Velocity = speed in a given direction e.g 3 m/s east or 3 mph north
(or -3m/s if the motion is one dimensional and the positive and
negative directions have already been specified).
Motion
can be described well with graphs or with equations. The three most useful types
of motion graphs are the displacement-time graph, the velocity-time graph and
the acceleration-time graph. Each graph type is interpreted differently:
Displacement-time
graphs
slope of graph = velocity
Steeper slope = higher velocity
Zero slope = stationary
Negative slope = moving backwards
Velocity-time
graphs
slope of graph = acceleration
Steeper slope = higher acceleration
Zero slope = constant velocity
Negative slope = deceleration (negative acceleration) = slowing down (or speeding up while traveling backwards)
Area under graph = displacement
Acceleration-time graphs
Area under graph = change in velocity
Scalars and Vector
4.1 Projectile Motion
'Projectile motion' is the motion
of an object moving near the earth and acted on by gravity alone (no air
resistance). The key to understanding projectile motion is to consider the
horizontal and vertical components of the motion separately, for they
are independent of one another
For objects in projectile
motion:
the horizontal component of
its velocity does not change (no horizontal force, so no horizontal
acceleration).
The
vertical component is subject to a constant acceleration due to the constant vertical
force.
4.2 Circular Motion
An object rotates when it
turns around an internal axis; it revolves when it turns around an external
axis.
Rotational speed is the
number of rotations or revolutions made per unit of time.
The SI unit for rotational speed is the hertz (Hz). 1 Hz = 1 rotation per
second.
An object moving in a
circle is acted on by a net force towards the center, called the centripetal
force.
The magnitude of the centripetal force is given by Fcent
= mv² / r
When an object moves in a
circle, there is NO force pushing the object outward from the circle.
From within a
rotating frame of reference, there may seem to be an outwardly directed
('centrifugal') force, but this is in fact an illusion, a 'pseudo-force'. In
reality, the net force is always inwards.
