What is Average speed: Definition and 202 Discussions

In everyday use and in kinematics, the speed (commonly referred to as v) of an object is the magnitude of the rate of change of its position with time or the magnitude of the change of its position per unit of time; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance travelled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as the duration of the time interval approaches zero.
Speed has the dimensions of distance divided by time. The SI unit of speed is the metre per second (m/s), but the most common unit of speed in everyday usage is the kilometre per hour (km/h) or, in the US and the UK, miles per hour (mph). For air and marine travel the knot is commonly used.
The fastest possible speed at which energy or information can travel, according to special relativity, is the speed of light in a vacuum c = 299792458 metres per second (approximately 1079000000 km/h or 671000000 mph). Matter cannot quite reach the speed of light, as this would require an infinite amount of energy. In relativity physics, the concept of rapidity replaces the classical idea of speed.

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  1. J

    Calculating the Average speed given two speeds

    $$ v_1= 115km/h $$ $$ v_2= 60km/h $$ solution: $$ \frac {v_1 + v_2}{2} $$
  2. vryC0nfused

    Average Speed Question for a Train that Accelerates and then Decelerates

    TL;DR Summary: A train on a level stretch of tracks, starting from a dead stop, steadily speeds up to 28 m/s in 2.0 minutes, and then slows to 18 m/s with a constant deceleration of 0.011 m/s^2. What is the average speed of the train for this whole thing (speeding up and slowing down)? I know...
  3. J

    I Car low average speed vs aero drag relation

    I drive car only at country roads allways at speeds 100-120km/h, no city and no idle time-heating engine etc. Why computer allways show very low average speed , 40-48km/h?I allways have feeling that this speed is too low because I allways drive way faster then this. Indeed all my friends have...
  4. T

    Average speed question about a hovercraft

    I assumed since both pass the points they must have the same speed at both instances, so the average speed would be 15 and the speed when passing the object should be 30, since the equation of average speed.
  5. F

    I Why we know average speed of single photon equal speed of EM wave?

    Why we know that average speed of a single photon(in point particle view) equal the speed of EM wave?If average speed of a single photon smaller than c then there exist massive photons?
  6. Huzaifa

    B Average Speed vs. Velocity: Explained

    Though average speed over a finite interval of time is greater or equal to the magnitude of the average velocity, Instantaneous speed at an instant is equal to the magnitude of the instantaneous velocity at that instant. Why so?
  7. mcastillo356

    Average speed (or maybe average velocity) of an athlete

    Hi PF It's a multiple choice exercise, and the answer is 10 m/s. I've been wondering why, but no way. Regards
  8. aspodkfpo

    Average speed with constant acceleration over a period of time

    Say that we have an instance where something falls down from a certain height with constant acceleration g. We know that the average speed with regards to the time period is less than (u+v)/2 since we spend less time at the higher speeds. How do we actually calculate the average speed over a...
  9. K

    Average speed and average velocity

    So thing that confusses me, is what is the difference regarding the calculation of speed and velocity? In think I should use the formula I wrote above to find the average velocity, but I don't understand what the formula for average speed will be then. By the way, when using the formula above...
  10. C

    Average speed needed to flip a vehicle

    Car A hits Car B's passenger side at intersection (T Bone). Car B flips over sideways in the direction of the driver's side. The intersection is basically level. Car A is an Audi 5, roughly 3,900 lbs. Car B is a Lexus NX, roughly 4,000 lbs. Car B had recently stopped and was traveling...
  11. S

    Average speed of a probability density wave and wave packets

    Show that ##v_{av}=\frac{\hbar k_2 + \hbar k_1}{2m}## is equal to ##v_{av}=\frac{\omega_2 - \omega_1}{k_2-k_1}##. Which of the identities listed above (if any) would make the sign change between ##k_2## and ##k_1##? One can attain a "wave packet" by superposing two or more sinusoidal waves...
  12. S

    Average speed of molecules in a Fermi gas

    My first most obvious attempt was to use the relation ##<\epsilon> = \frac{3}{5}\epsilon_F## and the formula for kinetic energy, but this doesn't give the right answer and I'm frankly not sure why that's the case. My other idea was to use the Fermi statistic ##f(\epsilon)## which in this case...
  13. Alexanddros81

    Calculate average speed/acceleration for the given function

    Homework Statement Serway Physics Chapter 2 20. An object moves along the x-axis according to the equation ##x=3.00t^2 - 2.00t+3.00##, where x is in meters and t is in seconds. Determine (a) the average speed between t=2.00s and t=3.00s, (b) the instantaneous speed at t = 2.00s and at t=3.00s...
  14. YMMMA

    How Do You Calculate Average Speed with Variable Acceleration?

