Describing Motion:

Kinematics in One Dimension

Sign Convention & Direction

Distance & Displacement

Distance (x) equates

Displacement equates to

Displacement

Displacement is written:

A person moves on the number line shown below. The person begins at B, walks to C, and then turns around and walks to A. For this entire range of motion DETERMINE: a)the person’s final positionb)the displacementc)the distance.

Example

0 5 m 10 m 15 m

A B C

Speed & Velocity

Speed: How far

Velocity: How

Average Speed & Velocity

A commuter drives 15.0km on the highway at a speed of 25.0m/s, parks at work and walks 150m at a speed of 1.50m/s from his car to his office.

Example

b) Determine the average speed of the entire commute

(a) Determine the total time of the commute.

EXAMPLEUsain Bolt holds the record for the 100m sprint completing it in only 9.58s!

a) Determine his average speed in m/s. (1.6km = 1mi)

b) Mr Sample (I hold no record) ran the Philly half-marathon (13.1mi) in 1hr55min36sec. Determine my avg speed in mph.

Did he run faster than this at some point?

Example: A woman starts at the entrance to a mall and walks inside for 185m north for 10minutes. She then walks 59m south in 3minutes to another store. She then leaves the store and moves south 155m in 8minutes to reach her car outside.Determine her average velocity during the trip.

Instantaneous Velocity

The instantaneous speed or velocity is how fast an object is moving at a single point in time.

Does the gauge on your dashboard give you speed or velocity?

Does this gauge give you an average or instantaneous value?

Acceleration

Acceleration is

Units?

Constant Acceleration

• Constant acceleration implies what about velocity?

• Constant acceleration or deceleration implies what about distance?

• Acceleration of zero implies what about the velocity?

Negative acceleration vs Positive acceleration:

Both can equate to slowing down. When sign of acceleration matches sign of velocity, object speeds up in direction of that sign. When signs oppose, object slows down in direction of ‘v’.

Graphical Analysis of Motion

Position-time graph:

Describes the position of object during a given time period.

Describe the position of the objects (A-D) over time. Use origin in your statement.

A

B

t

x

C

D

0

EAST

WEST

What does the intersection of A and B refer to?

Slope of x vs t graph

Recall that slope = Δy / Δx

Slope Interpretation

Describe the velocity of the objects (A-D) over time.

A

B

t

x

C

D

0

EAST

WEST

ExampleWhat was the total distance traveled?

What was the displacement for the entire trip?

What was the average speed for the first 6 sec?

What was the average velocity from B to E?

What was the velocity of the object btw 2-4 sec?

In which section(s) was there a constant + velocity?

In which section(s) was there a constant negative velocity? In which section had the maximum speed?

•

Instantaneous velocityUnlike vavg, instantaneous velocity occurs at a single point. How would we find vinst at t = 3.0s?

At what time(s) does the cart have a zero velocity?

Describe the velocity btw 0.0 - 0.80s?

Describe the velocity btw 2.6 - 3.2s?

Graphical Analysis of Motion (2)

velocity-time graph:

Describes the velocity of object during a given time period.

VELOCITY

time0

Describe the velocity of each object during its motion, including initial velocity

A

B

C

D

*Crossing t-axis = ?Intersection of lines on vt graph means ?

v-t graphs – part 2

Example

Determine the displacement of the object from 20s-38s.

vt graphs – part 3

Example

a) Determine the time(s) where object had - acceleration

b) Determine the time(s) where object had positive non-zero velocityc) Determine the time(s) where object was at rest

d) Determine the time(s) where object had constant velocity.

Example

Determine acceleration of object between 4-9s

At what time(s) did object turn around?

During what time period(s) did object slow down?

When did object reach maximum speed?When did object possess maximum + acceleration?

•

Instantaneous acceleration

Instantaneous acceleration occurs at a single point. To find ainst at t = 0.6s…

Constant Acceleration EqnsWe can write avg velocity 2 different ways:

Combining the two eqns yields:

Constant / Uniform Acceleration Equations

EXAMPLE

While driving along at 20m/s, you notice the light up ahead turns red (110m away). Assuming you have a reaction time of 0.5s,

a) How far from the light are you when you begin to apply the brakes?

b) What constant acceleration will bring you to rest at the light?

EXAMPLE 2

A car starts from rest at a stop sign. It accelerates uniformly at 4.0m/s2 for 6.0s, coasts for 2.0s, and then slows down at 3.0m/s2 for the next stop sign.

a) How far apart are the stop signs?

b) Determine the maximum velocity during the trip.

a vs t graph

a

t0

We will only deal with constant accelerations.

Reference Frames & Relative MotionAny measurement of position, distance, or speed must be made

In order to determine the speed of object moving in a particular RF, we use subscripts

VSG =20m/s

VSG =20m/s

VBG=6m/s

VCG= -30m/s

How fast is bike moving relative to bus?

How fast is bus moving relative to car?

Example: A railroad flatcar is traveling to the right at a speed of 13.0 m/s relative to an observer standing on the ground.

Someone is riding a motor scooter on the flatcar. Determine the velocity of the motor scooter relative to the flatcar if its velocity relative to the observer on the ground is 3.0 m/s to the left?

Falling & Acceleration

FREEFALL

Anatomy of a upwardly thrown object

A ball is thrown upward with an initial speed of 15.0m/s Assume negligible air resistance. 

EXAMPLE 1

a) Find the maximum height attained by the ball.

b) How much time does it take to reach the apex? 

c) Determine the velocity 2.2s into flight.

Example: Calculate the impact speed with the water after falling from 40ft above the water, from rest.

Assuming he stops in a depth of 12inches of water, determine his acceleration, assuming it to be constant .

EXAMPLE 2

As a part of a movie stunt a stunt man hangs from the bottom of an elevator that is rising at a steady rate of 1.10m/s.  The man lets go of the elevator and freefalls for 1.50s before being caught by the end of a rope that is attached to the bottom of the elevator. 

(a) Calculate the velocity of the man at the instant he is caught by the rope.  

(b) How long is the rope?

EXAMPLE 3

An honors physics student stands at the edge of a cliff that is 36m high. He throws a water balloon straight up at 12.5m/s so that it just misses the edge of the cliff on the way down.

Determine velocity of balloon as it strikes ground below (many ways to solve)

Three students are standing side-by-side next to the railing on a fifth floor balcony. Simultaneously, the three students release their pennies.  One student drops a penny to the ground below. The second student tosses penny straight downwards at 15 m/s while third student tosses penny straight upwards at 15 m/s. Assume freefall.

d) Which penny or pennies strike(s) the ground with the greatest acceleration?

a) Which penny or pennies strike(s) ground first?

b) Which penny or pennies strike(s) ground last?

c) Which penny or pennies strike(s) the ground with the greatest final velocity?

Collaborate with person next to you to answer following questions: