1. Kinematics in Two Dimensions
Chapter 3
Kinematics in Two Dimensions

2. 3.4 Relative Velocity

3. Example 11 Crossing a River
3.4 Relative Velocity
Example Crossing a River
The engine of a boat drives it across a river that is 1800m wide.
The velocity of the boat relative to the water is 4.0m/s directed
perpendicular to the current. The velocity of the water relative
to the shore is 2.0m/s.
(a) What is the velocity of the
boat relative to the shore?
(b) How long does it take for
the boat to cross the river?

6. Plane in the Wind VPG = VPA + VAG
To determine the resulting velocity - add the velocity of the airplane to the velocity of the air.
Resulting Velocity = Velocity of object wrt the air + velocity of air wrt the ground
VPG = VPA + VAG

7. What if the wind is blowing towards the north?
The resulting velocity of the airplane is relative to the ground.

8. What if the wind is blowing towards the West?
The resulting velocity of the airplane is relative to the ground.

9. The resulting velocity of the airplane is relative to the ground.
What if the plane is flying towards the Southeast and wind is to the West?
The resulting velocity of the airplane is relative to the ground.

10. Suppose he needs to fly south and the wind was blowing to the west?
The resulting velocity of the airplane relative to the ground must be south!
But what angle to fly at?

11. Heading Øo
100 km/h R
Ø = -75.5o
25 km/h
The pilot would have to fly his plane in a direction of -75.5o to compensate for the wind.

12. R = 96.8 km/h Resulting Velocity?
This velocity is relative to the ground and is sometimes called the ground speed.