What is Accelerating: Definition and 669 Discussions

In mechanics, acceleration is the rate of change of the velocity of an object with respect to time.
Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the orientation of the net force acting on that object. The magnitude of an object's acceleration, as described by Newton's Second Law, is the combined effect of two causes:

the net balance of all external forces acting onto that object — magnitude is directly proportional to this net resulting force;
that object's mass, depending on the materials out of which it is made — magnitude is inversely proportional to the object's mass.The SI unit for acceleration is metre per second squared (m⋅s−2,




{\displaystyle {\tfrac {\operatorname {m} }{\operatorname {s} ^{2}}}}
For example, when a vehicle starts from a standstill (zero velocity, in an inertial frame of reference) and travels in a straight line at increasing speeds, it is accelerating in the direction of travel. If the vehicle turns, an acceleration occurs toward the new direction and changes its motion vector. The acceleration of the vehicle in its current direction of motion is called a linear (or tangential during circular motions) acceleration, the reaction to which the passengers on board experience as a force pushing them back into their seats. When changing direction, the effecting acceleration is called radial (or orthogonal during circular motions) acceleration, the reaction to which the passengers experience as a centrifugal force. If the speed of the vehicle decreases, this is an acceleration in the opposite direction and mathematically a negative, sometimes called deceleration, and passengers experience the reaction to deceleration as an inertial force pushing them forward. Such negative accelerations are often achieved by retrorocket burning in spacecraft. Both acceleration and deceleration are treated the same, they are both changes in velocity. Each of these accelerations (tangential, radial, deceleration) is felt by passengers until their relative (differential) velocity are neutralized in reference to the vehicle.

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  1. E

    Atwood at an incline accelerating down

    Both myself and my TA gave up, but we found acceleration of the system 9g - 4gsin(30) = 13a a=5.27m/s^2
  2. xkcda

    Torque about an accelerating point

    The total force acting on the pulley is zero so: F=mg+T1+T2 (1)Analyzing the torque and angular acceleration about the actual axis of rotation, the axle of the pulley, gives: τnet=T1R−T2R=Iα (2)If we analyze about point P, the right edge of the pulley where T1 is applied, we get...
  3. J

    B Speed of light and this Accelerating Rocket scenario

    Hello, I have a question linked to gravity and speed of light : According to the Special Theory of Relativity, the speed of light cannot be exceeded because it would need an infinitive energy to accelerate the relativistic mass of a moving objet (a space craft for example) : indeed if the...
  4. chris25

    Rotation and torque about an accelerating point

    A solution was provided: We take torques about point B. Note that τ = MgL/2 = Iα so α = (3g)/2L. Everything from here is straightforward. I don't understand why in this step, you can take torque about B without accounting for a fictitious force due to the acceleration of the Rod.Thanks for...
  5. String theory guy

    Acceleration of a suitcase in an accelerating train after getting pushed

    I was doing one of MIT's 8.01.1x course and came across this question. In case 2, how would we be able to theoretically calculate the horizontal acceleration in this non-inertial frame? The course said that Newton's Law do not hold in accelerating frames. However, could we find the...
  6. theanswer2physicsisu

    Force and Kinematics -- Accelerating a 10kg box vertically

    I realize that there is a downward force of gravity weighing the object toward earth’s surface, equaling F = mg (downward). The upward force would have to be something at least as much as the downward force in order to lift the object up ”such that it is accelerated from rest to a velocity of 5...
  7. S

    Hydrostatic pressure at a point inside a water tank that is accelerating

    I draw this diagram: The formula for hydrostatic pressure is: ##P=\rho g h## so I just plug everything $$P=1000 \times 9.8 \times 0.1=980 Pa$$ Will the acceleration of the cart affect the hydrostatic pressure? Another thing that came to my mind was there would be extra force coming from...
  8. E

