Homework Help: HUGE help needed in RELATIVE VELOCITY!

1. Sep 27, 2006

wtf_albino

Hello all!

My physics teacher has been absent for about two days so i haven't been able to ask him any questions.

Basically I dont understand the concept of relative velocity. I couldnt even start one problem!

1) A Jet airliner moving intially at 300 mph due east enters a region where the wind is blowing at 100 mph in a direction 30.0 degrees north of esast. What is the new velocity of the aircraft relative to the ground?

For this question.. im thinking i can draw some vectors to represent the information and just manipulate them to find the resultant vector(?) and would that be the final velocity? You dont have to solve it for me.. would just appreciate a hint to a step in the right direction.

2) A rowboat crosses a river with a velocity of 3.30 mi/h at an angle of 62.5 degrees north of west relative to the water. The river is 0.505 mi wide and carries an eastward current of 1.25 mi/h. How far upstream is the boat when it reaches the opposite shore?

My book is very vague in explaining these types of problems! For this problem i drew it out successfully.. but i have no idea how to attack it! if someone could point me in the right direction im positive i could solve it on my own thanks!

thanks in advance!! i really appreciate your guys' help it's been a lifesaver.

2. Sep 27, 2006

Alt+F4

well i forgot how to do this but
http://online.physics.uiuc.edu/courses/phys101/fall05/physics_101_lectures.html [Broken]

I remb learning in the first few lectures and we did the same exact problems so just look at the completed PPT

Last edited by a moderator: May 2, 2017
3. Sep 27, 2006

wtf_albino

thanks a lot alt! ill definitely look through the lesson.

4. Sep 27, 2006

wtf_albino

i found the lesson about relative velocity.. the questions were rather simplistic though. are you sure it had similar questions?

5. Sep 27, 2006

Astronuc

Staff Emeritus
One simply adds the velocity vectors.

If one is given the velocity of a vehicle (windspeed of aircraft in air, speed with respect to water of a boat) with respect to the fluid, and the fluid itself has some velocity, then add the two vectors to get the resulting velocity of the vehicle with respect to the fixed reference (usually land). The coordinated system orientations must be consistent however.

See -

http://hyperphysics.phy-astr.gsu.edu/hbase/relmot.html

http://hyperphysics.phy-astr.gsu.edu/hbase/airpw.html

http://hyperphysics.phy-astr.gsu.edu/hbase/boatc.html

Last edited: Sep 27, 2006
6. Sep 28, 2006

civil_dude

Va + Vb = Va/b