What is Reaction force: Definition and 111 Discussions

As described by the third of Newton's laws of motion of classical mechanics, all forces occur in pairs such that if one object exerts a force on another object, then the second object exerts an equal and opposite reaction force on the first. The third law is also more generally stated as: "To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts." The attribution of which of the two forces is the action and which is the reaction is arbitrary. Either of the two can be considered the action, while the other is its associated reaction.

View More On Wikipedia.org
  1. Quasar100

    Normal Forces on a Sphere in a Non-vertical Groove

    neglect friction and motion (sliding) and G(sphere)=20N. In this question I reached two different result with two different solving method.But one of them is false according to answer key. My question is why first solving way is false? Because the first solution way makes sense to me. If we...
  2. Z

    Elbow Flexion Joint Reaction Force

    Homework Statement:: This is not a homework, but I want to know how to find the joint reaction force of the 2d elbow flexion, if I know the angular acceleration of the forearm, and the mass of elbow with a known weight on hand. I assume the forearm is a simple cylinder. Suppose the angular...
  3. F

    I Reaction force of and its relation to normal force and friction

    Hello, When we consider a block sitting on a surface, the gravitational force ##W## and the normal force ##F_N## are applied to the block. Both equal i magnitude and opposite in direction. We call the normal force the reaction force exerted by the surface on the block. Now we consider the...
  4. J

    Electromagnetic inertial reaction force?

    I accelerate charged particle ##A## causing virtual photons to travel to distant charged particle ##B## which feels an electromagnetic force proportional to ##A##'s acceleration (for a classical field description of this effect see https://www.feynmanlectures.caltech.edu/I_28.html Eqn 28.6)...
  5. D

    Engineering Finding the reaction force using the unit load method

    I just wrote down what I think and have no idea of it...please help me with this question
  6. Olivia Lam

    What is the reaction force between the ring and the hoop?

    a.)N cos θ=mg N sin θ=mrw^2 sin θ cos θ=g/rw^2 b.) My question is reaction force =N ? or =F=mg tanθ ? If it is N then N=mg cosθ =mg^2/r w^2 or N=mg/cosθ =mrw^2 ? Thank you
  7. TonyCross

    Calculating Theoretical Reaction Force of Ball Traveling in Semi-Circle

    Summary:: Theoretical reaction force due to ball traveling in a semi-circle circum Hi, Can anyone please help me with the following question: Setup... 1 x length of pipe bent 180 degrees with an inner diameter of 22mm, it's radius is 2mtrs, the mass of this object is 100kg. Total length of...
  8. 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°##...
  9. Adesh

    I How a rigid body causes a reaction force?

    The definition of rigid body says it cannot be deformed (theoretically). Now, Newton’s third law is caused (I mean the reaction force is caused ) due to the deformation of the body. What I have learned is that every body is like a spring, when we push on it we compress it and hence feel a...
  10. coldadler

    How can I calculate the reaction forces of a torsion spring on a wall?

    Hi guys, I am working on design where I am using a torsion spring. And I need make an analysis and see the deflection of the walls. How can I calculate the forces that applied from Spring's legs to walls. Thank you all in advance.
  11. Like Tony Stark

    Reaction force acting at the wheels of cars when turning

    Well, I considered the two wheels as two different bodies and I wrote Newton's equations for both of them I considered the wheel closer to the centre of the circle, we have: ##\mu N_1 =mR\dot (\theta)## So we can find ##N_1## Doing the same thing, we can find ##N_2## ##\mu N_2 =m(R+d)...
  12. Like Tony Stark

    Reaction force acting on a rod

    Hi, I have a doubt about reaction forces... I've attached a picture that shows two similar situations. The first one shows a rod left with an angle ##\beta## while it is on a smooth surface. The second one shows a rod leaning on a smooth surface and wall. My question is: why is the reaction...
  13. E

    How can a reaction force do work on someone who is jumping?

    Let R_{1} be the reaction force on the jumper from the ground. Assuming this is larger than the combined mass of the 3 sections of human, if we consider the entire system we will get an upward acceleration (a is labelled as the acceleration of the COM; it will of course vary from the bottom to...
  14. F

    I Do action force and reaction force lie on same line?

    Do action force and reaction force(Newton's third law) lie on same line?If not then the angular momentum will be violated.I don't know why a cat falling can rotate itself before contact land if the conservative law of angular momentum not be violated?
  15. Wein_Sim

    Reaction Force of Lorentz Force

    Hi everyone, in school we recently learned about the Lorentz Force, which says that when charged particles move through an electric field (perpendicular to the field lines), a force is excerted on them (called the Lorentz Force), which goes perpendicular to both the magnetic field lines and the...
  16. K

    Need help finding the restraining reaction force for a beam

    1. The illustrated structure is affected by a known couple, and try to figure out the restraining reaction force of the hinge A and hinge E. We should analyse ECD instead. Since arm CD is a two force members, so N(C) in in the direction where CD connects by these two points. The distance from...
  17. Raphael30

    Velocity of a falling folded rope and reaction force

    Homework Statement A rope of length L and mass m is attached to the ceiling and folded at mid length. The free end of the rope is held against the ceiling, then dropped at time t=0. a) Calculate the velocity of the free end as a function of y, its distance from the ceiling. b) Calculate R...
  18. U

    B Where's the action force if the normal force is a reaction force?