    Homework Statement In the file below. Homework Equations Speed= total distance over total time. Acceleration = change of speed over time The Attempt at a Solution [/B] I tought it is just E because of the first rule above. But then I doubt my answer since it moved with constant acceleration...
  15. opus

    Trying to get an average speed of 60 MPH

    Homework Statement I haven't taken a physics course yet, this is for independent study, so I don't have the strongest understanding quite yet. I was wondering if I could get some insight to this problem: Susie has planned a trip to a city 60 miles away. She wishes to have an average speed of...
  16. I

    Why total time taken s₁/v₁ + s₂/v₂ + s₃/v₃ in average speed?

    Homework Statement As we know the Average speed = Total distance / Total time but why in the case below Distance traveled = s₁ + s₂ + s₃ and total time taken = s₁/v₁ + s₂/v₂ + s₃/v₃ How is it possible? so, altogether Vav = s₁ + s₂ + s₃ / ( s₁/v₁ + s₂/v₂ + s₃/v₃)Homework EquationsThe Attempt...
  17. P

    What is the average speed for the entire duration? Trick?

    Homework Statement A motorcycle goes up a hill at a constant speed of 10 miles per hour and comes down at the speed of 25 miles per hour what is the average speed for the entire duration? Homework Equations Vavg =(vf-vi)÷2 The Attempt at a Solution It seems very straightforward that this...
  18. karush

    MHB Eb5 Calculate the average speed in km/h

    $\text{A person jogs eight complete laps around a $400 \, m$ track in a total time of $14.5 \, min$}. \\$ $\textit{a. Calculate the average speed in $km/h$}\\$ \begin{align*}\displaystyle \frac{(8)400 \, m}{14.5 \, min} \left(\frac{km}{1000m} \right) \left(\frac{60 \, min}{h}...
  19. R

    Estimate average speed of electrons in gas?

    Homework Statement Suppose that a heated gas comprised of electrons glows bright orange when it is in use. Estimate the average speed of the electrons in the gas. Model this gas as an ideal gas. Homework Equations None. I have no clue. This is a "review" question on a chapter titled "Current...
  20. N

    Why Isn't Average Speed Simply the Arithmetic Mean of Two Speeds?

    Homework Statement If you walk to school at 4 mph and jog home at 6mph, what was your average speed? Homework Equations S = D/T The Attempt at a Solution Is the average speed not just (4 + 6)/2 = 5mph ? If not, why not?
  21. NoahCygnus

    Kinematics: Find average speed from displacement-time graph.

    Homework Statement The problem is as follows; A boy begins to walk eastward along a street infront of his house and the graph of his displacement from the house is shown in the figure. His average speed for the whole time interval is equal to...
  22. LLT71

    Average Speed and Average Power confusion

    I found it somehow tricky to understand average speed and average power in mathematical fashion. I suppose general idea of taking average of some quantity is: sum of all values/number of values. how that idea fit's into idea of average speed=distance traveled/time traveled similarly, average...
  23. Gjmdp

    When is the average speed equal to the average of the speeds?

    Homework Statement In which conditions the average speed equals the Average of the Speeds? Homework Equations Average speed= (Xf-Xo)/(tf-to) Average of the speeds= (vf+vo)/2 The Attempt at a Solution I tried to set up a formula for vf and vo, but it would be false to determine that vf=xf/tf...
  24. Const@ntine

    Second opinion needed at UAV Exercise

    Homework Statement We're driving with a Uav of 80 km/h The duration Δt is 2 h 45 min On a rainy day, our Uav' is 65 km/h If the distance is the same, how much more is the duration of the drive? So, technically we want the result of Dt = Δt' - Δt Homework Equations Uav = Δx / Δt The Attempt at...
  25. M

    MHB What is the Average Speed of a Train Traveling 18 km in 20 Minutes?

