# What is Force: 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. ### B Is there any physics topic explaining how objects break into parts?

I am curious to know if there is any existing topic explaining how objects break into several pieces when force is applied.
2. ### B Force required to stretch training resistance bands

Hi all! I hope I'm posting in the correct section! I’m trying to work out the force required to stretch resistance training bands for two particular set ups shown in the pictures. I’m not a physics student or anything like that (I'm many years out of school!), however I am pretty good at maths...
3. ### B Compressive force of a shorter cylindrical bone vs a longer one

Hello, Why is it more difficult to break a shorter bone that is cut into a cylinder than a longer cylindrical bone by pressing the bone vertically from its top despite sharing the same cross-sectional area?
4. ### I Mechanics of an inertial balance

In the following diagram (from Taylor's Classical Mechanics), an inertial balance is shown. Intuitively, I totally understand that unequal masses would cause unequal accelerations and therefore rotational motion of the rod. However, how does one prove this mathematically? The first thing...
5. ### Force analysis of a spring-bob system

I dont need anyone to do the sum directly please. I just need some hints, please dont give me the solution. Im unable to understand how the bob will hit a maximum elongation, there are two forces accelerating the bob: Gravitational force and electrostatic force. There is one force...
6. ### Applying Force on a Cylinder at an Angle

A few months ago, our teacher gave us a problem regarding cylinder motion. He asked us to calculate the acceleration of a cylinder that's acted upon by a force F at an angle theta due to the x axis. Now, for some reason, he told us that the cylinder would rotate. Now, I'm not sure if he supposed...
7. ### Brake force calculations

I'll try and keep this as easy as I can. The Right Triangle above represents the forces acting through the trailing shoe of a automotive drum brake. Point A is the central position of the brake expander. When the foot brake is depressed, a force is created that pushes the brake expander piston...
8. ### Textbook 'The Physics of Waves': Calculating Work Done by Force

Reference textbook “The Physics of Waves” in MIT website: https://ocw.mit.edu/courses/8-03sc-...es-fall-2016/resources/mit8_03scf16_textbook/ Chapter 2 - Section 2.3.1 [Page 45] (see attached file) Question: In the content, it states that we need to use real force and real displacement...
9. ### I Producing enough force with a Solenoid

Hi, This is not a homework problem, but a small personal project where I am looking to create a small solenoid that can create enough pushing force of approximately 0.5 lbs. I am wondering what the best route is to create this amount of force. So far, I was able to lift about 32g with my...
10. ### General question about the definition of moment of force (Torque)

Definition: Moments is a measure of turning effect of a force upon a pivot. My questions: What does it mean if it is a measure of moments? Does it mean the angle of turn? Does it mean the speed of the movement? Does it mean the distance travelled? Let's take a door as an reference.
11. ### Reference source of study for welding clamping force calculation

What are the factors considered while calculating clamping force requirement ? Clamping force for welding based on thickness - 48*s^3 s- thickness what are other factors considered along with thickness and i need a detail source/reference books?
12. ### Engineering explanation please (pipe welded to a wall)

Hello. I work for a company and I need your help! The pipe is welded to the wall. However, the worker applied force to the right with the force of F, and the welding was cracked. As a countermeasure, welding is carried out in two layers as shown in the picture on the right. In this case, is it...
13. ### Force and torque question for two connected pipes

Hi. I need your help! I have a college physics question Suppose there are pipes A and B. [Figure 1] I had to connect pipe B to pipe A, but pipe B was short, so I bent A by about 30 degrees and installed it. [Figure 2] It's a picture of the pipe A being installed by increasing the length in...
14. ### B What is electromotive force? What's its relationship to Voltage?

This is the question: What is electromotive force? What's its relationship to Voltage? + clarification of confusion If you believe you have a good answer, please answer either in this website or in Stack Exchange.
15. ### What Force must be exerted on the trolley for constant speed?

My question is on the highlighted part in red. For the first part i have, ##F_{parallel} = 900 \cos 82^0 = 125.3N## For the second part, i have ##F= \dfrac{125.3}{\cos 15^0} = 129.7 N ≈130N##
16. ### Force between charged particle and conducting cylinder

Since ##x## is larger than the linear dimensions of the cylinder, hence it can be approximated to be a point dipole . $$\vec{F}=\left(\vec{\nabla}\cdot \vec{p}\right)\vec{E}$$ In our case : $$\vec{F}=p_x\frac{\partial E_x}{\partial x}$$ $$F=\frac{-2kQp_x}{x^3}$$ Of course , we can assume some...
17. ### Shape of Slinky being twirled in 0 gravity

I am able to understand the textbook solution, except for its very first assumption: We use the coordinate system shown in the figure, and find the shape ofthe spring (assumed to have already attained its stable configuration) in this frame. Why is it fair to assume that the slinky will ever...
18. ### How to calculate the force exerted by the "tendon" on the "tibia" bone?

