1D Motion

1
1D Motion
Physics 131 - Physics for Health, Life and
Environmental Sciences

• You drive a truck along a straight road for 8.4km
  at 70km/h at which point the truck runs out of
  fuel and stops. Over the next 30min, you walk
  2.0km back along the road to a fuel station.
• What is you overall displacement from the
  beginning of your drive to the station?
• What is the time interval from the beginning of
  your drive to your arrival at the station?
• What is your average speed and velocity from the
  beginning of your drive to the station?

2
1D Motion
Physics 131 - Physics for Health, Life and
Environmental Sciences

• Instantaneous velocity is the velocity of an
  object at an exact point in time.
• ?v
• v
• v (the slope of the s-t graph)

s
Positive Velocity
Stationary
Negative Velocity
Positive Velocity
Stationary
t
Instantaneous velocity is the rate of change of
displacement
3
1D Motion
Physics 131 - Physics for Health, Life and
Environmental Sciences

• When the velocity changes with time we say the
  object is acceleratingAvg. acceleration
  a

Differentiate
Differentiate
s
a
v
Slope
Slope
t
t
t
Integrate
Integrate
Area under Graph
Area under Graph
4
Equations of Motion
Physics 131 - Physics for Health, Life and
Environmental Sciences

• a assuming we always start the clock
  at ti 0, we have
• atf vf vi
• Rearranging this we have
• vf vi atf 1
• This assumes that a is constant

5
Equations of Motion
Physics 131 - Physics for Health, Life and
Environmental Sciences

• Average velocity v (vi vf)/2
• But v?t ?x s
• Therefore s (vi vf)?t/2 2
• From 1 we have vf vi atf
• Therefore ?x vi (vi atf)?t/2
• Recalling that ti 0 and simplifing
• we have s vi ½at2 3

6
Equations of Motion
Physics 131 - Physics for Health, Life and
Environmental Sciences
Gravity is a vertical acceleration of -9.8ms-2
7
Equations of Motion
Physics 131 - Physics for Health, Life and
Environmental Sciences

• Spotting a police car, you brake a Porshe from a
  speed of 100km/h to 80.0km/h during a
  displacement of 88.0m.
• What is your acceleration?
• How much time is required to decrease your speed?