What is F=ma: Definition and 192 Discussions

In classical mechanics, Newton's laws of motion are three laws that describe the relationship between the motion of an object and the forces acting on it. The first law states that an object either remains at rest or continues to move at a constant velocity, unless it is acted upon by an external force. The second law states that the rate of change of momentum of an object is directly proportional to the force applied, or, for an object with constant mass, that the net force on an object is equal to the mass of that object multiplied by the acceleration. The third law states that when one object exerts a force on a second object, that second object exerts a force that is equal in magnitude and opposite in direction on the first object.
The three laws of motion were first compiled by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), first published in 1687. Newton used them to explain and investigate the motion of many physical objects and systems, which laid the foundation for Newtonian mechanics.

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  1. N

    Problem with f=ma and E=0.5mv^2

    EDIT: The question has been answered, and I don't need help anymore. Thanks a lot everybody! Hi. This is something that has been puzzling me for a while, probably just due to my own stupidity. It seems to me that this should be really simple, but I don't know what I'm doing wrong though, so...
  2. P

    F=ma, no acceleration with constant velocity?

    I got a 30kg rock in space, no gravity, no friction. It is traveling at a CONSTANT speed of 100m/s. According Newton's second law, There should be NO force because there is NO acceleration. But I know if it hits another object it will either move the object or put a hole in it so I know there is...
  3. F

    Dynamics and F=ma, where a is 0

    Homework Statement A 24kg box is placed on a rough slope. Determine the magnitude of the frictional force exerted by the slope on the box if the box is moving downward at a constant velocity of 9.0m/s. The slope incline is 40° Homework Equations F=ma The Attempt at a Solution F = 24 x 0 a...
  4. K

    F=ma, but Gravity is a Force with no mass?

    I've always wondered why Gravity was described as a force. Force=mass*acceleration. Gravity has no mass, the pull of gravity depends on the mass of the objects it's pulling on. Right? Or does Physicist/Mathematicians see Force of Gravity depends on the mass of the objects it's pulling...
  5. L

    Acceleration of gravity conflicts with F=MA, why?

    I just started learning about forces. Here is what is confusing me. First, they say that no matter the mass of an object, it will accelerate towards Earth at exactly the same rate. So a car and a baseball would hit the ground at the same time even though their masses differ. However...
  6. X

    Solve Acceleration of 200-lb Block After 50-lb Force is Applied

    Homework Statement The 200-lb block is at rest on the floor (u=0.1) before the 50-lb force is applied as shown in Figure 1. What is the acceleration of the block immediately after application of force? Assume the block is wide enough that it cannot tip over. Homework Equations F=ma...
  7. K

    F=ma applied to slider crank piston motion

    I'm struggling to apply F=ma to the motion of a slider crank, more specifically the piston. I want to find out how much force is nessesary to keep the piston in motion. Essentially, in the mechanism there is acceleration and deceleration, does that mean only at some points in the cycle there is...
  8. T

    Calculating Acceleration from Tennis Serve Data

    Homework Statement The fastest served tennis ball, served by "Big Bill" Tilden in 1931, was measured at 73.14 m/s. The mass of a tennis ball is 57.0 g, and the ball is typically in contact with the tennis racquet for 30.0 {\rm ms}, with the ball starting from rest. Homework Equations...
  9. J

    Can one derive F=ma from kinetic energy ?

    I start with the expression for kinetic energy \Large E = \frac{1}{2}mv^2 Differentiate both sides by x \Large \frac{dE}{dx} = m v \frac{dv}{dx} Substitute the following expression for the velocity v \Large v = \frac{dx}{dt} To get \Large \frac{dE}{dx} = m \frac{dv}{dx}...
  10. T

    How Do You Calculate Acceleration in a Physics Experiment Involving Force?

    Homework Statement Problem for physics project (How morce force does a person use on another's head in a punch, kick etc) A 8lb medicine ball (to resemble a human's head weight) will be punched to accelerate X speed. Homework Equations (no clue what equations...) d=Vi t + 1/2 at^2 d=vtThe...
  11. M

    Calculating Friction Force from Velocity Equation

    Homework Statement A 12 kg box slides across the floor with an initial velocity of +8.4 m/s. It comes to a stop after sliding +13.8 m. How much friction was present? Homework Equations f=ma v^2=v(o)^2+2ad The Attempt at a Solution I did the ''v^2" equation to find out what the...
  12. M

    Can i get help with my F=ma problems?

