复习概念

Distance is a measure of how far an object has travelled from its starting point,
or how far away it is.
Distance is the same as length and is measured in
metres.
Displacement is also a measure of how far an object
has travelled from its
starting point but displacement also tells you the
direction of the object.
Distance is a scalar and displacement is a vector.
Speed is a measure of how fast an object is moving. It is measured in metres per second, written as m/s.
Speed = Distance ÷Time.
Velocity is similar to speed.
It is also a measure of how fast an object is moving
and is measured in m/s.
Velocity = Distance ÷Time.
If an object is stationary (not moving), then its speed
or velocity is zero.
The difference between velocity and speed is that velocity is speed in a certain direction (see scalars
and vectors).
If an object is moving in a straight line, then its speed and velocity will be the same.
If the moving object stays at the same speed but
changes direction,
then we say that the velocity has changed (because the direction has changed)
but the speed has stayed the same.
If the velocity in one direction is positive (for
example, forwards at 10m/s)
then the velocity in the opposite direction is negative (for example, backwards at -10m/s).
In most of the examples of motion that you will come
across,
the object will be moving in a straight line.
In this case, use the word velocity rather than speed
Either Speed or Velocity = Distance ÷Time.
If an object moving in a straight line travels 25
metres in 5 seconds,
then its velocity = 25 ÷5
= 5m/s.
You must always say what the units are!
(in this case m/s, called "metres per second"). Always check what units are given in the question.
If the time is given in hours or minutes,
then convert it to seconds before doing the
calculation.
If an objects velocity does not change, we say it has
a constant velocity.
In the above example,
we are not told whether the object has a constant
velocity,
or whether its velocity has changed during the 5
seconds.
If the velocity has changed,
then the answer we have calculated is an average
velocity of 5m/s.
If the velocity has not changed,
then the object had a constant velocity of 5m/s.
When an objects velocity changes, it is called
acceleration.
Acceleration = Change in Velocity ÷Time.
This equation is written as
a = (v-u) ÷t
where a = acceleration
v = final velocity (the one it ended up with)
u = initial velocity (the one it started with)
t = time
This equation is important!
The units of acceleration are m/s2, called "metres
per second squared".
If an object gets faster, it will have a positive
acceleration.
If an object gets slower, it will have a negative
acceleration
(this is sometimes called "deceleration"
but the term "negative acceleration" is preferred).
Acceleration = Change in Velocity ÷Time.
This equation is important!
a = (v-u) ÷t
where a = acceleration
v = final velocity (the one it ended up with)
u = initial velocity (the one it started with)
t = time
1. If a car changes from 10 m/s to 30m/s in 8
seconds,
what is its acceleration?
v = 30
u = 10
t = 8
a = (30 - 10)÷8
= 20÷8
= 2·5m/s2
2. If a bicycle moving at 15 m/s takes 10 seconds
to stop,
what is its acceleration?
In this example, the final velocity is zero because
the bicycle has stopped.
v = 0
u = 15
t = 10
a = (0 - 15)÷10
= -15÷10
= -1·5m/s2
The acceleration is negative because the bicycle
has slowed down.
Acceleration is the rate of change of an objects
velocity.
The object is said to have constant acceleration
if it gets faster (or slower, or its direction changes) at
the same rate.。