What is Free-fall: Definition and 75 Discussions

In Newtonian physics, free fall is any motion of a body where gravity is the only force acting upon it. In the context of general relativity, where gravitation is reduced to a space-time curvature, a body in free fall has no force acting on it.
An object in the technical sense of the term "free fall" may not necessarily be falling down in the usual sense of the term. An object moving upwards might not normally be considered to be falling, but if it is subject to only the force of gravity, it is said to be in free fall. The Moon is thus in free fall around the Earth, though its orbital speed keeps it in very far orbit from the Earth's surface.
In a roughly uniform gravitational field, in the absence of any other forces, gravitation acts on each part of the body roughly equally. When there is no normal force exerted between a body (e.g. an astronaut in orbit) and its surrounding objects, it will result in the sensation of weightlessness, a condition that also occurs when the gravitational field is weak (such as when far away from any source of gravity).
The term "free fall" is often used more loosely than in the strict sense defined above. Thus, falling through an atmosphere without a deployed parachute, or lifting device, is also often referred to as free fall. The aerodynamic drag forces in such situations prevent them from producing full weightlessness, and thus a skydiver's "free fall" after reaching terminal velocity produces the sensation of the body's weight being supported on a cushion of air.

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  1. davenn

    B Do Objects in Free Fall Encounter Air Resistance?

    from ... www.physicsclassroom.com/class/1DKin/Lesson-5/The-Big-Misconception "Free-fall is the motion of objects that move under the sole influence of gravity; free-falling objects do not encounter air resistance. Huh ?? of course it encounters air resistance, doesnt it ? An object...
  2. S

    B Explaining Variations of Free-Fall Gravity on Earth?

    I stumbled upon gravitational variations that do not coincide with altitude, latitude or lunar interference. The only logical explanation I have for this would be the geology or makeup of the earth below those locations. But what is the variation of the Earths makeup below the crust? From my...
  3. L

    Sandy Hill Plunge: A Lifted Bronco's Free-Fall

    I have a scene in a story where a full-size lifted bronco with 35” tires is sliding skewed down a steep sandy hill. The driver cranks the wheel upon seeing a ledge which fails to change the trajectory of the vehicle, now sliding on sandy gravel at about 20 MPH. The rim of the front wheel...
  4. Antarres

    A Proving Free-Fall in Rindler Spacetime is Finite

    Okay, so, while discussing Rindler space with my professor, I was asked to prove that for a free-falling observer, proper time for passing through the Rindler horizon is finite. That is at least how the question is phrased. So, the professor obviously assumes that it is clear and trivial to me...
  5. J

    Feeling Forces (like feeling gravity while in free-fall)

    a = (change in magnitude of velocity) + (change in direction of velocity) → Can you feel when you change direction? Even if force and acceleration are constant? ↑ ↓ ↑ ↓ ↑ ↓ o
  6. J

    How Do You Calculate the Meeting Point of Two Stones Thrown from a Cliff?

    Homework Statement Two stones are thrown simultaneously, one straight upward from the base of a cliff and the other straight downward from the top of the cliff. The height of the cliff is 6.00m. The stones are thrown with the same speed of 9.0m/s. Find the location (above the base of the cliff)...
  7. C

    Is jumping up in a falling elevator possible?

    Imagine that you are using an elevator and - due to some cause - all of the cables supporting it cracked and the brakes were not engaged - the elevator is in a free fall. Now, would it be technically possible to jump in that elevator? Jumping requires the force of reaction of the ground, this...
  8. J

    Help with a confusing free-fall motion physics problem?

    I'm taking physics for the first time, and I've been attempting the textbook problems, but I can't seem to get it. 1. West stands on the roof of a building, leans over the edge and drops a rock. Lindsay waits 1.25 s after West releases his rock and throws her own rock straight down at 28.0...
  9. S

    Kinematics question: free-fall distance traveled during a given second

    Homework Statement An object is released from rest in the gravitational field of the Earth. Air resistance is negligible. How far does the object move during the fourth second of its motion? A. 15 m B. 25 m C. 35 m D. 45 m Homework Equations v = u + at s = ut + 1/2 at^2 v^2=u^+2as...
  10. HRubss

