Forces Definition and 294 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. L

    B If gravity was a force wouldn't going back in time cause us to float?

    This might sound as a dumb and silly question but if you think about it, it makes sense. If we wrongly assume that gravity is a force just like any other, and given the fact that time is closely related to gravity and that gravitational time dilation is a thing, wouldn't reverse time travel...
  2. M

    A book rests at an angle against one side of a bookshelf...

    1.52cos(31)+Fbsin(Θb) = (0.305)(9.8) Fbsin(Θb) = 1.69 Now for horizontal direction: Fbcos(Θb) = FLsin(ΘL) cos(Θb) = FLsin(ΘL)/Fb cos(Θb) = (1.52 x sin31)/1.69 cosΘb=0.464 Θb = 62.35 I thought to find Fb I would just plug the Θb value into one of the trig functions, but apparently both of my...
  3. A

    How can work done by friction stop motion if it is less than applied force?

    The answer is .32m. I set the elastic potential energy as equal to the work, but at first I put the force in the work equation as (F elastic - F kinetic friction) times distance and rearranged. 1/2kx^2 = (kx-Ff) d (0.5) (22) (0.035)^2 = (22 x 0.035-0.042) d 0.013475= 0.728 d 0.013475/0.728 = d...
  4. Cody Livengood

    B Is there an experiment that shows strong force grows with distance?

    Is there an experiment that shows strong force grows with distance?
  5. A

    I My textbook is deriving wave speed on a string under tension

    My textbook is deriving wave speed on a string under tension with confusing thetas. It assumes ##\tan \theta_1 = \frac{-F_1}{F_T}## and ##\tan \theta_2 = \frac{F_2}{F_T}## which confuses me. I know for sure theta is the angle due to the position of y and x, ##\tan \theta = \frac{y}{x}##, but I...
  6. G

    To find forces in a framework

    A very simple (I thought!) question: I'm just looking at the first part, finding the reaction at the hinge A. Here is my annotated diagram, with the reaction and A resolved into it's X and Y components, the force at E labelled as Fe and the length of ED labelled as L. Considering the body...
  7. G

    How to find the forces in a particular framework of light rods

    Could I please ask for help in how to do this question. Is it in fact well formed, can it be solved as it is or do I need more information? Q. Find the external forces and the force in each rod in the following framework of light rods which is supported and A and C: So, I need to find Fa...
  8. Amphibia

    Forces on two different blocks with two pulleys

    I don't know what to do as I know the forces acting on it but not the scalar like I just don't understand the question with two different pulleys I know the tension same everywhere
  9. SpectraPhy09

    Plotting the graph of Friction Force vs. time (Laws of motion)

    F.B.D Of first block (I have shown only the horizontal Forces) f1(max) = μ (1kg)(g) = 0.5 * 10 = 5N F.B.D Of the second Block f2(max) = μ (3kg)(g) = 15N Now the string will become taut and the tension will start acting when f = t = 5N But for 0<f<5N there will be no motion between the 1 kg...
  10. I

    Forces on a slider crank

    I am in a mechanical design class that has been focusing on the slider crank mechanism. My professor tends to just provide derived equations without showing the analysis. I feel like I am missing out on some key understanding because of this. Specifically, I am trying to do what should be a...
  11. C

    Calculate dimensions of floating laptop stand

    I am planning to build my own laptop stand with an acrylic sheet. The final product should look like the one in the attached image (bottom right) Material: most laptop stands are made with 6mm acrylic thickness, hence this is what I will use. Also I already have this material at home. I am...
  12. L

    Mass m sliding without friction inside a rotating tube

    1) To be in equilibrium, it must be $$\begin{cases}F_{centr}-T=0\\ T-mg=0\end{cases}\Rightarrow F_{centr}=T=mg\Rightarrow m\omega^2 R_0=mg\Rightarrow R_0=\frac{g}{\omega^2}$$ 2) It is intuitive that this equilibrium is unstable but I don't know how to formally prove this. 3) In ##R_0## the...
  13. P

    Meaning of F = -dU/dx for capacitors

    I have noticed that F = -dU/dx in gravitation gives the attractive force experienced by both bodies. For capacitors, does F = -dU/dx give the force experienced by each capacitor?
  14. P

    I Does the Normal contact force act through the centre of mass?

    Consider the following situation: You have 1 rectangular block lying on a table, and an identical block is placed above the block on the table. Now, this new block is constantly pushed to the right, right before it topples off. Consider the torque about an axis passing through the rightmost...
  15. P

    I Why is my derivation of the catenary wrong?

