5-May-15 Physics 1 (Garcia) SJSU Chapter 3 Linear Motion (Motion in a straight line, such as falling straight downward)

Apr 18, 2023 Physics 1 (Garcia) SJSU

Chapter 3 Linear Motion

(Motion in a straight line, such as falling straight downward)


Demo: Cracking Nuts

What’s the best way to crack open a nut?


Tools you’ll learn today

• Speed and Velocity

• Acceleration

• Relationships among distance, velocity, and acceleration.

• Falling motion.


Speed

Define speed of an object as

For example, 30 miles per hour means object travels distance of 30 miles in an elapsed time of one hour. Write as,

(SPEED) =(Distance traveled)

(Time elapsed)

30 miles per hour = 30 miles

hour


Speed and Distance

From definition of speed,

Example: If speed is 30 miles per hour and time elapsed is 2 hours then distance traveled is (30)X(2) = 60 miles.

(Distance traveled) = (Speed) X (Time elapsed)

Match units: This time elapsed is also 120 minutes but it’s not correct to compute distance traveled as

(30)X(120) = 3600 miles {WRONG}.


Check Yourself

What is the average speed of a cheetah that sprints 100 meters in 4 seconds? How about if it sprints 50 meters in 2 seconds?

A car has an average speed of 100 kilometers per hour. How far does it travel in 60 minutes?


Average versus Instantaneous

Sometimes consider average speed, other times we speak of instantaneous speed.

For example, say it takes you one hour to drive the 30 miles from home to campus.

Average speed is 30 miles per hour.

Instantaneous speed (given by your speedometer) varies due to traffic, stop lights, morons driving in front of you, etc.


Demo: Ball Races

Which ball wins the race, A or B?A

BFinishLine

Which ball has the larger average speed?Which has the larger instantaneous speed at each point.


Check Yourself

A car has an average speed of 60 miles per hour. Is it possible for the instantaneous speed to always be less than 60 miles per hour?


VelocityVelocity is speed and

direction of object’s motion.

Examples:30 miles per hour, Northward25 meters per second,

Downward300 miles per hour,

Coming towards you

25 m/s, upward

25 m/s, downward

Same speedsDifferent velocities


Check Yourself

The speedometer of a car moving east reads 100 km/h. It passes another car moving west at 100 km/h. Do they have same speed? Velocity?

During a certain period of time, the speedometer of a car reads a constant 60 km/h. Does this indicate a constant speed? Constant velocity?


Changes in VelocityVelocity changes if speed or direction of motion change.

25 m/s, downward

10 m/s, downward

25 meters per second, 45 degrees upward

25 meters per second, 45 degrees downward

Velocity changesin both these cases.


Acceleration

Define acceleration as,

(ACCELERATION) = (Change in Velocity)

(Time interval)

Note: An object accelerates anytime its velocity changes. Examples include:

Object speeds up.Object slows down (speed decreases).Object speed constant but direction changes (curved path)

Best example of acceleration is objects in free fall


Demo: String of Falling Balls

Falling objects accelerate (speed increases).

Listen for the sound as balls hit the ground.

Time between “clicks” gets shorter & shorter (falling faster & faster).

String does not pull; no tension while falling.


Velocity in Free Fall (Down)

How fast do objects go when they fall?

Acceleration of gravity is 10 meters per second per second.

With each second of fall, speed increases by 10 meters/second

Zero meters per sec.

10 meters per sec.

20 meters per sec.

30 meters per sec.

40 meters per sec.

Release

1 second

2 seconds

3 seconds

4 seconds


Velocity in Free Fall (Up & Down)

Moving upward, with each second the speed decreases by 10 meters/second.

Going back down the motion exactly reverses itself.

See Fig. 3.8 (pg. 47)

Zero meters per sec.

10 meters per sec.

20 meters per sec.

30 meters per sec.

40 meters per sec.


Position in Free Fall

How far do objects go when they fall?

More complicated because speed is increasing.

There’s a pattern & Galileo figured it out.

But it wasn’t easy.

5 meters

20 meters

45 meters

Release

1 second

2 seconds

3 seconds

4 seconds80 meters

Higher than King library

Higher than this ceiling


Galileo’s Inclines

Galileo realized that rolling down an incline and falling were very similar.

It was much easier for him to study the slower motion of an incline.


Demo: Galileo’s Clicking Ramps

Roll balls down notched, inclined ramps and listen for the clicks.

Start

1 4 9 16 25 36 49 64

8 16 24 32 40 48 56 64

4=2x2 9=3x3 16=4x4 25=5x5 36=6x6 49=7x7 64=8x8


Position in Free Fall (cont.)

5 meters

20 meters

45 meters

Release

1 second

2 seconds

3 seconds

4 seconds80 meters

(Distance Fallen) =½ (Acceleration)(Time)(Time)

Galileo realized that:

Gravity acceleration is 10 meters per second per second so at a time of 3 seconds,

(Distance Fallen) =½ (10)(3)(3) = 45 meters

20=5x 2x2

45=5x 3x3

80 =5x 4x4

5=5x 1x1


Lab: Acceleration of Gravity

Record position of falling object using spark timer and paper tape.


Demo: Dropping the Ball

(Distance Fallen) = ½ (Acceleration)(Time)(Time)

How long does it take a ball to fall 3 meters?Using the formula,

Can check that it takes 0.77 seconds since

(3) = ½ (10)(0.77)(0.77)

Beauty of science: Predict, then verify by dropping balls!

Note: Do similar measurement in first experiment in the Phys 1L lab.


Demo: Catch a Buck

Put thumb and index fingers near Washington’s head. Can you react fast enough to catch the money?


Demo: Reaction Time

Release

Catch

Distance (inches) Time (sec.)1 0.072 0.103 0.124 0.145 0.166 0.177 0.198 0.2010 0.2312 0.2514 0.2716 0.2918 0.30

Documents

5-May-15 Physics 1 (Garcia) SJSU Chapter 3 Linear Motion (Motion in a straight line, such as falling straight downward)