    Calculating Initial Velocity of a Thrown Object Using Free-fall Equations

    Homework Statement A man stands on the edge of a roof 55m above the ground, he throws a stone upward and drops a second stone at the same time. If the dropped stone hits the ground 4.4s before the stone that was thrown upward, what was the initial velocity of the first one? Homework Equations...
  11. Deadawake

    Splitting/Exploding object & Momentum

    Hi, Homework Statement A free falling object of mass "m" falling from some height, collides the floor in speed of 20 m/s (perfectly elastic collision). In his 1/2 height back up he splits into 2 pieces- ¼m which going downward and ¾m keeping upward. The ¼m reaching the floor after ½ second. 1)...
  12. David Thomson

    Free Fall Arrest: Create Mathematical Model for Safe Thrill-Seeker Experience

    It is recorded that someone has survived a free-fall from an airplane by being lucky enough to land on snow-laden fir trees. My challenge is this: Create a mathematical model which describes the forces at play in order to determine the viability of creating a safe and fun 'thrill-seeker'...
  13. M

    Question on the free-fall of 2 bodies

    1. The probleas statement, all variables and given/known data Two stones are projected from the top of a tower 100m high each with velocity of 10m/s. One is projected upward while the other is projected downward. Find the ratio of velocity's by with which they strike the ground. Homework...
  14. N

    Simpson's rule with changing volumes

    Hi there, Homework Statement I am doing a simulation of a free-fall lifeboat trajectory into the ocean and there is a part where i need to get the volume of the lifeboat to obtain its buoyancy force. Buoyancy force is proportional to the immersed volume of the body and this volume can be...
  15. L

    Deriving an Expression of an Object in Free-Fall on Mars

    Hi everyone. I'm trying to derive an expression of the velocity of an object in free fall on Mars, and I am having trouble. Here is what I've done so far: Drag on Mars can be expressed as $$F_d = \frac{C_d A \rho_M v^2}{2}, $$ where C_d denotes the drag coefficient, A the reference area, and...
  16. T

    Calculate Speed of Red Ball Falling From 26m

    Homework Statement A red ball is thrown down with an initial speed of 1.3 m/s from a height of 26 meters above the ground. The force of gravity due to the Earth results in the balls each having a constant downward acceleration of 9.81 m/s2. What is the speed of the red ball right before it...
  17. Hijaz Aslam

    Cartesian Convention of Displacement Vectors.

    Homework Statement Q.[/B] A body dropped from a height H above the ground strikes an inclined plane at a height h above the ground. As a result of the impact, the velocity of the body becomes horizontal. The body will take the maximum time to reach the ground if : (a) ##h=\frac H4## (b)...
  18. H

    How Do Different Orders of Taylor Expansion Affect Free-Fall Landing Times?

    Hello fellow PF-goers As part of my high school curriculum I am required to write an extended essay on an academic field of my choice. I plan on doing a physics paper about the free-fall landing times of objects from high altitudes in a classical Newtonian potential. My plan is to separate my...
  19. W

    Free-fall acceleration vs gravitational acceleration

    Hi, experts I got a naive question about the relation between the free-fall acceleration vs gravitational acceleration. When you consider the rotation of the Earth around the axis which runs through the north and south poles, the free-fall acceleration "g" of an object with mass m is different...
  20. J

    Free-fall atomic model: Bohr with magnetic corrections

    We still often meet Bohr classical atomic model, e.g. for nearly classical Rydberg atoms. However, this model ignores the fact that electron has very strong magnetic moment: is tiny magnet (it wasn't known when Bohr introduced his model). To understand why there are needed corrections, let us...
  21. S

    Could Humans Survive Falling into Artificially Generated Gravity Wells in Space?

    I have a completely speculative sci-fi question I've been wrestling with, and I'd love some feedback from better minds than my own: Lets assume we develop the technology to artificially generate gravity without mass (maybe using electrogravitics). We build a spaceship that can project a...
  22. I

    Second order expansion of metric in free-fall

    Hello, I have read that, in a freely-falling frame, the metric/ interval will be of the form: ds2 = -c2dt2(1 + R0i0jxixj) - 2cdtdxi(\frac{2}{3} R0jikxjxk) + (dxidxj(δij - \frac{1}{3} Rikjlxkxl) to second order. Does anyone know where I could find a derivation of this result?
  23. J

    Time measured by free-fall observer near object?

