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.

View More On Wikipedia.org
Recent contents Top users
  1. Heisenberg7

    B Work Done By Conservative Forces

    I did classical mechanics a while ago and I was going over some stuff that I wasn't sure if I understood correctly and now I've come over this one. It says that work done by conservative forces is equal to the negative difference in potential energy. Or, ##W_c = - \Delta U##. And I've really...
  2. T

    Block on top of an inclined plane, with a rough surface, that moves with constant acceleration

    For the first point I need to draw a free body diagram of the block and balance the forces: I chose to use as axis the ones that have the same direction of the components of the weight force; y-positive upwards, x-positive leftwards ##\begin{cases} \Sigma F_y=0 \\ \Sigma F_x=0 \end{cases}## ##...
  3. A

    I Sliding a cloth beneath a block causing tipping

    For this freebody diagram showing a cloth being pulled from underneath a block, if we take the moment about the normal force: m*g*L/2 = 0, there is no force counteracting the moment from the weight of the block. But if we take the moment about the center of gravity: N*L/2 - Ff*H/2 = 0 How do...
  4. P

    I Question about Momentum vs. Kinetic Energy vs. Deforming In Collisions

    I'm currently reading Jason Thalken's Fight like a Physicist, and I think so far they gave one of the most intuitive explanations of the practical difference between Momentum and Kinetic Energy, but I'd like to just conceptually reinforce it and make sure I have it right. So from what I...
  5. maxelcat

    B How fast are gravitational forces transmitted?

    I understand that if the sun where to 'disappear' then it would take several minutes for the us on the earth to notice as the light takes time to travel. Does this mean that it would also take the same time for us to notice that the gravitational (or electromagnetic) force would disappear too...
  6. T

    Block connected to 2 real pulleys

    For point one I analyzed all of the forces, called the tension on the right ##T_2## and the left one ##T_1##, and taking as positive the clock-wise and upwards directions I found: $$\begin{cases} T_2 - mg = ma \\ N_2= T_2 + M_2g + T_1 \\ N_1= M_1g - T_1 \\ τ_{T_2} - τ_{T_1} = I_2 α_2\\ τ_{T1} =...
  7. Gunter1977

    Resolving Forces and Sizing Profiles in Structural Engineering

    I'm with a home study structural engineering as an electrical engineer. The assignment that I am trying to do is calculating a frame to support a large pipe with a weight of 32,000 kN distributed across eight legs. ( so it on a job site) I've performed a detailed analysis of the forces using...
  8. S

    Confused about how forces in a pulley balance

    The question is regarding this image, I understand the two 50N forces provide a clockwise moment of 40 x 10^-2 x 100 which gives option D, as this moment must be balanced by the moment given by T. Now my question is, how is the net force on this system equal to 0? It says it is in equilibrium...
  9. S

    Forces on Elliptical Current Carying Loop

    TL;DR Summary: Forces on Elliptical Current Carying Loop Please consider Current carrying elliptical 1-turn coil as shown in the picture. Is it true that due to forces from the current , the elliptical shape of the coil will tend to become circular ? Thank you...
  10. T

    Engineering Determining how far a body will move up an inclined plane and the time to reach the highest point

    15cos25 - 0.25N = ma.
  11. rosedog09

    Torque on a Pivot Point With Multiple Forces and Different Directions

    My AP Physics notes state that counterclockwise is generally positive and clockwise is generally negative. This makes sense to me and means forces on opposite sides of the pivot point which act in opposite directions will work together, and this is the logic I applied to answer the problems...
  12. DylanMurfly

    Cables, pulleys, 2 weights

    Fx = 0 = ABcos(a)+BCcos(b)-BDcos(30) Fy = 0 = ABsin(a)+BCsin(b)-BDsin(30) == Fx = 0 = ABcos(a) + 3.2737cos(b) - 3.738 Fy = 0 = ABsin(a) + 3.237sin(b) - 2.158 But i cant find a third equation to use. I've tried a+b = 90 but that produced a number of errors. Thank you. Edit: angle b is the...
  13. T

    Engineering Determine the resultant of the coplanar forces

    I used Rx = 100cos(0) + 200cos(90) = 100. Ry = 100sin(0) + 200sin(90) = 200. tanO = Ry/Rx = 2. Therefore O(theta) = inv tan (2) = 64 degrees.
  14. B

    Tension in a Two-Crate System on a Frictionless Surface

    for 4kg box: Fx = T m1a=T (4) (2.50) = T 10 = T for 6 kg box: Fx = Fa - T m2a = m2a - T 15 = 15 - T 15-15 = T 0 = T I assumed the tension would be the same for both boxes, they are supposed to be, so what did I do wrong and is the answer 10 or zero?
  15. tellmesomething

    Forces at play in a pendulum (conceptual doubt)

    .
  16. L

    Calculating the spring force constant K

    So, my question is pertaining more to a specific part of this problem than actually calculating ##P## which I get to be ##P = \frac{kh}{2} - mg##. But I need ##P## in terms of a multiple of ##mg## so I need to find ##k##. The solution attached uses the fact that when the object comes to a rest...
  17. K

    Cantilevered and line-loaded steel beam

    I found the internal force Vxy 25 +6 =31kn. , bending moment Mmin -qb2/2 , M min- 14 kNm. delta =M/W I can't find Mmax and tau= V/As. So I need help to find Mmax and As wall area, how to find.
  18. lola9

    Two blocks in contact -- find the force between them

    In this problem why do I need to use this formula? F – F12 = m1 a along the x-axis. What does F12=-F21 stand for? And I don't understand when they say to find the magnitude of the forces between the two blocks
  19. J

    I The direction of three forces acting on a line has to be concurrent?

