speed and velocity problem

[joonkimdds]

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This was one of the problem that I practiced for DAT exam and got wrong.

A horse canters away from its trainer in a straight line, moving 130 m away in 18.0 s. It then turns abruptly and gallops halfway back in 4.9 s.
(a) Calculate its average speed. m/s

i used average speed = (initial speed + final speed)/2.

initial = 130m/18sec = 7.222m/s
final = 65m/4.9sec = 13.265m/s
so 20.487/2 = 10.244m/s

but it says 10.244m/s is wrong. what did i do wrong??

 

[Richnator]

This was one of the problem that I practiced for DAT exam and got wrong.

A horse canters away from its trainer in a straight line, moving 130 m away in 18.0 s. It then turns abruptly and gallops halfway back in 4.9 s.
(a) Calculate its average speed. m/s

i used average speed = (initial speed + final speed)/2.

initial = 130m/18sec = 7.222m/s
final = 65m/4.9sec = 13.265m/s
so 20.487/2 = 10.244m/s

but it says 10.244m/s is wrong. what did i do wrong??

Sup Joon,
First of all, SPEED (EDIT) is distance over time. You can not just take the average of two Speeds (EDIT). So, this is how you solve the problem.

1. Disect the problem by noting that it takes 18 secs to move 130 meters away.

2. Notice that is turns back and only gallops half the distance 130/2= 65 meters in 4.9 seconds.

3. Lastly, you add these (Total distance)/(Total Time)= Speed(EDIT).
(130m+65 m)/ (18 + 4.9 sec)=
ANSWER: 8.5152 meter/second
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Edit.
If you need more help. Please post, I will be glad to be of service.

Good luck on your DAT,
Rich

 

[psiyung]

That statement is also incorrect. Velocity is displacement / time, not distance

1) Speed = distance / time
2) Velocity = displacement / time

displacement is basically the net distance that has been traveled. Since he moved 130 m and then back another 65m, the distance is 195m, and the displacement is 65m. So it depends if they are asking about velocity, or speed

 

[Richnator]

That statement is also incorrect. Velocity is displacement / time, not distance

1) Speed = distance / time
2) Velocity = displacement / time

displacement is basically the net distance that has been traveled. Since he moved 130 m and then back another 65m, the distance is 195m, and the displacement is 65m. So it depends if they are asking about velocity, or speed

AAh. Good thing you pointed that out. I stand corrected on that one. The answer should be right though. I just used speed and velocity interchangeably when it was actually two different things. Thanks for bringing it up.

Thanks bro,
Rich