    Important note: I only derived the differential equation, I did not solve it. What I think caused the mistake: - the tangent approximation (tan(theta+dtheta) ~ tan theta + d theta
  16. B

    Lifting a survivor into a helicopter with a rope

    We have 2 forces affecting the rope: 1. Gravitational force of the body ##=mg## and 2. Force of air = Force of drag= ##F_{AIR}##. The length of the rope is shortening with the velocity ##v_k##. So to figure out the angle ##\theta## I wrote: ##R##= force of rope ##R_x = F_{AIR}## ##R_y = mg##...
  17. SamRoss

    B Simple forces described in terms of fundamental forces

    My first question is actually, what happens when any two objects get near each other? This question is often phrased as "Why can't you really touch anything?" or "Why can't you walk through walls?" I have heard two answers: 1. the repulsion between electrons 2. the Pauli exclusion principle...
  18. G

    How to calculate the magnitudes of three forces around a hexagon

    Can anyone please help me with the following? Three forces which act along the sides AB, BC and CD of a regular hexagon ABCDEF of side 2a, have a resultant which acts along DF. When a couple of 4Pa in the sense CBA is added in the plane of the hexagon, the resultant acts along CA. Find the...
  19. goodOrBad

    Need to find forces in S1 and S2

  20. goodOrBad

    Two blocks connected by a rope over a pulley

  21. goodOrBad

    Feedback on the concepts surrounding pulley, ropes, friction

  22. E

    Pulley and Two Masses -- Newton's Law of Motion

    From this question, I do not understand why there are three forces exerted at Point C (2 of it being the tension by weight A and the other is the tension by weight B) I understand that there is tension by the two weights but why is there 2 forces exerted by weight A at point C? From the...
  23. AspiringPhysicist12

    2 masses and 2 pulleys (Kleppner and Kolenkow)

    For the official answer, I would appreciate an explanation as to why there's a division by 2 in the constraint equation for the total length of the strings, and why the way I wrote my constraint equation is incorrect.
  24. AspiringPhysicist12

    Basic problem involving forces (Kleppner/Kolenkow)

    I attached my diagram/work done so far. I believe that as long as the angular velocity is always less than sqrt(2g), the object must fall downwards when it reaches the top of the drum wall. Also, the angular velocity cannot be greater than sqrt(2g), as ω=sqrt((g/r)cosθ) at all times, and can...
  25. B

    Pressure generated by a force on a pump with a lever

  26. burian

    How to solve questions in pseudo force frames?

    $$ \frac{62 - .4mg + (.2) (m_{net} g)}{m} = a(t)$$ And, then I put ths into $$S=\frac{1}{2} at^2$$ But, my answer is not quite coming with what's in the options? So, my main confusions are, SHould the bottom most surface exert a force on the top most block, also where am I making mistakes...
  27. burian

    Two blocks on a third block which is split into two ramps

    So, for this question first I did a free body diagram in the perpendicular x-y axes, and, I got some equations with the normals, but the answer is independent of normal forces. So, I'm not sure how to eliminate the two normals. Further I find it quite weird that big block-A doesn't come into the...
  28. Achintya

    Trouble with the concept of tension....

  29. xWaldorf

    Newton's 3rd law -- Trying to understand the forces on objects with different masses

    So there's something I'm missing when I think of Newton's 3rd law of motion. If all forces between two objects exist in equal magnitude and opposite direction, how is it that, for example, when I'm driving my car, the car "runs through" all the air molecules, and they seemingly don't exert the...
  30. J

    Help Walk Me Thru Motor Sizing Calculations Please

    Summary: Hello; i am trying to work my way thru some motor sizing calculations on a semi-simple example so that i can more easily do it in the future. Could i get some guidance? Hello: I am trying to develop a good system for sizing motors (generally steppers). In the past i just guesstimated...
  31. Physics4Eva

    I Gravitons and the Basic Forces

    W and Z bosons, gluons, and photons are all gauge bosons that have been found. Since the graviton can be connnected to atoms and mass in certain ways, I think that CERN may eventually prove the graviton(if real) through smashing atoms that, supposedly, have a graviton orbiting them. Could...
  32. LilRubyKinz

    How to Solve Acceleration of Cart on 3 Different Slopes?

    I am trying to solve accelerations of a cart on these different slopes. I don't understand how it is possible without knowing the coefficient of friction, but my teacher says it is (very cryptically I might add). Can anyone help me understand this? Thanks.
  33. A

    Rigid object in equilibrium

    Here's the task: My attempt at a solution (I choose C as an axis): However, the textbook solution says D should be 58.8. What am I doing wrong?
  34. A

    Torque: find the force necessary for a body to be in equilibrium

    The question doesn't specify whether we're talking about translation or rotational equilibrium, so I suppose it's both: In order for the body to have translational equilibrium: 60 N + F2 = 0 F2 = -60N However, in order to have rotational equilibrium: 60 N * 3m + F2 *8 m = 0 60 N * 3m - 60...
  35. greenrichy

    Finding work and velocity for an object sliding up and down on an inclined rough surface

    $$\sum F_x = T - w_x - f_k = ma_x $$ $$ T = mg\sin(\theta) + mg\cos(\theta)\mu_k + ma_x$$ $$ T = (9.8 \frac{m}{s^2}) \cdot (\sin(41^{\circ}) + \cos(41^{\circ})) + (75kg)\cdot(0.25\frac{m}{s^2}) $$ $$T = 672.91 N $$ Having found the tension force, I can find the work done by the person who's...
  36. solarmidnightrose

    What is knowing the Tension Force good for?

