Relative Velocity

Ch 3. WOD are underlined.

Relative Velocity:Equations written to relate

motion to a frame of reference.

Motion that depends on velocity of an observer.

Motion relative to a frame of reference.

Relative Velocity:Equations written to relate

motion to a frame of reference.

Motion that depends on velocity of an observer.

Motion relative to a frame of reference.

How fast are you moving as you sit in your seat????

“Calculate this out for several F of R., Another students, center of earth, center of sun, etc. How fast are you moving???”

Section 3.3

What is this guy’s velocity?He travels 4 meters in 2 seconds going east.

Use Compass on watch to find east.

Section 3.3

What is this guy’s velocity?

1 Compared to the ground?2 What velocity does the earth spin at?3 What velocity do we revolve around the sun?

Section 3.3

What is this guys velocity? So what is his frame of reference?

3.4 Relative Velocity

What about now?

What about now?

Frame of referenceA coordinate system from which all

measurements are made.

Definition – a coordinate system within which objects, positions, and velocities are measured.

MUST PICK AN ORIGIN before you find speed and velocity.

Frame of reference

If two frames of reference are moving with constant velocity relative to each other, the objects appear to move with their own velocity and the frame’s velocity added together (remember that velocities are vectors).

Adding velocites.Remember – Velocities are vectors.

Question – a “wing walker” is walking across the wings of an airplane.

The airplanes velocity is 20 m/s North.

His velocity is 4 m/s East. What is his apparentVelocity to someone on the ground?

Adding velocites.Remember – Velocities are vectors.

Question – a “wing walker” is walking across the wings of an airplane.

The airplanes velocity is 20 m/s North.

His velocity is 4 m/s East. What is his apparentVelocity to someone on the ground?

Use Pythagorean Th. with legs = 20 and 4. Use cos -1 (4/hyp) and subtract from 90N to get 79Deg

An airplane drops a care package. Describe the path taken by the care package as seen from the airplane’s frame of reference.

What about from the ground’s frame of reference?

If the airplane is speeding up with a constant acceleration, what would the package seem to do?

Lets revisit a problem from last week

Falling Care Package

The airplane is moving horizontally with a constant velocity of +115 m/s at an altitude of 1050m. Describe the motion of the package from the ground’s frameOf reference.

Lets revisit a problem from last week

Falling Care Package

Describe the motion of the package from the airplanes frameOf reference.

Lets revisit a problem from last week

Show the WWII bomb bay footage on youtube.. Why can’t we show today’s planes?

3.4 Relative Velocity

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 relativeto 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?

(c) How far downstream does theboat come to ground?

3.4 Relative Velocity

WSBWBS vvv

What do these subscripts means? BS = Boat relative to ShoreBW = Boat relative to WaterWS = Water relative to Shore

3.4 Relative Velocity

sm5.4

sm0.2sm0.4 2222

WSBWBS vvv

WSBWBS vvv

θ = Cos-1 ( X / H)Cos-1 (2 / 4.5) = 63o

3.4 Relative Velocity

s 450sm4.0

m 1800t

θ = Cos-1 ( X / H)Cos-1 (2 / 4.5) = 63o

1800 Tan (90-63) =1800 Tan (27) = 900m.Also, 450s x 2 m/s = 900m

3.4 Relative Velocity

s 450t

Do in your Powerpoint notes.A canoe has a velocity of .6 m/s relative to still water. A

river has a current of .5 m/s.

Two docks are 1500 m apart on this river. How long will it take this canoe to make the round trip? (2 docks are on the same side of the river. Go down stream and then back upstream.)

How long would it have taken a person walking on land at .6 m/s?

Last comments on relative motion

Back to my first question about I’m walking. What is the correct frame of reference?

Ground, center of earth, center of sun?

How do I test to find out?

Last comments on relative motion

There is no experiment you can perform to determine (there is no way to tell) what frame of reference you are in.

So, there is no “correct” frame of reference. All are equally valid.

However, we usually pick the one that makes the math easiest to work.