    The total distance traveled by trains is 18 km & the time spent is 20 minutes. Find the average speed of the train in kilometers per hour. I' m stuck right in the beginning in converting 20 minutes to hours by dividing from 60 which I get this recurring decimal 0.33333
  26. Iron_Man_123

    Average speed, average velocity for a road trip

    Homework Statement [/B] I'm very stuck at answering parts (c) and (d) Homework Equations speed = Distance/Time The Attempt at a Solution
  27. Z

    What is the Average Speed of the Cyclist on a Multi-Stage Journey?

    Homework Statement So I have this homework exercise which I just can't seem to solve. It sound like this: "Half his time the cyclist was driving at the speed of 20km/h, half of the LEFTOVER way he drove at the speed of 16km/h. When his bike broke he finished his journey on foot at the speed of...
  28. X

    Car coasts to a stop: find acceleration, distance, average speed

    Its my first time ever being in Physics and I need help with this probably really simple problem. It first says, a car is initially traveling at 25m/s and then coasts to a stop in 12 seconds. What is its acceleration? **I took 25/12 and got 2.08 m/s. Hopefully that part is right? Then it asks...
  29. dyavol33

    Calculating Average Speed: Physics Homework Problem

    Homework Statement So here's a problem I need help with for my physics class that's due tomorrow night. On a road trip, a driver achieved an average speed of 76 km/h for the first 86.0 km and an average speed of 38 km/h for the remaining 54.0 km. What was her average speed (in km/h) for the...
  30. J

    Average Speed Calculation: Why 2x266.67 Doesn't Work

    1. Homework Statement Ok so for part ii) b) I calculated the average speed as follow, Average speed= 3792.6/ (2x266.67), where 3792.6m is the total distance, and 266.67s is the time when v=0 , i.e. the time from O to the turning point But it should be 3792.6/ 400, where 400s is the time...
  31. onemic

    Average Speed With Ambiguous Distance/Time

    Homework Statement A locomotive travels on a straight track at a constant speed of 40 mi/h, then reverses direction and returns to its starting point, traveling at a constant speed of 60 mi/h. What is the average speed for the round-trip? Homework Equations avgS = distance/time t = d/40 The...
  32. N

    Calculating Average Speed: Solving the v=8sin2t+3cos2t Equation

    Hello, I was wondering if someone out there could help me. The question: The speed, v metres per second, of a particle is given as v=8sin2t+3cos2t where t is in seconds. i) Find the total distance traveled in the time interval 0≤t≤1 ii) Find the average speed over this time interval.
  33. E

    What are the average speed formulas needed to solve this problem?

    Homework Statement A car is moving in a road and its average speed is 40 km/h. Then it moves back in that road by 60 km/h speed. What is the average speed of this car? ( The answer is not 50 km/h ) Homework Equations Average speed= d/tThe Attempt at a Solution I can't solve this. I get 50...
  34. CMATT

    Average velocity? Average speed?

    Homework Statement A cow runs in a straight line, moving 108 m east in 16.4 seconds. It then turns and runs 93 m north in 7.64 seconds. a) What was the cow's average speed over the whole trip? b) What was the cow's average velocity over the whole trip? Homework Equations average speed =...
  35. a lone fishy

    Finding time from average speed

    Homework Statement A golfer rides in a golf cart at an average speed of 3.1 m/s for 28s. She then gets out of the cart and starts walking at an average speed of 1.3 m/s. For how long must she walk if her average speed for the entire trip, riding and walking, is 1.8 m/s ? Homework Equations...
  36. I

    Average Speed in Kinetic-Molecular model

    Hello, In the Kinetic-Molecular model for gas molecules in a container it is assumed that the square of the x-y-z velocity components is the same, how is it possible that we assume this?
  37. RaulTheUCSCSlug

    Average Speed for Maxwell's Distribution of Molecular Speed

    Using the Maxwell-Boltzmann equation above, there is an example in my book (Giancoli 4th edition p. 481) where they use this to find the average velocity. I understand that it would just be the sum of all the speeds of the molecules divided by the number of molecules. But then I'm having...
  38. wein7145

