required to push the block up the incline with constant velocity. The force is applied at an angle of 15° as shown in the picture. The coefficient of kinetic friction (μk) is 0.26. What...
  50. Shardul Khare

    B Climbing Rope: Physics and Forces on Ceiling

    When a perosn is climbing up a rope with mass a ( attached to ceiling) with acceleration a how does the physics work and what forces will be applied on ceiling? I know when the rope is massless all the force gets transferred ,but now hwat will happen
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