Do now

• Conceptual Challenge, p. 107.

Section 3-4 Relative motion

• Objectives1. Describe situations in terms of frame of

reference.2. Solve problems involving relative velocity.

Checking Your References

Relative Motion

and

2-D Kinematics

How would Homer know that he is hurtling

through interstellar space if his speed were

constant?Without a window, he wouldn’t!

All of the Laws of Motion apply within his FRAME of REFERENCE

Do you feel like you are motionless right now?

ALL Motion is RELATIVE!

The only way to define motion is by changing position… The question is changing position relative to WHAT?!?

You are moving at about 1000 miles per hour relative to thecenter of the Earth!

The Earth is hurtling around the Sun at over66,000 miles per hour!

MORE MOTION!!!

Example #1• A train is moving east at 25 meters per second.

A man on the train gets up and walks toward the front at 2 meters per second.

• How fast is he going?– Depends on what we want to relate his speed to!!!

• +2 m/s (relative to a fixed point on the train)• +27 m/s (relative to a fixed point on the Earth)

vtrain = +25 m/svperson = +2 m/s

Example #2• A passenger on a 747 that is traveling

east at 230 meters per second walks toward the lavatory at the rear of the airplane at 1.5 meters per second.

• How fast is the passenger moving?– Again, depends on how you look at it!

• -1.5 m/s (relative to a fixed point in the 747)• +228.5 m/s (relative to a fixed point on the Earth)

Non-Parallel Vectors• What happens to the aircraft’s forward

speed when the wind changes direction?

vthrust

No wind – planemoves with velocity

that comes from engines

vwind

Wind in same directionas plane – adds to overall

velocity!

Wind is still giving the planeextra speed, but is also

pushing it SOUTH.

Wind is now NOT having anyeffect on forward movement,

but pushes plane SOUTH.

Wind is now slowing the plane somewhat AND pushing it

SOUTH.

Wind is now working againstthe aircraft thrust, slowing itdown, but causing no drift.

Perpendicular Kinematics

• Critical variable in multi dimensional problems is TIME.

• We must consider each dimension SEPARATELY, using TIME as the only crossover VARIABLE.

Example• A swimmer moving at 0.5

meters per second swims across a 200 meter wide river.

200 m

vs = 0.5 m/sHow long will it take the swimmer

to get across?

t

dv t = 400 s

t =0

The time to cross is unaffected! Theswimmer still arrives on the other bank

in 400 seconds. What IS different?

• Now, assume that as the swimmer moves ACROSS the river, a current pushes him DOWNSTREAM at 0.1 meter per second.

vc= 0.1 m/s

The arrival POINT will be shifted DOWNSTREAM!

• A motorboat traveling 4 m/s, East encounters a current traveling 3.0 m/s, North.

1. What is the resultant velocity of the motorboat?

2. If the width of the river is 80 meters wide, then how much time does it take the boat to travel shore to shore?

3. What distance downstream does the boat reach the opposite shore?

practice• A motorboat traveling 4 m/s, East encounters a

current traveling 7.0 m/s, North.1. What is the resultant velocity of the motorboat? 2. If the width of the river is 80 meters wide, then how

much time does it take the boat to travel shore to shore?

3. What distance downstream does the boat reach the opposite shore?

4 m/s7 m/s d = ?

80 m

Class work

• Page 108 – sample problem 3F• Page 109 – practice 3F