    Average speed over time interval

    Edit Background: The teacher doesn't teach we just do activities and have whole chapters worth of 20 mini assignments due every few days. Yes I did read the book found nothing. 1. Homework Statement The average velocity of a car over a certain time interval is 35 mi/h. If the velocity of the...
  39. Lana Elcic

    Calculating Average Speed on a Road Trip | Physics Help for Beginners

    New to physics, so I'm looking for guidance and help. Here is the following question: A=19 B=0 On a road trip, a driver achieved an average speed of (48.0+A) km/h for the first 54.0 km and an average speed of (43.0-B) km/h for the remaining 86.0 km. What was her average speed (in km/h) for...
  40. H

    How Do You Calculate Average Speed in Different Driving Conditions?

    Homework Statement You drive on Interstate 10 from San Antonio to Houston, half the time at 58 km/h and the other half at 103 km/h. On the way back you travel half the distance at 58 km/h and the other half at 103 km/h. What is your average speed (a) from San Antonio to Houston, (b) from...
  41. AHashemi

    Calculate the average speed and average velocity

    1. a man drives from a to b with car and speed of 20 km/h and returns from b to a immediately with speed of 30 km/h. what's the average speed and average velocity? 2. V=at+Vi etc 3. I think avg. speed will be 25 km/h and avg. velocity is -10 km/h. Is that right? thanks :)
  42. Tonia

    What is the Average Speed and Acceleration of a Car on a Straight Road?

    Homework Statement On a straight road you start your car and increase the speed at a steady rate to 45 miles per hour during a 5 second interval. Calculate: a)the average speed during the 5 seconds b) the acceleration in miles per second at t = 3.2 sec. after you start your car c) the speed of...
  43. E

    What is the Average Speed of an Object Traveling 12 Meters in 9 Seconds?

    1. I'm a middle school science teacher without a science or math background. My goal here is to avoid teaching an explanation that kids will have to unlearn later in their academic career. I'm designing a lesson to teach students how to answer this question: 2. Limitation: students are NOT...
  44. Sarlacc

    Average Speed Over Given Distance - Driving me Nuts

    "A car accelerates from rest over 400 metres in 19 seconds. The driver then brakes and the car stops in 5.1 seconds with constant deceleration." Part one of the question is "calculate the acceleration for the first 400m" which went just fine: x=ut+(1/2)at^2 400=180.5a a=2.22 m/s Now the...
  45. B

    Neon Atom Speed Ratio at Different Temperatures

    Homework Statement What is the ratio of the average speed of an atom of neon to another atom of neon at twice the temperature? (A) 1:1 (B) 1:1.4 (C) 1:2 (D) 1:4 Homework Equations KE = 3/2RT The Attempt at a Solution I used the above equation and substituted 2 for T for twice the temperature...
  46. R

    Average speed? -- Homework problem

    Ok let me just type the question and get to the real question I have. If you traveled one mile at a speed of 100 miles per hour and another mile at a speed of 1 mile per hour, your average speed would not be (100 mph+ 1 mph)/2 or 50.5 mph. What would be your average speed? (Hint:what is the...
  47. P

    Can Distance Traveled be Calculated Mathematically in Average Speed?

    It is my understanding that while instantaneous speed is the magnitude of instantaneous velocity, average speed is , in general, not the magnitude of average velocity, since average speed is total distance traveled divided by change in time. Why is the mathematical definition of average speed as...
  48. N

    Average speed question: forwards then backwards

    Homework Statement An object starts at the origin, position A; then travels 20 m to position B; then returns to position A. From A to B the average speed is 10 m/s; on its way back the average speed is 6 m/s. What is the average speed for the entire trip? Homework Equations average speed =...
  49. L

    Kinematic Equations Used to Find A.) Average Speed & B.)Acceleration

    Homework Statement A truck covers 40.0m in 7.15 s while uniformly slowing down to a final velocity of 3.50 m/s. a.) Find the truck's original speed b.) Find it's acceleration Homework Equations Δx = Vav(Δt) = (V+Vo/2)Δt a=(V-Vo)/(Δt) The Attempt at a Solution Part A.)...
  50. S

    Is Average Speed a Scalar quantity?

    Is Average Speed a Scalar quantity?