TL;DR Summary: how to calculate the force exerted by the "tendon" and the resistance occurring on the "tibia" if a 70kg man leans on his tip of the foot. Hello, This is the question: "A 70 kg person stands on the tip of the foot. Assuming a configuration as in Figure 5.6 calculate a) the force...
19. ### B How's it possible to push on systems with 2 parts? (Newton's third law)

An alternative title could have been "how do forces propagate" but google searches bring up things related to waves only. Initially my problems started with a mass on a spring but I was able to boil it down to any general system comprising at least two 'parts' in succession. Suppose you have...
20. ### Lorentz force and induced electric field

As far as I know, there are two causes of the rotational motion of the charged ring. T he first torque is from Lorentz force: $$T=Q\mathbf{R\times ( v} \times \mathbf{B}_{r}) =-\frac{QR^{2}}{2}\frac{dz}{dt}\frac{d\mathbf{B}_{z}}{dz}$$ (where ##\mathbf{B}_{r}...
21. ### How is centripetal force involved here? (charged mass sliding down a conducting hemisphere)

I have some questions... Firstly I drew the FBD of the said block at an angle theta from the vertical Which force causes the block to lose contact.ie the normal to become zero... Is it because the forces in the horizontal direction surpass some limit value of the net force in the radial...

23. ### Find the direction and the magnitude of Frictional force

In my working i have, the attached. My question is Why is the Reaction force not being considered here, My equations are; Frictional force = ## 30\cos 50^0 - 20\cos 40^0 = (19.28 - 15.32)N = 3.96N## The direction will be downwards.
24. ### Does a shaking hammer blow cause less damage than a stiff blow?

They always talk in boxing and martial arts to stiffen at the moment of impact, but how crucial is this? By way of analogy, how much of a power leakage results from a hammer shaking at point of impact compared to a stiff one? Less concentrated force, surely.
25. ### B Is the time derivate of force equal to the position derivative power?

Is this wrong?
26. ### Force Between 2 Coaxial Current-Carrying Loops

I have attempted to solve it as follows: Using the Biot-Savart law, I found the flux density at the centre of the smaller coil due to the bigger coil as: $$\frac{\mu_0 I b^2 N_2}{2(a^2 + b^2)^{1.5}}$$ where a is the distance to the coil (10cm), N2 is the number of loops in the larger coil (50)...
27. ### What would the slope of the force between 2 charged particles vs 1/r^2 give me?

I can get the slope of the line, but then I dont know what the slope is (units or whatever it is) then i get lost from there.
28. ### Calculate electric force using Coulomb's law (vector components are the struggle)

So for this problem I think I am doing something weird with the trig and/or vector components. I calculated the problem like this: First drew a picture, q1 and q2 on the x axis. q3 located equidistant between them but negative .300m in the y direction. First finding magnitude of Electric...
29. ### Blowing Into Your Own Sail

C. There is no net change in force, momentum, or velocity of the sail craft because the fan exerts a forward force on the air; however due to Newton's third law the air exerts an equal and opposite force on the fan. This air then exerts a forward force on the sail which cancels out the rearward...
30. ### B What forces act on a bar when you do pull-ups?

I am not sure here, even though it is probably simple. If you just hang on the bar and don't move up, you act on the bar with a force equal to your weight. But when you want to do a pull-up, I would intuitively say that you act on the bar with a higher force. But when I think about it I would...
31. ### Minimum force to tip the bin over

I am stuck on part b) i). I understand that there is no normal force from the ground as the bin is on the point of being lifted off the ground, that is all fine. That leaves R, F and W. I know W is 40g, and I am required to calculate F, therefore it makes most sense to take moments about the...
32. ### B How to convert ft-lbs/sec to Newtons?

Hi, How to convert 51065 foot pound per second to Newtons, not Newtons meter per second?
33. ### B Why don't cars accelerate according to F = ma?