    I Calculation of gravitational redshift in an accelerating elevator

    When we derive the formula of the redshift by the equivalence principle we imagine a light ray which goes from the bottom to the top of the elevator and which would take a duration t = h/c to make the journey, with h = height of the elevator. I don't understand why t = h/c, because while the ray...
  9. N

    Change of ellipse while accelerating the rocket

    When the rocket accelerates in space does its trajectory which is an ellipse change in size and not the focal points because the Earth is still in one of two and also the current height doesn't increase, right?
  10. P

    I Energy of Light in Accelerating Universe: Comparing Cases

    Assume two observers very far from each other, so far, that the accelerating expansion of the universe matters. (edit: But not outside of each others event horizons.) They will send light beams each other, and measure the energy of it. Also tie them together with a very long rope to fix their...
  11. S

    I Receding movement of a stopped object in an accelerating universe?

    Imagine we attach an imaginary cosmological scale rope to an object that is very far away from us. Before attaching the string, the object would be receding from us due to spacetime expansion. After attaching it, tension would form in the string and we would eventually stop the object. After...
  12. MattGeo

    I Exploring Collisions w/ Accelerating Objects

    I am not sure why it never occurred to me before despite actually having taken an advanced classical mechanics course in college, but how do we treat a collision where the objects involved are actually accelerating? In the case where colliding objects move at constant velocity it is standard...
  13. AspiringPhysicist12

    Block on an accelerating wedge (part c only)

    My solution for part b) (i.e. minimum acceleration) in case it's needed: https://ibb.co/D8CCQMM I'm confused about part c. 1) Since the block will travel up the incline if a > a_max, acos(theta) is up the incline. But based on my attached FBD diagram, acos(theta) should be down the incline...
  14. D

    B Accelerating to a stop in the Milky Way Galaxy

    If the sun is traveling around the center of the Milky Way at a speed of 536,865 mph and we are tagging along with it, couldn’t a spacecraft travel to another solar system by stopping? Would it be possible currently to slow down from that speed to 0 mph? And lastly, why does physics make my...
  15. rudransh verma

    Accelerating wedge with a block at rest on it

    I have drawn a fbd and the logic I think is that at rest the block moves down the wedge but when a force P is applied vertical force becomes zero and the horizontal force ##F_N\sin \beta## = P?
  16. Gumby The Green

    I Can time run backwards in an accelerating frame?

    I'm trying to make sure I understand how the traveling twin tracks the time of his stationary earthbound sibling and the time of another stationary observer who's farther away. From what I've understood until now, it's pretty straightforward with the earthbound twin: In the traveler's frame, the...
  17. Y

    Friction on a box in the bed of a pickup truck accelerating up an incline

    hey i would like to get some help with this question i try to answer it but it was not correct i like to know what i did wrong and how i can fix it:
  18. A

    I Why don't we "fly up" in an accelerating elevator?

    Earlier I was doing a sample problem for class that involved the work done by an elevator, and the problem gave us the normal force experienced by the person in the elevator (to calculate the acceleration of the elevator-person system). I had done this wrong because I had wrongly assumed...
  19. rudransh verma

    Does Acceleration Affect Tension in a Sphere and Car System?

    If I draw the fbd then some force will accelerate the car in horizontal direction which I think does not effect the string in vertical direction. So same tension regardless of acceleration. But we know it will increase. So what will be the correct physics behind it?
  20. M

    I Equivalence of Universe Accelerating Expansion?

