Download Speed - TGHSLevel1Science

Speed, Distance
and Time
Speed and Velocity
• An object has speed when it travels a distance
in a time interval.
• At any moment in time, a moving object has
instantaneous speed. Since this is difficult to
calculate, we usually use average speed.
Speed and Velocity
• Since velocity is speed in a given direction,
• the equation for average velocity is:
final distance – initial distance
Velocity =
final time – initial time
∆d
Vav =
∆t
• The unit for speed or velocity is metres per
second (ms-1) or kilometers per hour (kmh-1).
Distance and Time
• Distance is a measurement of how far apart
two points are.
• The unit for distance is the metre (m), or
kilometer (km).
• Displacement is a measurement of how far an
object has moved from its starting point.
Distance and Time
• The equation is:
displacement = final
– initial
distance
distance
• Time is a measurement of the duration of an
event.
• The unit for time is the second (s), or
hour (h).
Distance-Time Graphs
• Distance is plotted on the vertical axis and
Time on the horizontal axis.
• The distance-time graph for an object moving
at a constant speed is always a straight line
with a slope.
Interpreting Distance/time graphs
•the gradient of a distance/time graph is the
velocity
d
t
Stationary Constant (v=0)
distance remains the same)
(the
Interpreting Distance/time graphs
d
Faster constant velocity
Slow constant velocity
t
Constant velocity (the distance changes at a
constant rate)
Interpreting Distance/time graphs
Slowing down (-ve
acceleration)
d
Speeding up (+ve
acceleration)
t
Changing velocity (the distance moved
per second changes)
Interpreting Distance/time graphs
d
t
An object that is returning to its
starting point. (at a constant speed)
Interpreting Distance/time graphs
RC Car Journey
B
d
C
D
A
t
• A -fast constant speed (steep gradient and line straight)
• B –slower constant speed (shallow gradient and line
straight)
• C –stationary (line horizontal)
• D –Very fast constant speed backwards (steep gradient,
straight line, sloping down)
Calculating Gradient
• The slope or gradient of a line can be
calculated using the formula:
• Gradient = Increase in vertical height (of line)
Increase in horizontal length
Velocity (speed)
Velocity (speed) is a measure of how quickly the
position of an object is changing (units ms-1)
• If the object is a constant velocity (balanced
forces) the formula is;
distance
velocity 
time
d
v
t
Velocity (speed)
• If the velocity of a journey changes (some
slowing and speeding up) the formula for the
average velocity is;
Total distance
Ave.velocity 
Total time
d
vave 
t
Velocity Calculations
1. A cyclist travels 50km in 1.5 hours. Calculate
her velocity.
2. An athlete records a time of 64s for a 400m
race. What speed was he running at?
3. An Otago student takes 2.5 hours to return
to Dunedin from Timaru (190km), with a
short stop at Hampden from fish and chips.
What was her average velocity for the
journey?
Acceleration
• An object changing its speed is said to be
accelerating. If the acceleration is:
– positive (eg. 2ms-2) = object speeding up
– negative (eg. -2ms-2) = object slowing down
or decelerating
Acceleration
• The equation is:
acceleration =
a
=
final speed – initial speed
time taken
∆v
∆t
• The unit for acceleration is metres per second
squared (or ms-2).