If the engine is constant, then the wheels of the car exerts a constant force on the floor. And F = ma, So the car should be accelerating rather than maintaining the same speed. What is going on here?
34. ### I The answer for the total mass of the Universe?

it requires less energy to impart acceleration to an object than to the entirety of the universe, This would break the principle of relativity since the amount of energy to move an object should be the same as the amount of energy to move the rest of the universe. This observation implies a...
35. ### Graph of viscous force and velocity

my answer was A but i dont understand because when the viscous force equals the weight speed becomes constant but why does the viscous force needs to be equal of weight? is it weight = viscous force? or speed = viscous force?
36. ### Center of mass acceleration for an inclined plane and mass m

Why is the center of mass acceleration zero? Did I do anything wrong?
37. ### Minimum work done to push the cube over

my attempt: centre of mass of standing block is at 0.6 and the fallen block is at 0.25 so then change in height = 0.35 and mg = 4000 N so change in gpe = 4000* 0.35 = 1400J but wouldnt we take into account that we only have to apply force till the centre of mass is outside of the block so...
38. ### Finding spring constant k based on diagram -- Don't understand how to interpret force as drawn

$$F=kx$$ $$k=\frac F x= \frac {50+50~N} {5+5~ cm}= \frac {100~N} {10~cm}= 10~N/{cm}$$ However, the answer is ##5~N/cm##, because the force on the spring is ##50~N##. I am having trouble understanding why the force isn't ##50~N## + ##50~N##. The diagram looks as though the spring is experiencing...

40. ### Horizontal force on hinge is affected by moving the block farther away?

my attempt: no effect
41. ### Centripetal force - Ferris wheel... Opposite results

I don't understand why am I getting opposite answers. I get 250N for the lowest point and 750 for the highest point. For the highest point: ##F_{net}=F_{cp}+F_{mg}-F_N## and then ##F_N=ma_{cp}+mg = 50*(25/5)+500=750N## because I've been told that ##F_{cp}## always acts towards the center of the...
42. ### Finding force between solenoid and magnet

$$\mu = 4\pi \times 10^{-7}$$ $$n = \frac{180}{0.135} = 1333.33$$ $$I = 1.85$$ $$L = 22.619$$ $$r = 0.014$$ $$F= \frac{\mu \times 1333.33 \times 1.85^{2} \times 22.619}{2 \pi \times 0.014}$$
43. ### I Force between opposing magnets

Following a conversation with @Tom.G , I’ve come up with the following question, to which we’d both like an answer: Imagine two drilled magnets threaded onto a frictionless upright shaft, set N-N. One fixed down, the other floating above. Now, if I move an iron gate in and out of the gap (the...
44. ### Estimating the force exerted by a water jet on a flat plate - Bernoulli vs Momentum Equation

Hi, Case 1: Imagine a steady liquid water jet with unit cross sectional area - incompressible and inviscid. density D and uniform velocity V. Operating atmospheric pressure is 0.0 Pa. The jet is in air ( no air resistance ) and hits a stationary flat plate perpendicularly. Applying momentum...
45. ### Does a Slowing Piano Exert More Force Than the Man Pushing It?

Albeit the simple question, I am a bit confused on whether the correct answer choice is (B) or (C). When the piano is slowing down, shouldn't the force received by the piano be a bit greater than the force received by the man?
46. ### How Does Stress Affect a Hollow Pipe When Force Is Uniformly Applied?

Hi, I've problem thinking about this problem isn't the force acting on "nothing"? how can there be any stress, I'm lost!
47. ### B Can the force of gravity overcome the Planck force?

Can the force of gravity overcome the Planck force? I remember that a long time ago I read that in the universe the Planck force was the maximum force that could be reached. But when Sagittarius A and the black hole at the center of the Andomeda galaxy approach, I have made the calculation that...
48. ### I Work done by a non constant force over a non constant angle

I was just wondering if I needed to set up this double integral or not. I've never seen anywhere people setting up double integrals for calculating the work.
49. ### Magnetic Force in Parallel Magnet Setup: Exploring the Strength of Attraction

Imagine two parallel lines of five magnets each, spaced a few millimeters apart. In the first line, all magnets have their north poles facing one direction. In the second line, each magnet's south pole faces the corresponding north pole of the first line, but the magnets are offset by one along...
50. ### B Newton's 3rd Law interactions with 2nd and 1st Law Partners

I was over on the stack but got asked to move over here. https://physics.stackexchange.com/questions/804903/newtons-3rd-law-force-on-a-rocket I've been at it all day trying to understand what partner forces are and how you can have a change in force with regard to them, or why gravity is...