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.

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  1. J

    Think the Mass is correct, unsure on Reaction Force

    Homework Statement [/B]Homework Equations [/B] kg/m3 = mass / volume Downward forces on the column = Weight + Additional force (F=ma) The Attempt at a Solution [/B] kg/m3 = mass / volume Therefore, 2400 = mass / (0.3 x 0.4)...
  2. T

    Torque Reaction Force of Transmission and Drivetrain

    Hello, As I understand it there is an equal and opposite reaction for every mechanical action. In the case of a vehicle drivetrain the engine generates a torque at the flywheel or flex plate which becomes an input to the transmission and ultimately the wheels. The rollong resistance of the...
  3. L

    Is the friction "reaction force"?

    Hello, members. When a vehicle moves on the road, it applies a force to the road (action force according to the Newton's 3rd law), road also applies an equal and opposite force on the wheels of the vehicle to cause it to move forward. My question is, is this reaction force frictional force...
  4. PhiowPhi

    Linear reaction force on motors?

    I have two motors(m1 &m2) placed on a vertical position(on the ##y-axis##), that have gears connected to their shafts, and I placed the gears next to one another. Fixing them to move together. I now powered both motors allowing their torques to oppose one another and cancel out each other's...
  5. terryds

    What Is the Correct Direction of Normal Force in a Stick-in-Well Scenario?

    Homework Statement http://www.sumoware.com/images/temp/xzmbharheaqshjjf.png A homogeneous stick with mass M is placed in a well with smooth surface (see picture). θ is the angle between the stick and the floor. Determine the force the well gives to the stick. Homework Equations ΣF = 0 Στ =...
  6. N

    Gravity's reaction force in the relativist model?

    this is related to the following thread: https://www.physicsforums.com/threads/gravitys-reaction-force.28395/ (but it doesn't seem to accept more replies so i posted here) my cuestions are: - ¿if gravity depends on mass, how come we define a black hole as an object so massive that photons (mass...
  7. K

    Reaction force on a leaned beam

    Homework Statement A beam of 12[kN] is on a smooth floor on one side and on a rough edge on the other. The total length of the beam is 10[m]. What is the reaction on the edge. Homework Equations Moments (torque): F1L1=F2L2 The Attempt at a Solution I drew the forces. the reaction on the floor...
  8. D

    Reaction force at the top of a rollercoaster loop

    I understand that in a rollercoaster loop, two forces combine to make the centripetal force required to keep the cart moving in a circular path - the cart's weight, and the reaction force of the track against the cart (except in the special case where the cart's weight alone is sufficient for...
  9. B

    Tough question about reaction force Newton's third law -- Please help

    Homework Statement . question is as attached. answer is 2f/5..how do u get this? Homework Equations F=ma F= (5m)a a= F/(5m) The Attempt at a Solution gotten a=F/5m , but why the answer is 2f/5?
  10. R

    Reaction Force: A Comparison of Masses

    Homework Statement Two blocks with different masses are in contact on a frictionless surface. A force, Fp can be applied to either block. If the magnitude of Fp is the same on both cases, the force exerted by the heavier block on the lighter box is A) The same B) Greater when force Fp is...
  11. 2

    Force parallel to a smooth wall?

    Hello! My question is quite a quick one- I was wondering whether it is ever possible to have a smooth wall exerting a force parallel to it (and not just perpendicular to it). For example, if you were to place a see-saw by a smooth wall so that the wall is holding one of the see-saw ends below...
  12. V

    Reaction force due to the curvature and gradient drift

    We know that a charged particle will have a drift velocity in both a curved magnetic field and when there is a transverse spatial gradient in the magnitude of the magnetic field. This drift velocity is added to the rotation velocity around the the field line. In both cases the force vector on...
  13. sergiokapone

    Does the Bernoulli Effect Apply to the Jet Reaction Force in a Leaking Bucket?

    If in the bottom of the bucket with water we made a hole, then water flows out of it. Bucket with water is the system with ##m(t)##, thus we have to use equation: ##m(t)\frac{dv}{dt} = F + u\frac{dm}{dt}##. Where ##u\frac{dm}{dt}## -- is jet reaction force. But, I think the jet reaction force...
  14. R

    Ground Reaction Force: What Causes It?

    This may seem like a really basic question (I'm teaching lower school science, and it's not my specialism!), but: What causes the force which balances gravity, which I think is called "ground reaction force?" All the descriptions I've seen say that the ground "pushes" up on a stationary...
  15. B

    Reaction force direction during reflection

    I have a simple question let's say object hits the wall at angle and reflect. Will reaction force from that impact exerted on wall be red line or green line ? ( I would like explanation why in each case)
  16. Greg Bernhardt

    What are Reaction Forces and how do they relate to Newton's third law of motion?