  20. C

    Calculate how much weight the crane could lift

    I think the first part is F = mxa? But given the jib is resting on a turntable at a certain distance, does that mean the longer and shorted ends experience different loads? Especially when the shorter end has a 16T counterweight? The second part, I have no idea how to calculate this nor the...
  21. M

    How Do You Incorporate Drag Force in Free Body Diagrams for Multiple Objects?

    I don't even know how to begin this. I know that I need to somehow account for the drag force that duck 3 is causing on the first 2, but I don't know how to deal with that. I am asking for someone to help me get started, not to give me the answer.
  22. theegyptian

    I Brownian motion and the resultant of forces

    Hello, Why isn't the forces resultant on a "large" molecule (by small molecule: water for example) zero? The reason for this Brownian motion is the thermal agitation of the water molecule. If we talk about white and Gaussian noise in electronics (due to the thermal agitation of the electrons)...
  23. internationallove089

    I A question on momentum conservation of bodies in combined motion

    Frictions are neglected in the system pictured. Here, a 1kg object released from above moves the car underneath. When this object falls off the car, what is its speed in m/s? This question was asked from momentum conservation, but I didn't want to do it from there. Can you solve the question...
  24. L

    Two point masses, where is the force zero?

    Hi, I had no problems calculating parts a and b, but I am having problems with task c For ##F## I got the following, with ##\vec{r}= \left(\begin{array}{c} x \\\ y \\\ 0 \end{array}\right)## , ##\vec{r}_1= \left(\begin{array}{c} -a \\\ 0 \\\ 0 \end{array}\right)## and ##\vec{r}_2=...
  25. J

    Why are the forces on torque tension and not the weight of the mass?

    Let the left string be T1 and the right string be T2. Pretend that the masses are NOT equal and that the total mass on the left is 3mg and the total mass on the right is 2mg. My first thought: Net torque = 3mgR1-2mgR1 Actual solution: Net torque = (T1-T2)*R Once again, the force that's used...
  26. tellmesomething

    Understanding Stress and Strain: The Relationship Between Two Variables

    I was going through the stress-strain curve and I realized that strain is taken as the independent variable and stress as the dependent variable. In reality is this true or is it the other way around?I saw a lot of answers on Quora that say that strain is the cause and stress is the effect. But...
  27. A

    I Permanent Magnetic Dipole in an electromagnetic field

    I've been trying really hard to calculate the forces between a permanent magnet that is within an electromagnetic field. I have tried every formula under the sun, but it seems I am just not using the right ones, as my results always end up nonsensical. To be clear, I am trying to understand the...
  28. nateTheaweseome

    Understanding and Troubleshooting Handlebar Load Calculations

    Hey all, I'm either overthinking it or too tired but this problem should be an easy one to figure out but I'm stumped embarrassingly. Say in the photo above is a lever arm. I'm trying to backtrack to calculate the force a user must input (yellow) against a force (red). It can pivot around...
  29. Hak

    I Dynamics of systems of material points

    I have difficulty understanding the extension of the fundamental laws of material point dynamics to systems. Example 1: Consider a system consisting of two material points. Suppose that the two forces acting on the two constitute a pair of forces of nonzero arm. The resultant of the forces...
  30. G

    The period of a simple pendulum

    While not exactly correct, we will continue to use Newtonian gravitational force and tension force in the lab frame. We will not concern ourselves with GR, besides the approximation is reasonable for low velocity and small mass. In the lab frame, the forces acting on the pendulum is weight and...
  31. Juanda

    The Relationship Between Masses and Angles in a Pulley System

    The problem is based on a similar thread. In fact, the first question is extremely similar. However, the second question is the one I consider more interesting but I posted the first one too for context. If this was just 1 pulley and two masses, then equilibrium is only possible if both masses...
  32. sarah123

    Describe how you could use a pith ball as an electroscope

    Would it still work in a similar manner as an actual electroscope where you would charge it either positively or negatively and then observe attractive or repulsive forces or is there an easier and more efficient method?
  33. S

    I All possible models to explain the hierarchy problem?

    There is an interesting paper by Arkani-Hamed and collaborators (https://arxiv.org/abs/1607.06821) to address the hierarchy problem. There, they consider many possible models of fundamental particle physics where they all have an exact copy of the Standard Model but with different Higgs...
  34. F

    Springs connected to a mass in series?