    What is the time measured by an observer who is in free-fall near an object? The Schwarzschild metric is given by: \large c^2 d\tau^2 = (1 - \frac{r_s}{r}) c^2 dt^2 - ( 1 - \frac{r_s}{r})^{-1} dr^2 - r^2(d\theta^2 + \sin^2 \theta d\phi^2) Now for an observer at a fixed position one uses...
  24. V

    Free-fall kinematics on a plane: Displacement from Window to Ground?

    Homework Statement A ball is thrown vertically upward from a window at 10m/s. It hits the ground 5.0 seconds later. What is the height of the window from the ground? Given Vi = 10 m/s Vf = 0 Δt = 5.0 m/s Δa = -9.8 m/s^2 Find Δd(w-g) = ?Homework Equations Δd = Vi(Δt) + Δa(Δt)^2 And the...
  25. PhizKid

    Beginner free-fall motion problem

    Homework Statement A ball at rest is dropped from height 'h' and travels a distance of 0.50h in the final 1.00 seconds before it hits the ground. Find the time and height of its fall. Homework Equations The Attempt at a Solution I drew a picture with the knowns: Obviously...
  26. PhizKid

    Max Height of Ball Thrown Up in 0.5 Seconds at 2m Window

    Homework Statement A ball thrown up falls back down in front of a window from where an observer sees the ball for a total of 0.5 seconds. The window is 2 m. tall. What is the maximum height of the ball? Homework Equations 1D Kinematic equations The Attempt at a Solution I got...
  27. I

    Calculate Cliff Height: Free-Fall Problem with Rock Dropped from a Cliff

    A rock dropped from a cliff falls one-third of its total distance to the ground in the last second of its fall. How high is the cliff? I'm not asking for answer, I know the Physics Forum rules, but I am totally unsure where to start. I guess I have to use the equation d = Vot + at²/2 ...
  28. C

    Does Throwing a Clock in Free-Fall Affect Time Dilation Calculations?

    If I am in free-fall and then I throw a clock above me so that it is moving away from me at a constant speed, can I just use regular time dilation to see how time is flowing in the clocks frame relative to my frame? Or do I need to worry about gravitational time dilation.
  29. J

    Does a clock in free-fall slow due to gravitational time dilation?

    I imagine I have two clocks that are synchronised and are sitting a long way from a large massive body. I hold on to clock A and let clock B fall towards the mass. Let's assume that the falling clock B gets close to the massive body but somehow misses it, swings around and travels back to me...
  30. H

    Help with Impulse, Free-Fall, and Momentum?

    Hello everyone. In Physics class, we did a lab where we dropped golf balls into a box of sand from a height of 9 ft (or 2.743 meters). The golf ball had a mass of .046 kg. We were not allowed to time the drop so these were the only givens. We also measured the depth of the "crater" the golf...
  31. M

    Calculating free-fall acceleration of other planets

    Homework Statement Here is the text of the question: "A satellite circles planet Roton every 2.8 h in an orbit having a radius of 1.2 X 10^7 m. If the radius of Roton is 5.0 X 10^6 m, what is the magnitude of the free-fall acceleration on the surface of Roton?" Homework Equations...
  32. T

    How Do You Calculate Displacement at Reduced Velocity in Free-Fall Motion?

    Homework Statement A stone is thrown straight upward with a velocity of 10V and travels upward a distance X. What is the displacement when the object is traveling at 5V? Homework Equations xf=x0+v0t+1/2at^2 vf=v0+at vf^2=v0^2+2ax The Attempt at a Solution .75X? Further...
  33. nukeman

    Free-fall Problem: Ball 2 Thrown Upward with Gravity - Homework Solution

    Homework Statement I am just trying to get confirmation I am correct. Refer to the picture below, which shows the question and diagram. I am working on 2, a) Homework Equations The Attempt at a Solution So I am to draw for ball 2 as if I was throwing the ball upwards against gravity...
  34. N

    Motion/Acceleration: free-fall gravity vs. ball being push downward

    Motion/Acceleration: free-fall gravity vs. ball being "push" downward Homework Statement If by accident a ball were given an initial downward push instead of being freely released, would the resulting g be different? Explain. (this is ignoring any air resistance) Homework Equations N/A...
  35. 5

