Kinematics

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Kinematics

• Displacement, Velocity and Acceleration

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Mechanics The study of how matter and forces
interact, on a macroscopic scale. Kinematics Th
e part of mechanics concerned with the study of
motion, without consideration of its
causes. Dynamics The part of mechanics
concerned with the study of motion and its
causes, notably forces.
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• Kinematics
• Speed and Velocity
• Both are measured in ms-1
• Speed is a scalar while velocity is a vector
• These are both averages as they may vary over the
  period of time taken.
• Note Many of the examples we study involve
  linear motion things moving in straight lines.
  Also, the direction is not always stated for a
  vector quantity if it is clear.

Average Speed Distance travelled
Total time
v s t
Average Velocity Change in displacement
Total time
v ?s ?t
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• Q. Arthur takes a walk from home, around the park
  and back to where he started. If he walked 2.7km
  over fifteen minutes, determine
• his distance travelled.
• his total displacement.
• his average speed.
• his average velocity.

2700 m 0 3 ms-1 0
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Frames of Reference Measurements can be made from
different viewpoints, known as frames of
reference. E.g. To an observer, a building
appears to be at rest. However to a driver of a
passing car, the building appears to be moving
away at high speed. The stationary observer, and
the driver of the car, both have a different
frame of reference.
Frames of reference link
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Relative Velocity If two moving objects have
different velocities, their relative velocity
depends upon which frame of reference you are
using. If we know the velocities of two bodies
in the same frame of reference, the velocity of
body A with respect to body B is given by
vAB vA - vB
E.g. Car X moves at 20ms-1 to the left, car Y at
16ms-1 to the right. What is the velocity of X
w.r.t. Y?
(taking to the left as the ive direction)
vXY vX vY 20 - (-16) 36ms-1
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E.g.2 A bird flyi
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Acceleration Acceleration is what happens when
the velocity of a body changes. It is a vector
quantity. Q. Rearrange this formula to make
v, u and t the subjects.
Average Acceleration Change in velocity
Total time taken
a v u t
a ?v ?t
or
( v final velocity, u initial velocity )
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• Some typical apparatus for linear motion
  experiments
• One light gate and a double interrupter card
• Two light gates and a 10cm card.
• Ticker timer
• Distance sensor, interface and computer
• Stopwatch
• Trap door and digi-timer

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