    Without suggesting that there actually is a massive spherical shell enclosing the universe, I am trying to explore whether the observed expansion of the universe would be the same if there was a spherical shell with a diameter greater than the universe, with some arbitrary mass producing a...
  21. J

    B Accelerating expansion of the Universe

    Will the universe keep expanding faster and faster for all eternity or is there a limit to how fast the expansion will get?
  22. Yossi33

    Accelerating pulleys (3 pulleys and 3 masses)

    hi,i have this question : m1=3kg m2=6kg m3=20kg there is no friction between m3 and the floor. what is the acceleration of each block? my attempt is : the pulley that moves is moving downward at the acceleration of m3. so the system of m1,m2 is moving downward at the acceleration of m3, then i...
  23. J

    I Energy input\losses for a cyclist accelerating from a standing start

    I have been analysing the power that a cyclist puts out during a standing start and the numbers don't seem to match the effort that is going in! Here's a real example: Cyclist (93kg incl bike) moves 15m from standing start in 4secs (average velocity 3.75m/s). Assuming a constant acceleration, of...
  24. Franklie001

    Newton's second law -- Crate sliding in the back of an accelerating truck

    Hi everyone is able to help solve this question for my assignment in university? I've draw a free body diagram for each component of the question but now i am stuck. [Mentor Note -- Poster has been reminded to show their work when starting a new schoolwork thread]
  25. cianfa72

    I Principle of relativity for proper accelerating frame of reference

    Hi, I've a doubt about the application of the principle of relativity as follows. Assume as principle of relativity the following statement: It is impossible by any experiment performed inside a "closed" laboratory to say whether we are moving at constant velocity or staying at rest. Consider...
  26. E

    When an object is accelerating, does speed, velocity, or force change?

    qn3: why is statement 2 the only answer? isn't speed one of the factors in velocity too? then shouldn't statement 1 be true too?
  27. Mike holden

    B Accelerating Universe Expansion: Age & Hubble Constant

    In my article - The accelerating expansion of the Universe at: [Unacceptable reference delted by the Mentors] I discuss how the age of the Universe depends on the rate of acceleration, the common factor being the value of the Hubble Constant at the present time. I go on to show how the expansion...
  28. Ranku

    I Accelerating Universe and spherical distribution of matter

    If we take a spherical distribution of matter wherein gravitational force and cosmological-constant force are equal upon an object on its surface, then does the time that it took for that volume to grow to the size wherein the two forces are equal match the time it took for the universe to start...
  29. E

    B Transform to accelerating coordinates

    It's a silly example, but hopefully it will help me to understand the maths. Two guys ##A## and ##B## are initially at the same spacetime event ##O##, and then ##B## receives an impulse along the ##x##-direction giving him an initial coordinate velocity ##\dot{x}_B = v_0## as measured by ##A##...
  30. S

    Will EMF be induced in a coil that is accelerating in a uniform magnetic field?

    My answer will be no for both (a) and (b) because there is no change in magnetic flux experienced by the circular coil. Am I correct? Thanks
  31. W

    Motion equation problem -- Car accelerating with a constant power engine

    So I tried to solve this in two methods, but I keep getting different results, and I don't know why.
  32. Sj4600

    Electric Potential Energy Question: Electron and Proton accelerating between charged plates

    Ve=0m/s Vp= 0m/s Qe/Qp= 1.60E-19 Me=9.11E-31 Mp-1.67E-27 Ive pretty much gathered all of the equations I think I need to solve the problem. I just am stuck. The last step I realize that the forces would be equal to each other so I have mp x ap = me x ae but then when I try to solve for the...
  33. S

    Does the current speed matter when accelerating?

    Neglecting the air resistance, is it more difficult for a biker with a proper gearing in his bike, to increase his speed by 5 kph if his current speed is 20 kph rather then if his speed is 10 kph? It seems that the answer is yes according to the energy formula but I've asked several bikers and...
  34. AN630078

    Mechanics: Elevator Accelerating Downward

    The acceleration of the elevator is downwards and therefore negative. The overall acceleration of the man is downward with the the direction of the elevator meaning that ma is negative. The external forces acting on the man are the force of gravity acting downwards (-W=-mg) and the supporting...
  35. badluckmath