    I know that normal force is not a reaction force to the gravitational force; - First, because the gravitational force is from Earth on the object (action force) thus the reaction of force is simply form the object on Earth according to Newton's third law. - Also because in certain situations...
  19. Jeviah

    Can the only reaction force (to gravity) be friction?

    Homework Statement If you have an object say in a vertical pipe, which only has a gravitational force acting on it would the reaction forces only be friction forces? If this is the case how would you calculate the forces? would you calculate the friction force using the friction coefficient or...
  20. F

    Mechanics of materials: Statically Indeterminate member

    Homework Statement Find reactions at ends and deflection at C Homework Equations RA + RE = 115kN ##\delta##Total = ##\sum## (PiLi)/(AiEi) The Attempt at a Solution RA + RE = 115kN Then apply superposition: (Get rid of RE) divide whole member into 4 elements and add. ##\delta##Total =...
  21. F

    Determine the vertical reaction force

    Forgive my current brain fart, but just posting here for some confirmation of my thoughts. I have a cart with a similar setup to a rear wheel wheelbarrow. Assuming static conditions, I am trying to determine the vertical reaction force (FA) from the ground when applying a horizontal force to...
  22. L

    I Caculate the reaction force of a radially moving object

    Hello, An object moves outward in shaft radially due to centrifugal force. lest say it weights 1kg . in a timestep=0.0001 it travels 0.0017 meters this is actually the difference of two radius, let say first radius r1 = 0.25 meters second r2 = 0.25 + 0.0017 meters rpm of the rotating system...
  23. N

    Why is the reaction force in this direction?

    I want to understand why the reaction force of the short cable and link cable is in this direction? I want to understand it instead of memorizing. Your help would be much appreciated. Thanks in advance!
  24. H

    How does the reaction force of friction affect the ground?

    Suppose I am trying to push a heavy block on ground and I haven't applied a force stronger than the maximum static friction. The force of friction acts on the block and its equal and opposite force acts on the ground. Does the reaction force of friction(acting horizontally on the ground)...
  25. O

    Reaction force be 0 at top of circular path swing?

    Homework Statement Hi, I have a question about a bucket filled with water being swung in a vertical circular path. I'm wondering why at the top of this swing, the reaction force of the bucket on the water can be 0? (ie. why is the minimum centripetal force required only the weight of the...
  26. Abdullah Kacar

    APDL - Transient Structural - Exporting stresses over time

    I'm writing my master thesis and need to export forces over time in Transient analysis. I tried using the force reaction but it shows the result as zero. BTW I'm not trying to get the forces from the nodes which is supported, with these i get results. But with the nodes with no support i get...
  27. S

    Finding magnitude and direction of reaction force

    Homework Statement A uniform horizontal rod of mass 4 kg and length 500 mm, is hinged at the left hand side and supported at rest by a chain (S) attached 10 cm from the hinge. Calculate the magnitude and direction of the reaction force on the rod at the hinge. State the principle you have...
  28. A

    B Will there be a reaction at the hinge B in this question?

    I have solved this question by completely disregarding the reaction at the hinge B. And surprisingly, answer turns out to be correct. Why is that so?
  29. G

    Skier, curved motion on slope/ reaction force?

    Hi folks, I was thinking about an old physics olympiad question I read. Can't remember exactly but it was about minimising the reaction force of a skier as she took a curved path down a ski-slope. That got me thinking generally about what is going on with the reaction force in such a situation...
  30. N

    Rod is toppling. Find reaction force as a function of theta

    Homework Statement Rod of length b and mass m topples from vertical position at theta = 0. Calculate the reaction force f as a function of theta. What is the minimal and maximal values of f for theta between 0 and pi Homework Equations Third law?[/B]The Attempt at a Solution I never really...
  31. T

    Pressure Gradients along an angle pipe

    I have a high pressure volume coupled with a low pressure volume by a pipe and valve . If the pipe was straight I would expect the isobars of the pressure gradient to be parallel to the pressure differences and perpendicular to the pipe , I would expect a reaction force (red arrow) on the wall...
  32. K

    Max reaction force - ferris wheel

    Homework Statement What is the maximum reaction force exerted by a seat on the passenger of a ferris wheel? There's a diagram showing the seat at 6 o'clock, 12, 3, and 4:30... on a 12 hour clock. Homework Equations N/A The Attempt at a Solution I guess the reaction force is the normal force...
  33. L

    Calculating Reaction Forces in ANSYS: Step-by-Step Guide"

    Hey, I have a simple ANSYS model shown in link: https://dl.dropboxusercontent.com/u/104865119/hand_calc.PNG . Ansys reaction force in z axis is +121 N and -121 N. Can someone clarify how do you get to this result using hand calculations?
  34. K

    Reaction Force, Centripetal Force

    Does reaction force only occur when it is in contact with another object? And also if rollercoaster goes around in a circle, at the maxium height, why isn't there any reaction force (got this from a book). Personally, I think that rollercoaser have clamps attached between the path and the...
  35. S

