Frames of Reference
 

• an object or point from which movement is determined
•  the most common frame of reference is the Earth or those things that are attached to the Earth

Motion - a change in position relative to a frame of reference

Speed - the distance traveled by an object per unit of time

     Speed = Distance / Time    or  v = d / t

                                   - it is usually measured in km/hr or m/sec

Problem:  A car travels 300 km in 4 hours.   What is the average speed the car travels at?

 = 300 km
       4 hr.

v = 75 km
           hr.

velocity -- speed in a given direction
 

Problem:  Aaron walks North 25 meters in 5 seconds.   What is his velocity?

v = ?

d = 25 m

t = 5 s.

v = d
       t

v = 25 m
       5   s

v = 5 m   North
         s

Note:  Velocity to be properly calculated must be given a numerical value and a direction

acceleration -- the rate of change in velocity  acceleration

To change acceleration an object may be:
 

• speeding up
• slowing down
• changing direction

Acceleration = Final velocity - Original (initial) velocity
                                             Time

OR

Acceleration =     ^  v  (change in velocity)
                                t         time

     - if an object is speeding up its acceleration is >0
     - if it is slowing down its acceleration is <0

Problem:  A tricycle has an initial velocity of 0 m/s.   10 s later it has a final velocity of 5 m/s.   What is the
                acceleration of the tricycle?

Acceleration =   ^  v
                              t

v (initial) = 0 m/s

v (final) =  5 m/s

a = 5   m/s  ÷  10 s  

a = 0.5 m
            s ²

Note:  Acceleration units will always be expressed as distance divided by time squared.
 

Circular Motion

centripetal force -- any force that causes an object to move in a circle
 

•  since acceleration can be produced by just changing direction then whenever an object is moving in a circle it is
•  always changing direction and therefore constantly accelerating
•  
•  this is why a weighted string will always be swinging out to the outside of the motion
•  amusement rides take advantage of this by making their rides move relatively slowly but with many changes of
•  direction to make it feel as though you are going faster (accelerating)

• satellites in orbit around the earth continually fall toward the earth, but because the earth is curved, they travel around it

Momentum -- the mass of an object multiplied by its velocity

p = m (v)   p = momentum
                 m = mass
                 v = velocity
 

     - takes into account the amount of energy necessary to get an object moving at a certain velocity

     - it would take more energy to move a large object than say a smaller one

     - it would take more energy to move one object faster than another of the same size moving slower
 

     - momentum is the reason why it takes so long for a moving object like a car to come to a stop once it has been
       moving

Conservation of Momentum

     - the total momentum of any group of objects remains the same unless outside forces act on them

     - one object may lose momentum but another object will gain the momentum that was lost

     - when a bat hits a ball its momentum is transferred to the ball and since the ball's mass is less its velocity is
        increased

Hitting a baseball involves some complex physics including knowledge concerning momentum and its conservation
from ball to bat.   Two interesting links discussing this are:

The Science of Baseball

The Physics of Baseball