    Four questions that i don't know how to do at all. 1. A student standing on a scale in an elevator at rest sees that it reads 650 N. As the elevator rises, the scale reads 700 N. Find the acceleration of the elevator. 2. A sign in an elevator states that the maximum occupancy is 20...
  13. L

    Newton's second law F=ma and torque do not agree?

    Homework Statement Suppose a sphere is rolling on the ground and there is friction. Friction is the only net force acting on it, and by Newton's law, it will accelerate the object in the direction of the force (which is backwards). But the object is actually accelerating forward because...
  14. V

    Basic physics forces problem (f=ma)

    Homework Statement A 28.0-kg block is connected to an empty 1.00-kg bucket by a cord running over a frictionless pulley. The coefficient of static friction between the table and the block is 0.450 while the coefficient of kinetic friction is 0.320. Sand is gradually added to the bucket until...
  15. N

    GR Analog to F=ma: Exploring the Geodesic Equation

    I am interested to know what Newton's second law (F=ma) looks like in general relativity. Looking at the geodesic equation, it appears to have some similarity. Multiply the mass of a particle by both sides and it looks like this...
  16. S

    Why is vector division considered undefined in F=ma?

    Hi, I have seen a discussion similar to the question I am asking right now on the forum before, but I did not understand it and do not know if a conclusion was reached. In F = ma, F, or force net, is a vector and acceleration is a vector. When solving for m, or m = F/a, you are dividing...
  17. 5

    How Does Doubling Mass and Force Affect Acceleration?

    Homework Statement Two rubber bands stretched to the standard length cause an object to accelerate at 2 m/s^2. Suppose another object with twice the mass is pulled by four rubber bands stretched to the standard length. The acceleration of this second object is: ?? The correct answer is...
  18. S

    F=ma, kinematics in vector notation

    I'd like to go over my approach to a question I am doing right now, but I've posted my general approach instead of the question itself. Say there is some object with three forces acting on it, and these forces are given to me in vector notation (i.e. with unit vectors "i" and "j"). I am also...
  19. U

    How to Best Prepare for the USAPhO (F=ma) Exam?

    Hello, I'm new to this forum, and I'm not sure if this is the right place to post this. If that is the case, could someone tell me? I have several questions regarding USAPhO prep, so I'd appreciate suggestions from people who have done USAPhO before. So, I'm relatively new to physics (I'm...
  20. R

    I don't understand f=ma So far so good

    Alright so f=ma I know the difference between mass and weight so that's alright. Now I'm heavy (96Kg); If I'm moving forwards at a rate of one metre per second (with the passing of every second I move forward exactly one metre) then is 'a' equal to one? This confuses me in that I'm not...
  21. Alain De Vos

    What is the Relationship Between Vectors and Newton's Second Law?

    Let F be a force vector and a an acceleration vector. A vector v(x,y,z) in 3 dimensional space xyz is the set of objects ((x1,y1,z1),(x2,y2,z2)) with conditions : x2-x1=x , y2-y1=y , z2-z1=z Writing vectorial F=ma , the equality means an equality between the sets. When a ball falls in...
  22. L

    Can't Figure out what I am doing wrong F=MA (easy problem)

    Homework Statement An elevator (mass 1020kg) is to be designed so that the maximum acceleration is 0.062 g. What is the maximum force the motor should exert on the supporting cable? Homework Equations F=MA The Attempt at a Solution (.062)(9.8 m/s^2) = .6076 m/s^2 F =...
  23. M

    The Strong Force on a Neutron Captured by a Nucleus

    Homework Statement A nucleus that captures a stray neutron [in a nuclear reactor, for example] must bring the neutron to a stop within the diameter of the nucleus by means of the strong force.That force, which "glues" the nucleus together, is approximately zero outside the nucleus. Suppose...
  24. M

    How Does Subtracting and Adding Masses Affect Acceleration in a Force Equation?

