Who’s Moving and How do we know?

Physics - Daubert9/8/11

Who’s moving and how do we know?

•when observing experiments in class, make sure you describe and draw pictures of what’s happening in your notebook.

•Watch the orientation of your telescope as you follow the beach ball.

•Watch the orientation of my telescope as I follow the beach ball.

•In General, How do we know if something is moving or not?

The real problem here...

•Copernicus & Galileo

• “Earth moves around Sun!”

• Ptolmeny & Catholic Church

• “Sun moves around Earth!”

VS.

Who’s Moving?

•Name two observers who see you as moving right now.

•Name two observers who see you as stationary right now.

What if...

•Chris is sitting on the sidewalk, watching a car and moves his head to the right to follow it. Which direction did the car move?

•Jodi is watching the same car but can follow it without moving her head at all. How could this be? (Multiple Possibilities)

•Dave is watching the same car but has to move his head to the left to follow it. How could this be?

Red Car vs. Grey Car• A grey car moves

along the street with two passengers - one in the passenger seat, and one in the back seat.

• A red car moves in the same direction and is passing the grey car.

• A pedestrian stands along the side walk and watches the cars.

Observer What (s)he sees

The person sitting in the back seat of the grey car

The pedestrian standing on the sidewalk as the grey car passes

The driver of the red car moving in the same direction and passing the grey car

Giving Directions•Greg lost

his phone and maps!

•Give him directions to get from GWU to the smithsonian by 8:30am

Giving Directions•What was the

object of reference that you used to give directions?

•How did the map legend help you?

•What assumptions did you make?

Assumptions•Assumptions - Issues we take for granted in

an experiment. They can help us explain why the results weren’t exactly as we expected. They are factors that could affect our results, but have not been included in our calculations or reasoning. It’s important to note your assumptions so others can analyze your results.

•How did assumptions affect the time you told Greg to leave?

Defining Motion• Motion: An object is in motion with respect to

another object (reference object) if, as time progresses, its position is changing relative to the reference object.

Reference frame: A reference frame includes three essential components:

1. An object of reference, which is a real object in the physical world.

2. A clearly defined coordinate system. The coordinate system includes labels for the direction of the axis along with units.

3. A zero clock reading that serves as a reference for future clock readings.

Relative Motion Practice

• Imagine you are somewhere in the picture, where should you be and what should you be doing to observe....

• 1. The student not moving.

• 2. The street sign moving.

• 3. The police car moving right.

• 4. The police car moving left.

• Bonus! What city is this?

What did you learn today?