    Definition/Summary Between two bodies in contact, there is a pair of Reaction Forces. They are always equal but opposite (this is precisely Newton's third law), and so it is usual (though slightly misleading) to refer to "the Reaction Force", in the singular. The component of the Reaction...
  17. K

    Reaction Force of Rod at 90 and 180 Degrees

    Homework Statement a uniform rod of mass m and length L is free to rotate in the vertical plane about a horizontal axis passing through its end. the rod is released from rest in the position shown by slightly displacing it clockwise. Find the hinge reaction at the axis of rotation at the...
  18. M

    Tension in cable and reaction force

    Homework Statement Homework Equations The Attempt at a Solution For this problem, what I am confused about is the fact that the Z direction parts of the tension will tend to rotate in a direction different from the Y components of the tension and the 850 lb vertical force. I am not sure how to...
  19. P

    Finding the reaction force - moments and equilibrium

    The problem statement A uniform rod of length 2m and mass 5kg is connected to a vertical wall by a smooth hinge at A and a wire CB as shown. If a 10 Kg mass is attached to D, find: a) the tension in the wire b) the magnitude of the reaction at the hinge A. attached is my copy of the...
  20. B

    Wheels Reaction Forces: Solving for Ra and Rb?

    Homework Statement In the diagram below, work out the reaction force in each of the wheels. Note there are two wheels that make up Ra. The Attempt at a Solution 0 = 25,000*2 + 5,000*15 -5 Rb - 8,000*2 0 = 50,000 + 75,000 - 5 Rb - 16,000 0 = 109,000 - 5 Rb 5 Rb = 109,000 Rb...
  21. D

    Reaction force for force exiting a tank through a clean hole

    Homework Statement Determine the reaction force from water flowing through a clean 0.02cm (Diamater assumed) hole in tank depth 3m Homework Equations P = rho.g.h A = Pi.R^2 P=F/A The Attempt at a Solution P = rho.g.h = 29400 Pa A = Pi.R^2 = 0.00031415 m^2 P = F/A -> F = PA =...
  22. G

    Radiation reaction force and Newton's third law

    There seems to be a problem with radiation reaction force. Let's look at charged sphere, attached on a rod to an object with larger mass, and rotating around its centre together with it. The rotating charged sphere emits radiation with power P. The sphere's speed is V. Assuming energy is...
  23. E

    Reaction Force On Ladder and Wall Case

    Homework Statement https://dl.dropbox.com/u/63664351/Physics/MECH1.PNG Homework Equations Sum of Forces and Torque = 0 The Attempt at a Solution I found similar problem here: http://farside.ph.utexas.edu/teaching/301/lectures/node133.html Why is there no vertical reaction on a...
  24. R

    Torque due to reaction force at contact point of two objects

    Hi there, I would like to know that if we have an leaning object supported by another object, both are supported by ground floor to satisfy static equilibrium condition, does the reaction force at the contact point produce cancelling torque (for the leaning object)? In general, can a reaction...
  25. R

    Reaction force of freely rotating rod

    Hi guys! I'm working some problems for our exam in a few weeks and there are a few problems we are unable to solve. One of them is: "A homogenous rod is free to rotate without friction around one of it's ends. Find the reaction force in function of the position of the rod." As you can see the...
  26. K

    Find tension force of a cable and components of reaction force of pivot

    Homework Statement Please see attached photos. I've been drawing and re-drawing diagrams but I'm at a loss. For part a, I've tried a right triangle using the top cable as the hypotenuse. I've tried the force equation (ƩF=ma) but I don't think that's right. I just can't get a start...
  27. A

    Action reaction force quick question

    Homework Statement A firewoman opens the fire hose, and water sprays forward. What is the action force and reaction force? Homework Equationsnone The Attempt at a Solution I was thinking that the action force would be water pushing on air and reaction air pushing on water? But how...
  28. G

    Reaction force, Bending Moments on a sub frame

    Hi, I've been working on a tubular sub frame, that I want to attach to a main frame. The attached shows the sub frame in black lines. The main fram is in purple. There are only four connection points and a force pushing down on each side. When I check for the reaction forces and make...
  29. A

    Reaction Forces for Tension in a Rope

    What is the reaction force for a tension force. For example someone pulling a brick by a rope.
  30. C

    Calculating Reaction Force in High Volume Air Blow through Piping Circuits

    When Executing an air blow through a circuit of piping at high volumes and resonable pressures. How would you calculate the reaction force that the piping circuit will generate by the open end exit blow of air? Need a detailed method of how to calculate and what the numbers used represent and mean.
  31. Z

    Reaction force on a semi-cubical parabola

    Homework Statement A particle of mass m moves without friction along a semi-cubical parabolic curve given by y2=4ax3 with a constant speed v. The reaction force of the curve on the particle when it is at the point (x = 1, y = 2) is given approximately by The Attempt at a Solution...
  32. K

    What Are the Reaction Forces in These Physics Diagrams?

    Homework Statement One or more external forces are exerted on each object enclosed in a dashed box shown in the figure below. Identify the reaction to each of these forces. (Assume that (a), (b) and (c) occur on Earth while (c), (d) and (e) occur far from any outside gravitational influences...
  33. M

    Finding tension and reaction force for a boom

    Homework Statement A 1100 N uniform boom is supported by a cable perpendicular to the boom, as in the figure below. The boom is hinged at the bottom, and a 2000 N weight hangs from its top. (a) Find the tension in the supporting cable. (b) Find the components of the reaction force...
  34. E

    How do action reaction force pairs arise?

