Chapter 2: Motion in 1D



Chapter 2: Motion in 1D

Concepts:

• Displacement

• Velocity

Background:

- Aristotle (384-322 BC) – Greek natural philosopher

- Galileo (1564-1642) – Italian scientist

Kinematics = The description of motion (not concerned with the cause)

Any motion involves three concepts:

1) Displacement

2) Velocity

3) Acceleration

Displacement:

- Before knowing displacement, we need to answer “where is it?”

- To determine position, you need a frame of reference

- A frame of refence contains:

• A starting point

• A direction (coordinate axis)

- Displacement is a change in position (final minus initial)

- Change is written symbolically as the Greek letter delta ((), so

Displacement = change in position = (x = xfinal-xinitial

- Note that displacement is not the same as the distance traveled!

- Note that displacement can be positive or negative!

Velocity:

- Velocity is how fast an object is moving in a particular direction

- First, lets distingish average velocity from instantenous velocity

- Average velocity is displacement (distance) over how long (time) it took to occur

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- You can obtain an average velocity over any desired amount of time

- If your time interval is really small, we call it instantenous velocity

- In physics velocity is not the same as speed!

- Speed does not have direction, velocity can be positive or negative

Test your understanding:

1) What is the frame of reference for the markings on this part of a football field?

2) On the same part of the football field, how would you describe the position of a player midway throught the end zone (10 yds long)?

3) What is the displacement for the cars in the Indy 500 race (500 miles) where the starting point is the same as the finish line?

4) If it takes you 2 hours to go to San Diego (120 miles away), what was your average speed (miles per hour)?

5) During your way to San Diego, a cop stops you for going 90 mph, could this had been true given your previous answer?

6) During the Chatsworth tragedy of two trains colliding head-on, the news anchor said “both trains were going at 50 miles/hour”. From the reference frame onboard the train, how fast was the other train going?

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