1. Relative Velocity
Physics on the Road
Lesson 6

2. LI…
 Add velocities as vectors, tip to tail.
 find relative velocity
 Identify collision courses

3. What’s the next move?

4. Air Traffic Control
At busy airports air traffic control need to track
multiple aircraft. Each aircraft is identified and
it’s height, distance, direction and speed
monitored by radar.
Distance 20 km
Speed = 100 m/s
Distance ? km
Speed = 100 m/s
What assumptions
and variables are
important?

5. How do you catch a ball?
1. Keep your eye on the
ball
2. Run at the right speed
Must keep the angle of your line of
sight the same. This becomes trickier
as the ball gets closer.
The Physics
You keep the vector of the relative motion between you
and the ball in the same direction.

6. Collision course
 Predicting a collision is looking ahead.
Computing the next move. A calculation of the
future.
1000 km/h 1000 km/h
Time to collision = distance
approach velocity
Approach velocity is the relative velocity how do we work
it out?
10 km

7. Collision course 2
1000 km/h 1000 km/h
10 km
AvB = vA -vB
AvB = vA +(-vB)
A
B
1000 km/h
vA
1000 km/h
vB
AvB
2000 km/h
We are
looking at
A from B

8. Collision course 3
1000 km/h 1000 km/h
Time to collision = distance
approach velocity
= 10/2000 = 0.005 h
= 18 s
10 km

9. Relative velocity
 To find relative velocity you subtract the
vectors.
 That is the same as adding an opposite vector.
225 m/s
212 m/s
225 m/s
212 m/s
2v1 = v2 – v1
2v1 = v2 + - v1
vr
If the relative velocity is in a line between the objects…
1
2

10. Are we heading for Xing
disaster?
60o