    A simple pendulum is suspended from the ceiling of an accelerating car

    Here is an image of the problem: The problem consist in finding the moviment equation for the pendulum using Lagrangian and Hamiltonian equations. I managed to get the equations , which are shown insed the blue box: Using the hamilton equations, i finally got that the equilibrium angle...
  36. badluckmath

    A simple pendulum is suspended from the ceiling of an accelerating car

    Here is an image of the problem: The problem consist in finding the moviment equation for the pendulum using Lagrangian and Hamiltonian equations. I managed to get the equations , which are shown insed the blue box: Using the hamilton equations, i finally got that the equilibrium angle...
  37. DarkMattrHole

    I Virtual Tidal Effect in Accelerating Ship - Does it Exist?

    Does a sort of virtual tidal effect exist inside an accelerating body (space ship) similar to the way the tidal effect is present while rest on a massive body? Or could an accurate enough 'thought experiment' measuring device figure out if it's in an accelerating ship or resting on a planet, by...
  38. brotherbobby

    "Distance" between events in uniformly moving and accelerating frames

    (1) Uniformly moving frames I begin with a drawing of the situation. The events are labelled as ##\color{red}{E_1}## and ##\color{red}{E_2}##. We note the time of those events : ##t_1 = t'_1 = 30s## and ##t_2 = t_2' = 30+60 = 90s##. I attempt the problem in two different ways. (a) By...
  39. Leo Liu

    Normal force acting on a block on an accelerating wedge

    We have a wedge whose surface is ##\theta## from the horizontal surface. After a block is placed on its frictionless slant surface, the wedge starts to accelerate due to a force F. What is the normal force acting upon the block? I have been trying to solve it but I got no clue. Could someone...
  40. Leo Liu

    A block on an accelerating wedge

    For question b, the official solution sets up a non-inertial coordinate on the block and writes out the following two equations: $$\begin{cases} \begin{align*} f\cos(\theta)+N\sin(\theta)-mg=0 \qquad \hat\jmath \\ N\cos(\theta)+f\sin(\theta)=ma \qquad\quad\;\;\, \hat\imath \end{align*}...
  41. C

    Frequency of EM waves produced by linearly accelerating charges

    I was wondering about EM waves produced by linearly accelerating charges, as opposed to oscillating charges. With oscillating charges, the frequency of the wave depends on the frequency of the oscillation of the charge. But what determines the frequency of the wave produced by a linearly...
  42. D

    B Why the stay-at-home twin is not considered to be accelerating?

    I am not a physicist. I need your kind help in removing my following doubt about twin paradox. What I have been able to understand about twin paradox is this- 1. Special relativity deals with non-accelerating (inertial) motion. 2. The traveling twin (A) moves at a high speed in relation to the...
  43. LCSphysicist

    Easy problem about a pulley accelerating

    That is the classic problem involving pulley and masses, I can find everything that is requested, but there is a big problem, i don't know how to justify that the tensions in both side are equal to half of the force.
  44. E

    Does the Relation ##T_2 = T_1 e^{\mu \theta}## Hold for an Accelerating Capstan?

    For a massless rope wrapped around a pulley with friction, it can be shown via integrating over string elements that the tensions on either side of the pulley are related by ##T_2 = T_1 e^{\mu \theta}##, if ##\mu## is the coefficient of friction and ##\theta## is the angle subtended by the rope...
  45. J

    B Accelerate Light-Clock Simulation: Proper Time & Coordinate Time

    So this what I found out: One cycle takes less coordinate time, and less proper time according to the lower mirror when the clock accelerates. Proper time per one cycle does not seem to get larger with more cycles, although coordinate time per one cycle does get larger. So what do you think...
  46. I

    Travel time for an accelerating car

    Hi guys, i hope i find the right Sub-Forum. Can you please tell me if i calculated it right. A car speed's up with 5 m/s² to a speed limit of 50Km/h. After it arrived the speedlimit it drives for 30s with the speed of 50km/h. After that it break with a negative acceleration 3.0m/s² to stop...