• Support PF! Buy your school textbooks, materials and every day products Here!

Khan academy video mistake?

  • Thread starter m_scott
  • Start date
  • #1
10
0

Homework Statement


http://www.khanacademy.org/video/2-dimensional-projectile-motion--part-3?playlist=Physics [Broken]

Skip video to 4:40. He says that the average velocity in the horizonatal direction is the same as the initial velocity (7.07m/s) because it doesnt change. but wouldnt the velocity change from 7.07m/s to 0? which means the average velocity should be 3.5m/s, right?

which means the change in displacement should equal 10m, not 20m. right?
 
Last edited by a moderator:

Answers and Replies

  • #2
rl.bhat
Homework Helper
4,433
7
You are right.
 
  • #3
1,860
0
Hmm, I missed the part you are talking about, after fast forwarding, but as the other users pointed out his time was off by a factor of 2 based on when he first started.

You are wrong that the horizontal direction (did you mean to say vertical?) changes to zero though. Of course, once the ball hits the ground it may splat right in place and go to zero velocity, but we are concerned with the infinitesimal second before it hits.
 
  • #4
SammyS
Staff Emeritus
Science Advisor
Homework Helper
Gold Member
11,307
999

Homework Statement


http://www.khanacademy.org/video/2-dimensional-projectile-motion--part-3?playlist=Physics [Broken]

Skip video to 4:40. He says that the average velocity in the horizonatal direction is the same as the initial velocity (7.07m/s) because it doesnt change. but wouldnt the velocity change from 7.07m/s to 0? which means the average velocity should be 3.5m/s, right?

which means the change in displacement should equal 10m, not 20m. right?
The horizontal component of velocity is constant up until the moment of impact! So the person in the video is absolutely correct.

BTW: You can only be sure that vavg=(vinitial+vfinal)/2 if the acceleration is constant!
 
Last edited by a moderator:
  • #5
10
0
so when you find displacement in the horizontal direction, you shouldn't ever use 0 as your final velocity? You can only use 0 as your final velocity for the y componenet?


And SammyS, he used vavg=(v0+v)/2 to get his answer. is the acceleration not constant?

(btw, i realize his time is wrong. i am looking at this problem based off his time)
 
  • #6
1,860
0
For vertical velocity it depends on where you look at the trajectory. If you want to know how high the ball goes the final vertical velocity to get to that height will be 0. If you want to know the vertical velocity before it hits the ground the vertical velocity will be the same as when it was launched (at least given the ball is launched from the ground).
 

Related Threads on Khan academy video mistake?

  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
7
Views
632
  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
0
Views
1K
  • Last Post
Replies
6
Views
691
  • Last Post
Replies
1
Views
978
  • Last Post
Replies
15
Views
2K
  • Last Post
Replies
1
Views
7K
Replies
5
Views
4K
Replies
4
Views
2K
Top