    Free-Fall Physics Homework: Find Velocity After 2s

    Homework Statement If a boy dropped a rock off a high cliff, how fast would it be falling after 2.0 seconds had elapsed? (ignore air resistance) a) -2.0 * 10^1 m/s b) 2.0 * 10^2 m/s c) 35 m/s d) -35 m/sHomework Equations vf = vi + g∆t vf = final velocity vi = initial...
  36. G

    Ballistic free-fall - rock slide

    Homework Statement At 6:52 pm PDT Wednesday, July 10, 1996, a large block of granite with an estimated volume of 78,000 cubic yards detached from the cliff between Washburn Point and Glacier Point in Yosemite Valley. The estimated mass of the granite is 80,000 metric tons...
  37. R

    Physics Free-Fall Kinematics Flowerpot Problem

    Homework Statement A flowerpot falls off a windowsill and falls past the window below. You may ignore air resistance. It takes the pot 0.420 seconds to pass this window, which is 1.90m high. How far is the top of the window below the windowsill from which the flowerpot fell? Homework...
  38. P

    Free-Fall Physics Problem: Position, Height, Time, and Speed of a Falling Brick

    Homework Statement A load of bricks is lifted by a crane at a steady velocity of 5.0 m/s when one brick falls off 6.0 m above the ground. (a) Sketch the position of the brick y(t) versus time, from the moment it leaves the pallet until it hits the ground. (b) What is the greatest height the...
  39. S

    Solving Free-Fall Kinematics: Find Acceleration from Initial Speed

    Homework Statement A ball is thrown downward with an initial speed if 20 m/s on earth. What is the acceleration of the ball? Homework Equations vf=a (change in time)t + vi The Attempt at a Solution well you have to find the velocity of then find acceleration. and acceleration is...
  40. H

    Free-Fall Acceleration and the Gravitational Force

    When a falling meteoroid is at a distance above the Earth's surface of 3.00 times the Earth's radius, what is its acceleration due to the Earth's gravitation?
  41. B

    Solving Free-Fall Problems: Find the Time of Fall!

    Homework Statement An object falls into free falling from a hight such that during the last second of falling it traverses 1/4 of the total hight.Calculate the time of the fall knowing that there's no initial velocity. Homework Equations I know solve this problem fallowing these steps...
  42. N

    Inital acceleration to free-fall

    Homework Statement A model rocket blasts off and moves upward with an acceleration of 12 m/s^2 until it reaches a height of 26 m, at which point the engine shuts off and it continues its flight in free fall. A)what is the maximum height attained by the rocket? B) What is the speed of the...
  43. B

    Free-fall acceleration problem

    hello there, i had a question about a free fall question. Problem states that a stone is thrown vertically downward with an initial speed of 12.0 m/s from some tower. well, i was wondering if i should use 12.0m/s or if i should use gravity (9.8 m/s) will the rock have an initial speed...
  44. L

    Solving a Free-Fall Problem with Height & Time

    I am wondering if I did this right. Homework Statement An object falls a distance h from rest. If it travels 0.57h meters in the last 1.00 s, I need to know the time and the height of its fall. Homework Equations The Attempt at a Solution Here what I have: Since it...
  45. M

    The shape of an object affects it's free-fall density

    Homework Statement Identify evidence that the shape of an object affects it's free-fall acceleration Examples would help. Homework Equations N/A The Attempt at a Solution Wind resistance would be different for an egg as compared to a rubix cube
  46. D

    Free-Fall WITH Variational Gravity

    Problem background: Body free-falling towards the earth. Account for variation in gravitational force with height. Neglect air resistance. Initial conditions summarized by total energy E. I'm having trouble understanding part of my textbook's solution to this problem, so I will...
  47. R

    Accelaration and Free-fall problems

    Question A: This one is easy but my answer is coming out wrong, maybe my signs are wrong. The question is as follows: A ball is launched directly upward from ground level with an initial speed of 19 m/s. How many seconds after launch is the ball 8 m above the release point? I set...
  48. R

    Accelaration and Free-fall problems

    Question A: This one is easy but my answer is coming out wrong, maybe my signs are wrong. The question is as follows: A ball is launched directly upward from ground level with an initial speed of 19 m/s. How many seconds after launch is the ball 8 m above the release point? I set...
  49. R

    Precision during free-fall experiments.

    I'm not sure how to answer this question, so I'm turning to you guys for help. The question, specifically, is: "When timing the fall of a ball, how would you achieve greater precision, with larger or smaller heights?" Any help would you appreciated.