    So, I'm working on calculating the tension force in vertical circles-and I understand how to calculate the tension force at different points in the vertical circle (via vector sums). But what I don't really understand is how to interpret this information. What can I further do with these...
  37. L

    Sinking a cylinder with varying hole sizes

    I'm trying to write up some theory for this experiment I'm carrying out. I'll be changing the radius of the hole at the bottom of the cylinder, and I think the time taken for the cylinder to completely submerge is inversely proportional to the size of the hole. Problem is that I'm stuck at...
  38. Benjamin_harsh

    How to represent this statement in a diagram?

    Statement: My friend represented this sentence "Two forces have magnitude of 3N and 5N, and have an angle of 60 between them" into this diagram: But why didn't he represent like this?:
  39. somasimple

    Ion migration by diffusion in an electric field

    Hi, A solution contains some ions (charged particles). We are only interested in my exemple to positive ions. It is assumed that these ions acquired some mobility under a concentration gradient. Their direction is A to B. Then these ions encounter/cross an electric field which is oriented from B...
  40. bigbosswilly

    High school Physics - Simple Harmonic Motion

    I started off by finding when Fg=Fx: (72)(x)=(31)(9.8) x=4.2193m After this I'm stuck and have a few things I'm confused about: When the penguin's jumping, is there elastic energy? (because the rope's getting compressed? Or maybe not). Also, I know you can use energy conservation, but...
  41. Sian R

    Curvilinear Dynamics and forces

    Homework Statement A wedge with face inclined at an angle θ to the horizontal is fixed to a rotating turntable. A block of mass m rests on the inclined plane and the coefficient of static friction between the block and the wedge is µ. The block is to remain at position R from the centre of...
  42. bardia sepehrnia

    I Wrong solution in the book? Calculating force projection.

    The question asks to calculate the determine the projection of the resultant force of F1 and F2 onto b-axis. However instead, the solution is a projection of F1 on b axis plus F2. Shouldn't the solution involve the projection of R which is 163.4 on to be axis? and for answering that, don't we...
  43. some4544

    I Force required to tip an object

    So I need to find equations for the force required to tip over a cylindrical object. This is fairly straight forward in the case when the force is applied above the centre of mass, by taking moments about the pivot corner. However, in the case where the force is applied below the centre of mass...
  44. T

    Forces - Space Shuttle Takeoff Calculations

    Homework Statement [/B] The total mass of a space shuttle and its launch vehicle is M=2000t. a) What must be the minimum size of the thrust force, to make the rocket move? b) The actual thrust of the rocket is F=30MN. What is its acceleration in the beginning? c) Assume that a mass of a...
  45. S

    Calculating tensions and acceleration

    Homework Statement A mass M1 is sliding across a table with coefficient of kinetic friction μk. A string is tied to this mass and runs over a pulley, drops vertically and is tied to another mass M2 which is falling. The pulley is connected to the table by a support. The pulley is a solid...
  46. thebosonbreaker

    Resolving forces: Mass on a string

    Homework Statement A mass of 50 grams hangs in equilibrium on a string. The mass is pulled aside and upwards by a force of 0.3N which makes an angle of 30° with the horizontal. Find the angle that the string makes with the vertical and the tension in the string. Homework Equations Body is in...
  47. lyn83716

    What will the FBD of 2 blocks being pulled look like?

    Homework Statement Block A is on top of Block B, both are of equal mass and size. Block B is being pulled by a hook at a constant velocity. There is friction between the table and block B. Homework Equations [/B] What will each FBD look like? Is there friction between Block A and Block B...
  48. CheesyPeeps

    Tension in a Cable

    Homework Statement Homework Equations ∑MA = 0, where A is the pin. Tx = Tcos150 Ty = Tsin150 The Attempt at a Solution I've drawn in the 100kN force of the beam's weight, and identified that ∑MA = 0. I attempted to find the moment arm, but really wasn't sure how do to it, or if it would even...
  49. N


    Homework Statement Two identical trucks traveling at different constant speeds are about to collide. The trucks are traveling in opposite directions with truck A going at a velocity of 3m/s right and truck B going at 4m/s left. During the collision, will the magnitude of the acceleration of...
  50. A

    A charged cork ball is suspended on a light string in the presence of a uniform electric field

    Homework Statement A charged cork ball is suspended on a light string in the presence of a uniform electric field. E=(3×10^5 N/C)i+(5×10^5 N/C)j. The ball is in equilibrium in the field. Find the charge on the ball. The acceleration due to gravity is 9.8 m/s2 . The mass of the ball is 0.7g. The...