Unit 1: Kinematics

Physics 20: Unit 1 Kinematics

Or Describing Motion

Introduction

• a scalar is a quantity that has no direction,

such as distance (ex. 56 m) and time

(magnitude only)

• a vector has magnitude and direction, i.e.

velocity 56 m east

Units and directions

• Units and direction of vectors are always

important!

• Vectors in opposite directions must be

given opposite signs in equations

• vectors that are parallel or point in the

opposite direction are collinear

• Vectors are indicated by arrows on

diagrams

Example:

A person walks 100 m east and then 210 m east. The

vector diagram would look like this

A person walks 200 m east and then 400 m west

Angle between these vectors is 180o

100 m east 210 m east

200 m east 400 m west

Angle between these vectors is 0o

• position is a distance measured from some

origin or starting point, plus the direction,

symbol is

d

• distance and displacement are not the

same

• displacement is the change in position, the

change in any quantity is always the final

value minus initial value

d

Start Point

(reference

point)

Finish point

• the magnitude of vectors are indicated by

using the absolute value symbol ex.

|v|

• speed is the rate at which an object moves

• velocity is a vector

• speed and velocity do not always have the

same magnitude

time

cetandisv

t

dv

Example A golfer hits a ball as shown. Assume the golfer is

at the reference point. Determine the distance

traveled and the displacement of the ball

if ddd

= 10 m - 0 m = 10 m right.

Distance = 15 m

Displacement = final position – initial position

• If the ball was sunk in 6.2 s, determine

the average speed and average velocity

of the ball.

tan 152.4 /

6.2

dis ce mv m s

time s

101.6 / right

6.2

d mv m s

t s

Uniform Motion

• uniform motion is motion in a straight line at a constant speed, the velocity is constant: no change in magnitude or direction

• the position-time graph for uniform motion is always a line

• the slope of the line is velocity

position

position

Object is moving

away from the

reference position

Object is moving

towards the

reference position

• velocity-time graph is horizontal

• slope = acceleration

• area = displacement

Horizontal line: slope = 0

Area= Lw

=velocity x time

Average Velocity

• The velocity

for the interval

0 – 6.0 s is

not uniform

Dis

tan

ce

(m

)

Average Velocity

• The average

velocity for the

interval (t)

0 – 6.0 s is

101.7 /

6.0

ave

ave

dv

t

mv m s

s

Dis

tan

ce

(m

)

Example Chainsaw, the psychotic

physics feline, walks

12.3 m in 39.7 s, stops to

scratch the couch for

28.0 s, then runs at 11.4

m/s for 12.9 s. Calculate his

average speed.

1

2

tan

12.3

11.4 / (12.9 ) 147.06

dis ceaverage speed

time

d m

d vt m s s m

147.06 12.31.98 /

39.7 28.0 12.9

speed

m mm s

s s s

Example 1. Describe the

motion for the 8.0 s.

2. Determine the average velocity of the object between 2.0 s and 7.0 s

1.2 m/s south

3. What is the maximum distance travelled by the object?

Displacement north as a function

of time