    Homework Statement A certain force gives an object of mass m1 an acceleration of 11.9 m/s^2 and an object of mass m2 an acceleration of 2.8 m/s^2. What acceleration would the force give to an object of mass m2 - m1 and m2 + m1?Homework Equations F=ma Force = mass*accelerationThe Attempt...
  25. C

    ?How to derive the equation of motion F=ma, given this info

    ?How to derive the equation of motion F=ma, given this info Homework Statement http://img861.imageshack.us/img861/1557/2009b5a.gif Homework Equations The Attempt at a Solution Is my attempted solution correct? [PLAIN][PLAIN]http://img42.imageshack.us/img42/2465/2009b5asol.gif
  26. J

    How Does Time Impact the Force Calculation in F=ma for a Falling Object?

    i am an idiot. i am going to drop a rock from the top of a building. i want to know the force with which the rock will impact the ground. i know the rock's mass is 1kg. i know that the rock will accelerate at 9.8m/sec/sec. if i use the equation F=ma, i get a fixed result/answer - 9.8N...
  27. I

    Breaking a Wall: What Determines its Threshold for Crumbling?

    I was thinking about this today: When a force acts on an object, the object exerts on equal and opposite force - Newton's third law. Imagine this situation: I am breaking down a wall with a hammer. I begin hitting the wall gently and this wall doesn't break. I then hit harder and the force...
  28. A

    Should be very simple F=ma question

    Homework Statement A group of folks are riding in a hot air balloon 60 m above the ground. The total mass of the balloon and its occupants is 300 kg. The balloon is stationary in the air due to the upward buoyant force of the hot air. Do a free body diagram of the balloon. If the buoyant force...
  29. J

    F=ma Physics Olympiad Exam question 19: Constant power

    Homework Statement A car has an engine which delivers a constant power. It accelerates from rest at time t = 0, and at t = t0 its acceleration is a0. What is its acceleration at t = 2t0? Ignore energy loss due to friction. Homework Equations P=W/t, W=Fd, F=ma The Attempt at a...
  30. R

    How to calculate force and acceleration in a space shuttle launch?

    Homework Statement Right after clearing the launch tower, the space shuttle is accelerating upward at 6 m/s/s. a) Calculate the net force on an 80 kg astronaut during this acceleration. \ b) Calculate the force that the seat must exert on the 80 kg astronaut during this acceleration...
  31. G

    Find torque to lift 100kg F=Ma

    Hi all, Please note this is not a home work. Working on a NPD.Please look at the jpeg file I have done few basic calculations to find torque to lift 100kg F=Ma F=100x9.81 F=981N Torque=Force x Distance T=981x.77 T=755Nm The torque on the gearbox output shaft required to lift is...
  32. Q

    F=ma is not making sense here.

    Normally we are used to the fact that if "a" in F=ma doubles, then Fnet as well should double and if "m" is halved then F is also doubled. Can anyone explain why it isn't so in this case? "A horizontal force accelerates a box from rest across a horizontal surface (friction is present) at a...
  33. V

    Did i do this f=ma problem right

    1. Homework Statement A body of mass m = 10 kg initially at rest has force F = 20Ni + 30Nj applied to it. Find its velocity at t = 3 s f=ma 3. The Attempt at a Solution m(ai,aj) = (20i,30j m*ai= 20N m*aj = 30N ai = 2*m/s^2 aj = 3*m/s^2 to find the velocities...
  34. Z

    Trouble understanding Newtons first law and f=ma

    In reading Newtons first law, An object at rest or in motion will remain the same unless acted on by an outside force. also known and understood in Newtons 2nd is Force = mass x acceleration. An object "A" moving at a constant velocity has no acceleration therefor no force. Object "B"...
  35. F

    Calculate Net Acceleration of Masses: F=ma, t=mg+ma, t=mg-ma

    Homework Statement Two masses of 40kg and 120 are connected by a light inextensible string Calculate the net acceleration of the system of masses magnitude and tension of the string Homework Equations F=ma t=mg+ma t=mg-ma The Attempt at a Solution 120x9.8 = 1176N 40x9.8 = 392N 1176 - 392...
  36. A

    Measuring Mass: F=ma - kg or g?

    m=F/a then is mass measured in kg or g. Thanks
  37. H

    Proof of F=ma from second law of Newton

    from the second law of Newton we know... applied force is proportional to the change of momentum...that means F= k( mv-mu) where k is constant ...from here now proof F=ma
  38. R

    U.S. Physics Olympiad F=ma 2008: Problem 12

    12. A uniform disk rotates at a fixed angular velocity on an axis through its center normal to the plane of the disk, and has kinetic energy E. If the same disk rotates at the same angular velocity about an axis on the edge of the disk (still normal to the plane of the disk), what is its kinetic...
  39. A

    Analyzing Acceleration Components in a Horizontal Projectile Trajectory

    Homework Statement A small point-like object is thrown horizontally off of a 50 meter high building with an initial speed of 10 m/s. At any point along the trajectory there is an acceleration component tangential to the trajectory and an acceleration component perpendicular to the trajectory...
  40. B

    F=MA + Average Acceleration etc.

    Homework Statement A. A Volkswagen Rabbit has a mass of 1090 Kg and can go from 0mi/hour to 65 mi/hour in 12.0 seconds with an average acceleartion of 2.42 m/s^2. Disregarding friction, what force must the engine be producing? B. Disregarding friction, how much mass would need to be...
  41. B

    Projectile F=ma: -475 N & 3.00x10^3m/s^2 calc force

    Homework Statement what force is required to give a projectile weighing -475 N an acceleration of 3.00x10^3m/s^2 Homework Equations f=ma The Attempt at a Solution f=ma -175 N=m (-9.81m/s^2) m=48.4 Kg f=ma f=(48.4kg)(3.00x10^3m/s^2) f=145000 N hi thanks so much for...
  42. B

    Did I do this right an F=ma need to find Initial Speed?

    Did I do this right an F=ma need to find Initial Speed? Need help on this. A car whose mass is 2600 kg can produce an unbalanced braking force of 2765N. Calculate the car's initial speed in order to have a stopping distance of 3036.3m. Vi=? So... m=2600kg F=2765N s=3036.3m Vf=0 So I...
  43. W

    Need help on a problem F=ma and i have gravity

    F=? m=295kg a=75m/s^2 an object is falling with a mass 295kg and has an acceleration of 75m/s^2 what is the force of the collision. F=295(75 * or + gravity)
  44. C

    What Do F=ma and the Definitions of Force and Mass Really Mean?

    F=ma How do we define m and F? If their definition both come from this equation, then the equation doesn't really mean anything...We can also definite a F', and say F'=ma^2, and there isn't anything wrong... However, we have F=ma as something true, then F and m must be something true...
  45. B

    The Force of Wind and the Debate on Heat, Energy, and Mass

    i'm currently discussing this with a friend. anyway. here is the conversation: he said It started as "If something doesn't have a mass, it cannot have a force", due to Newton's second law, F = ma (Force = Mass x Acceleration). Logically, if you substitute 0 in for the Mass, you will get a...
  46. E

    F=Ma Problem Help: Kinetic Energy of Rotating Device

    Hi, I'm practicing for the physics olympiad qualifier and I came across this problem: A spaceborne energy storage device consists of two equal masses connected by a tether and rotating about their center of mass. Additional energy is stored by reeling in the tether; no external forces are...
  47. N

    Would like to double check my work. Mostly F=ma if I'm right.

    Homework Statement A) A car of mass 1000 kg is acted on by a net force of 2500 N. What is the resulting acceleration? B) A car of mass 1000 kg pulls a caravan of mass 800 kg; the driving wheels of the car exert a force of 8000 N on the road. The total resistance to motion is 3000 N. B1) What...
  48. W

    Dymanics, F=ma, F function of position & velocity (it's really important)

    Homework Statement The proposed design for an energy- absorbing bumper for a car exerts a decelerating force of magnitude bs + cv on the car when it collides with a rigid obstacle, where s is the distance the car travels from the point where it contacts the obstacle and v is the car's velocity...
  49. T

    F=ma , p=mv , EK=1/2 x m x v^2

    f=ma p=mv ek=1/2 x m x v^2Not exactly homework, but some guys on the net are wondering about what formula to use for a boxer throwing a punch. I know its the EK one. But some dude said F=MA and I don't have a clue how to explain he is wrong. Also I've just been doing this stuff (linear motion)...
  50. F

    Is a bicycle or dirtbike an example of f=ma?

    Hey all - this is my first post so I mght as well say hi - I'm not very smart, but find everyday physics interesting. I tried to look up Newton's Laws, but I can't figure out: Is a dirt bike or bicycle a good example of the f=ma principle? All the info I found states that the force is...
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