    I found the answer for the springs in parallel, but not for the ones in series. I believe I don't understand how the forces are interacting properly. Here's a force diagram I drew. Everytime I try to make equations from this though my answer dosen't make sense. The mass m has a gravititoanl...
  35. Nova_Chr0n0

    Engineering Statics - Determine the Reactions on this bent bar levering between two surfaces

    The figure is shown below: Here is my FBD for the figure with assign +x and +y directions I started off by summing up the forces in the x-direction: Next is the summing up of the forces in the y-direction: After this, I solved for the moment at point A: assuming that counter-clockwise is +...
  36. JohnnyLaws

    Where Does a Bar on a Fulcrum First Tip When a Person Walks Across It?

    I think that explained all in "Relevant equations". Here is the image of this exercise: This is my Free Body Diagram:
  37. JohnnyLaws

    Finding the angle that a string makes with a wall and its tension

    For a better understanding of this exercise here is the image illustrating the scenario described in the statement: So to solve this exercise I began by drawing a forces diagram: I believe I have explained everything in the "Relevant equations" section. What am I doing wrong? The book that...
  38. JohnnyLaws

    Basics of forces/torques and fulcrum

    TL;DR Summary: When a cube is supported at the fulcrum and remains stationary due to a balloon exerting a force in the opposite direction of its weight. So the exercise is as follows: We have a homogeneous cube with an edge length of 2 meters, weighing 98N. On the other hand, we have a balloon...
  39. jaketodd

    B Transfer speeds of forces affected by time dilation?

    Are forces subject to the Lorentz transformation? Not force carriers; I already got that question answered, thanks to @PeterDonis. But forces. The different forms of them, such as the contact forces etc.: https://en.wikipedia.org/wiki/Contact_force With time dilation, does the rate of force...
  40. jaketodd

    I The speed of forces, and applying relativity to force carriers

    Are there any limits to how fast forces can exert influence... or are they all instantaneous? How fast are the forces of a collision? I think frame dragging, with gravity, shows that at least gravity is not instantaneous, because if it is instantaneous, then how would it produce drag - it...
  41. Nova_Chr0n0

    Calculating torque about a point with given forces

    I've inserted a photo of the figure, hope everyone can see it. SOLUTION: 1. I first solved for the angle of 100 N and 50 N since I need the force that is perpendicular to point A. >> Angle of 100 N theta = arctan(3/4) theta = 36.870 degree >> Angle of 50 N...
  42. H

    Forces for bodies connected by a wire on inclined surface

    Hello PhysicsForums! Here is my attempt at a solution for the problem stated above: Where m1 and m2 are the masses Where Ff1 and Ff2 are friction for each mass Where a1 and a2 is the resulting acceleration Where S is the fore of the wire (threadforce) Where FN is the normal force The answear...
  43. S

    A Forces on rotating disk object

    Forces on rotating disk object Hi. Is it convenient to ask following question. Suppose we have solid circular object and 5 different moments like in the picture:In moment 1 we apply force (downwars direction) so as to start rotating the object around center of the mass (green dot) , Only...
  44. V

    External forces required to move an electric dipole quasi-statically

    If the dipole is to be in equilibrium at all positions as it's moved so that it's always inclined at 60° to the horizontal, then the torque due to electric field needs to be balanced by torque due to external forces ##F_{ext}## as shown in above diagrams. But such external forces will not make...
  45. vedant_krish

    Tipping Point of Stationary Barbell

    TL;DR Summary: Please Help. I need an answer for an Investigation if a uniform bar of length 2200mm is supported evenly on 2 points (1 and 2) 1190mm apart. The bar is comprised of 3 sections (A, B and C) of varying masses. Section A and C have equal mass and volume and is comprised of the...
  46. O

    I Uncovering the Forces on Drum Lugs: An Engineering Design Exploration

    I am currently designing an alternative to normal acoustic drum lugs and I need to find the forces that are felt by the tuning rods of a drum for the engineering background to my design. I have been able to use the 2D wave equation assuming uniform tension on the membrane and found values of the...
  47. brochesspro

    I Does a person standing up on a swing in oscillatory motion do any work?

    Also, I wonder if a system can have work done to it due to its internal forces, as it is generally said that the work done due to internal forces is zero or at least I have learnt that(cannot trust my knowledge anymore, lol). I got this question in my mind because of a situation in another...
  48. bo reddude

    What are the forces on a cell in a tree vs on the surface of a tree

    Hey everyone. Let's say you have a tall tree. say, 10 m tall. Take a cubic cell 1mm in dimensions. Suppose the cell is at 1 m high in the center of the trunk of the tree. What are the forces acting on the cell? let's say tree cell's mass is t grams. its weight is F= ma = t (9.8) = 9.8 t...
  49. paulimerci

    Three equal forces applied to a rectangle, find net torque direction?

    This is how I interpreted the problem, a) The net torque about point A is zero. This is because the forces F1 and F2 are equal and opposite, and they act at the same distance from point A. Therefore, they produce torques that cancel each other out.. The force F3 doesn’t does not produce any...
  50. H

    Forces in a Line: Examining the Relationship Between Mass and Force

    Hello! In the following image, is it true that S1 = mg/2. Thanks for answears!
Back
Top