    Hi guys, What is the reason for action-reaction force pairs to arise? Is it a consequence of electrostatic repulsion due to interaction of the 2 bodies, or can it be explained using energy conservation or momentum conservation. Thanks!
  35. J

    Balancing Weight (mg): Exploring the Normal Reaction Force

    The weight (mg) is balanced by the normal reaction acting on the body. But What is the reason for this force(normal reaction)?
  36. C

    Reaction force of ladder against wall

    Basically I don't know where to even start. Does anyone know a good page that explains this topic? I would be very grateful if someone could provide a solution, but I know that's too much to ask. Thank you very much for your time.
  37. X

    Proving what the reaction force equals with moments in mechanics.

    Homework Statement A horizontal piece of wood that has a weight 40N is held in equilibrium by two brackets, A and B. The wood is 1.6m long (CD). A is 0.4m from C and B is 0.2m from D. Meaning that there is 1.0m between A and B. At rest the reaction force at A is 24 N and at B 16N. When an...
  38. L

    How Do Reaction Forces Change as a Man Walks Across a Beam?

    1. A man of mass 75kg walks along the beam shown in the attached file, of length 5m and mass 100kg, from pivot A to pivot B. Draw graphs showing how the reaction forces FA and FB vary as a function of the man's position. Homework Equations 3.I'm not so sure what should be on the...
  39. B

    How does a laser beam propel objects in air?

    I'm aware that photons carry momentum while technically having no mass. However I recently saw a video of a laser used to propel a small silvered object up several meters in the air just using the momentum of the light. So what about the reaction force? As the light is coming out of the laser...
  40. N

    Reaction force on disk by pivot

    Homework Statement A uniform disk of mass M and radius R is pivoted about a point on its edge. The disk is released from rest when its center of mass is at the same height as the pivot. What is the reaction force exerted on the disk by the pivot the initial instant the disk is released? The...
  41. L

    Joint Reaction Force: Estimate Net Moment & Force Vector in Right Knee

    Homework Statement A dancer stands on his left leg (knee extended), torso and arms forwards outstretched horizontally. The right leg (knee extended) is backwards outstretched horizontally. The dancer weighs 55 kg. The length of the thigh is 35 cm, the shank 45 cm. Draw the free body diagram...
  42. R

    Beam Reaction Force: Calculating Final Value

    Homework Statement http://users.on.net/~rohanlal/udl.jpg If the middle restraint has a vertical reaction of 50kn when the structure is first built, how do i determine what the reaction would be if the middle restraint was to settle by 13mm? Homework Equations Is this a simply...
  43. M

    Free Body diagram & reaction force

    Homework Statement This is just a little part to a bigger problem I am having trouble with. I have simplified it to get to the point. In the free body diagram shown, I would like to find the value of the horizontal reaction force (RH) applied to the wheel from the surface, assuming there is...
  44. M

    Normal reaction force of a roller to a wedge and the velocity

    Homework Statement http://img32.imageshack.us/img32/6505/p450.gif Cam mechanisms are used in many machines. For example, cams open and close the valves in your car engine to admit gasoline vapor to each cylinder and to allow the escape of exhaust. The principle is illustrated in Figure P5.66...
  45. N

    Reaction force between a Sphere and particle

    http://img13.imageshack.us/img13/9112/70948888.th.jpg I do not understand how circular motion can be occurring a point B - surely at B, the reaction force occurs in a direction away from O. Therefore, there is no resultant force that can be acting towards O, since the weight is...
  46. V

    What is the direction of the Reaction force

    Hey I am confused about the direction of reaction force.. When a ball is on a vertical track at the bottom most of the track, I understand that the reaction is opposite the weight of the ball, mg. But when it is directly above the vertical track the reaction force is towards the center...
  47. 1

    Inclined plane + Inclined force. Normal reaction force = ?

    ok I'm desperate now lol n this is my first time on PF. I've just had an exam and I'm sure i got this question wrong. But i'd like to know: the normal reaction force and the tension/Force F. The question is: There is this object that has a weight of 20N. It is on an inclined plane that is 30...
  48. R

    Inclined plane and normal reaction force

    A block of mass M slides down a frictionless plane inclined at an angle è with the horizontal. The normal reaction force exerted by the plane on the block is a Mg. b Mg sin è. c Mg cos è. d zero, since the plane is frictionless Homework Equations Fn=mg cos è The...
  49. S

    Understanding the Reaction Force of Gravity and Air Resistance on Falling Leaves

    When a leaf begins to fall to a ground from a tree, what is the reaction force ot the force of gravity on the leaf? Is it air resistance?
  50. tiny-tim

    Net Reaction Force: Where do the Forces Act?

    I'm trying to write a Library entry on Reaction Force, and I've discovered I can't answer this very simple question. :redface: Two equal blocks are on a horizontal table, with a gap between them. A straight rod is placed above the gap, but not symmetrically, resting on both blocks, with an...
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