Lesson 11/12

Lesson 11/12

Relative Velocity

Quarantine - Day 2

• The dogs have taken over the bedroom

• They seem to think they have control of me.

• I don't know how much longer I can hold out.

Open Ended

For today's journal, add to my quarantine journal...

Relative Motion in one Dimension

• Suppose you are in a school bus that is traveling at a velocity of 8 m/s in a positive direction.

• You walk with a velocity of 1 m/s toward the front of the bus.

• If a friend is standing on the side of the road watching the bus go by, how fast would your friend say that you are going? (Assuming you friend can judge velocity of objects just by looking.)

Relative Motion in One Dimension

• Relative to the bus, you would be traveling at 1 m/s

• Relative to the street, you would be traveling at 9 m/s

• How is this possible?

Relative Motion in One Dimension

• It's all about perspective!

• The different velocities mean that motion is viewed from different frames of reference.

• Maybe a better way to ask the question would would to be more specific.

Relative Motion in One Dimension

• What is your velocity relative to the bus?

• or

• What is your velocity relative to the road?

Relative Motion in One Dimension

• Let's change up a little just for fun..

• What would be your relative velocity relative to the bus on the same bus if you were walking to the back of the bus at 1 m/s?

• -1 m/s.

• Relative to the road?

• 7 m/s

Relative Motion in One Dimension

• Surely, there is a formula for this...

• Yes, I'm glad you asked, there is!

• Relative velocity formula

• v_a/b + v_b/c = v_a/c

• v_you/bus + v_bus/road = v_you/road

• The means the relative velocity of you to the road is the vector sum of you relative to the bus and the bus relative to the road.

Relative Motion in One Dimension

• It's easier than it looks

• In our Original problem, you were moving forward at 1 m/s relative to the bus and the bus was moving at 8 m/s relative to the road.

• 1 + 8 = 9 m/s

Relative Motion in Two Dimensions

• Adding relative velocities also applies to motion in two dimensions.

• Vector diagrams can be very useful in solving relative velocities.

• Remember to always draw the velocities tip to tail.

• The resultant will show the relative velocity

Relative Motion in Two Dimensions

• Let's look at an example of an airplane.

• Pilots can't just aim their planes along a compass direction and expect to reach their final destination.

• They have to take into consideration the plane's speed relative to the air as well as the velocity of the wind.

• These velocities must be combined to obtain the velocity of the airplane relative to the ground.

• The resultant vector tells the pilot how fast and in what direction the plan must travel relative to the ground to reach its destination.

Combining velocities

We'll use the digital whiteboard for this.

Open Ended

Exit Ticket - If you are walking the wrong way on a conveyor belt that is moving at 5 m/s E and you are moving 5 m/s W, what is your relative velocity relative to the ground?