    Calculating reaction force for sliding plank

    Homework Statement A plank of length 2L leans against a wall. It starts to slip downward without friction. Show that the top of the plank loses contact with the wall when it is at two-thirds of its initial height. (from Kleppner&Kolenkow,An Introduction to Mechanics 1st ed., problem 6.41)...
  36. V

    Forces on abseiling (rappelling) rock climber: find vertical reaction force

    Homework Statement Can someone please help me with this A level physics question on Mechanics. Help will be appreciated. The picture of question is attached. :D Thank you Homework EquationsThe Attempt at a Solution [/B]sin20x590=201.79N or cos70x590=201.79N
  37. M

    Reaction force to resist rotation

    Homework Statement There is a object traveling at 0,2m/s with a mass of 75,000kg. The object impacts a fixed point at a distance of 3.5m from its center of gravity (COG) and rotates around this point. What is the reaction force at the point of rotation? Homework Equations I think it is...
  38. Zack K

    B Action Reaction force on the earth

    "As you sit in your chair and study your physics (presuming that you do), the force of gravity acts downward upon your body. The reaction force to the force of the Earth pulling you downward is the force of your body pulling the Earth upwards". I read this somewhere just a bit ago and it's...
  39. D

    Speed bump reaction force on a car

    Homework Statement Hi I need to solve the next problem: a car with a total mass of 1950KG hits a speed bump at 19,4m/s. The speed bump is 0,05m high and has a angle of 45°. The car hits the bump with the front axle first. How much force acts on the front axle on the moment of impact. Homework...
  40. Blockade

    B Why is there a negative in Hooke's Law (F = -kx)

    For Hooke's Law, why is there a negative in F = -k*x? Is it because k is the reaction force against the force applied on the spring and not the force pushing or pulling on it? For example, let's say that the origin is set where the spring ends when it's in equilibrium with no forces applied to...
  41. Jonski

    Calculating Reaction Force at A for a Beam with Pin Joint | Beam Homework

    Homework Statement Calculate the reaction force at A? Homework Equations ΣMb = 0 The Attempt at a Solution As there is a pin joint at B it is possible to consider the bar only from A-B. From here I applied ΣMb = 0 120 + 20*2 + Ay*4 = 0 This gives Ay = -40kn Also since there are no forces in...
  42. M

    Normal reaction force on banked track

    If a car us on a frictionless banked track what causes the normal reaction force to be greater than the weight force? My understanding from what I've read is that the vertical component of the reaction force cancels the weight force which makes sense as the object doesn't leave the ground. And...
  43. E

    How to Calculate Bolt Reaction Forces Under Different Loading Conditions?

    *Note: Even for 1 answer for any of the question, I am highly appreciate it. I am understand it quite a burden to answer all. Thank you. Hi guys, I am currently seeking a correct approach of getting the reaction force developed on bolt due to the Forward or Upward force experience by the...
  44. V

    Finding the Reaction Force in a Truss

    Homework Statement Homework Equations In static equilibrium the forces are = 0 Method of Joints The Attempt at a Solution i don't know how to find the reaction forces of A and B. i know how to calculate the force of each member by working backwards from where F is applied but once I do...
  45. A

    What is the formula for calculating water jet reaction force?

    Hi all, my first post on here and just wanted to check something I'm working on for a project. The project is to use a pump to create a jet of water to manouvre small boat. I have found a formula to work out the jet reaction force in a fire fighting textbook which gives: R = 0.157 * P * d^2...
  46. Jonski

    Reaction Force, find tension, a in the tangential, or alpha

    Homework Statement The diagram shows the instant when a long slender bar of mass 4.9 kg and length 4.9 m is horizontal. At this instant the mass m= 6.5 kg has a vertical velocity of 4.0 m/s. If the pulley has negligible mass and all friction effects may be ignored, what is the magnitude of the...
  47. Z

    What is the reaction force of centripetal force

    According to Newton's 3rd law, force exists in pair. When I attach a ball to a rope and rotate it, I am giving the ball an centripetal force to keep it moving in circular motion, so is the tension in the string the reaction force I am asking? I also want to ask about the reason why centrifugal...
  48. G

    What happens if the normal reaction force is less than zero?

    Homework Statement why the object will fall of when the normal reaction force is less than zero ? P/s : the normal reaction force is in downward direction... How if if the normal reaction force is less than zero ? the normal reaction is in upward direction ? Homework EquationsThe Attempt at a...
  49. G

    Direction of reaction force in a circular motion

    Homework Statement Thewheel is rotated in the anticlockwise direction...The correct reaction force is drawn using the blue pen . I don't understand why the reaction force isn't directed to the left ? which is drawn using the red colour ? IMO, the red colour should be correct because the...
  50. Alexander1

    Find Torque Caused by Beam Pulling Down on Another

    Homework Statement Hi guys, I'm currently working on a homework question but I'm a little bit stuck although I do know what I'm meant to be doing. My question asks me to find the torque being caused by a beam pulling down on another. I'll explain a little more the context of it - there is one...