Average speed (or maybe average velocity) of an athlete

[mcastillo356]

Homework Statement

An athlete takes 10 seconds to travel some distance, then reduces his gait traveling 50 meters backwards, towards the starting point, during 30 seconds. Find out the average speed of this athlete during the total course

Relevant Equations

Kinematics, Linear Movement

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

 

[willem2]

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

It would be useful for the discussion if you wrote what you think the answer should be and why,
I don't get 10 m/s either. Average speed would be total distance traveled divided by the total time traveled.
What do you get for total distance traveled and total time?

 

[Lnewqban]

If I understand the problem correctly, the athlete consumed 40 seconds to go from starting point to another one, which was [Certain distance - 50 meters] away.
Doesn't the average speed depend on that "certain distance", which is not provided?

 

[mcastillo356]

Hi willem2, Lnewqban, I will post tomorrow. We must consider accelerations, I guess:
x0=0----------------------------------------------- xf -----------------------------------------x1
t0=0------------------------------------------------ tf------------------------------------------t1
v0=0 ------------------------------------------------vf ---------------------------------------- v1
Information provided:
t1-t0=10 s
xf-x1=-50 m
tf-t1=30 s
We must deal with very few information
Regards

 

[jbriggs444]

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

Suppose that we accept the given answer of 10 m/s and accept the problem as posed.

If the average speed over a 40 second interval is 10 meters per second then the total distance covered must have been 400 meters.

If the man went backward for 50 meters then the distance traveled forward must have been 350 meters.

350 meters in 10 seconds is well beyond world class pace. Here in the U.S. that would be 78 miles per hour. Overseas it would be 126 kilometers per hour.

 

[mcastillo356]

Well, I don't know what to say. Sorry, jbriggs444, I didn't trust you, and I feel I've lost all the day. Well, let's suppose he goes by car, and he is going to receive a punishment, a fine, soon or later... Sorry, I'm annoyed... It's not the first time... exercises that oppose to reality.
Thanks and regards, jbriggs444, Lnewqban, willem2

 

[Lnewqban]

Sorry, Castillo; I believe that you deserve better problems with more information.
The presentations of these last problems that you have had to endure have been atrocious, in my humble opinion.

 

[mcastillo356]

Hi, Lnewqban
A Physics Graduate told me to write her, but I didn't. The delay in reponding you is due to explain why. I've slept for a lot of hours, and now I have things clearer: if I want to become a physicist, I will have to deal with few information, inconsistent results, or misinformation. The teacher knows it. She is good,
This month I will enroll in Physics Degree; only Calculus and Physics 1... Well, I must enroll in at least 36 credits; maybe two subjects won't be enough.
Regards

 

[mcastillo356]

Hi PF
There was a missing measure in the statement:
"A runner runs 100 m in 10 s and then turns back, covering 50 m towards the point of departure in 30 s. Wich are the average speed and the average velocity of all the travel"
$$Average\; speed=\frac{Distance\; Traveled}{Time\; of\; Travel}=(150\; m)/(40\; s)=3,75\;m/s$$
$$Average\; velocity=\dfrac{\Delta\;position}{time}=\dfrac{displacement}{time}$$
$$Average\; velocity=\dfrac{50\;m-0\;m}{40\;s}=+1,25\;m/s$$
10 m/s=(100 m)/(10 s)
Salutes!

 

[mcastillo356]

Hi PF
I did not say that the statement I wrote in the first post was the one I had at that moment. I've had to go to Santurtzi, where the Uned is at Bizkaia, and look for the exercise in the bibliography. I've been a lucky guy. I've found the same exercise, but the statement included the first 100 m the runner runs. Actually, we are talking about a good athlete, close to Usain Bolt's 9,58 s. But it is still nonsense: can anybody run 100 m in 10 s and then turn back inmediately, to run 50 m in 30 s? Well, I might be too maniac, and there must be exercises like this.