Physics 101: Lecture 7, Pg 1

Constant Acceleration and Constant Acceleration and Relative VelocityRelative Velocity

Physics 101: Lecture 07

Physics 101: Lecture 7, Pg 2

Relative VelocityRelative Velocity Sometimes your velocity is known relative to a

reference frame that is moving relative to the earth. Example 1: A person moving relative to a train, which

is moving relative to the ground. Example 2: a plane moving relative to air, which is

then moving relative to the ground.

These velocities are related by vector addition:

bcabac vvv

» vac is the velocity of the object relative to

the ground

» vab is the velocity of the object relative to a moving reference frame

» vbc is the velocity of the moving reference frame relative to the ground

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Physics 101: Lecture 7, Pg 3

Relative VelocityRelative VelocityYou are on a train traveling 40 mph North. If

you walk 5 mph toward the front of the train, what is your speed relative to the ground?

A) 45 mph B) 40 mph C) 35 mph

25

40 5

45

Physics 101: Lecture 7, Pg 4

Relative VelocityRelative Velocity

You are on a train traveling 40 mph North. If you walk 5 mph toward the rear of the train, what is your speed relative to the ground?

A) 45 mph B) 40 mph C) 35 mph

27

405

35

Physics 101: Lecture 7, Pg 5

Relative VelocityRelative VelocityYou are on a train traveling 40 mph North. If

you walk 5 mph sideways across the car, what is your speed relative to the ground?

A) < 40 mphB) 40 mph C) >40 mph

40 mph N + 5 mph W =

30

405

Physics 101: Lecture 7, Pg 6

Example: airplaneExample: airplane

vp= 50 m/s

vT

vw= 20 m/s

•An airplane needs to fly due south to reach it’s destination, but there is a wind blowing from the east. Will this increase, decrease or have no effect on the travel time?

Physics 101: Lecture 7, Pg 7

Physics 101: Lecture 7, Pg 8

Three swimmers can swim equally fast relative to the water. They have a race to see who can swim across a river in the least time. Relative to the water, Beth (B) swims perpendicular to the flow, Ann (A) swims upstream, and Carly (C) swims downstream. Which swimmer wins the race?

A) Ann

B) Beth

C) Carly

ExampleExample

A B C

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x

y

Physics 101: Lecture 7, Pg 10

Tractor Demo 1Tractor Demo 1

Which direction should I point the tractor to get it across the table fastest?

A) 30 degrees left

B) Straight across

C) 30 degrees right

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1 2 3

Physics 101: Lecture 7, Pg 11

Tractor Demo (moving table)Tractor Demo (moving table)

Which direction should I point the tractor to get it across the table fastest?

A) 30 degrees left

B) Straight across

C) 30 degrees right

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1 2 3

Physics 101: Lecture 7, Pg 12

If the width of the river is 50m and Beth can swim at 3m/s, how long does it take her to get across? If the river is flowing at 2m/s, How far downstream is she swept?

ExampleExample

A B C

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x

y

50m2m/s

Physics 101: Lecture 7, Pg 13

What angle should Ann take to get directly to the other side if she can swim 3 mph relative to the water, and the river is flowing at 2 mph?How long does it take her to get across?

Swimmer Example Swimmer Example

A B C

48

x

y

2m/s

Physics 101: Lecture 7, Pg 14

Summary of ConceptsSummary of Concepts

X and Y directions are Independent!Position, velocity and acceleration are vectors

F = m a applies in both x and y direction

Relative Motion (Add vector components)

wgswsg vvv