Mechanics is the branch of physics that deals with the motion of objects and the forces that cause this motion. You will sometimes see the terms Kinematics and Dynamics used in various texts. Kinematics describes the motion of an object without explicit reference to the forces involved. Dynamics deals with the forces and their effects.
When we are describing the motion of anything the direction in which it moves is as important as how fast it is moving. There is no point is sending a rocket up to Mars that can escape the earth's gravitation field if it misses and hits Saturn!
The terms Vector
and Scalar are used to describe quantities. A scalar quantity has size
only (e.g. Mass and Time) A vector quantity has size and direction (e.g. Velocity
and Force).
The difference
between scalar quantities and vector quantities is so important
We will need to redefine some common terms with more precise definitions.
Distance:
a scalar quantity
Displacement: The distance travelled in a certain direction (Vector)
Speed: The rate at which distance changes with time (Scalar)
Velocity: The rate at which displacement changes with time
Acceleration: The rate at which velocity changes with time (Vector)
The difference between distance/displacement at speed/velocity may seem insignificant but it is important - trust me!
In both cases it is the rate at which something moves
|
Speed
= Distance / Time
Velocity = Displacement / Time |
In both cases the unit is ms -1 (So you can't tell from the units!)
Now this is fine
for things moving at constant speeds in nice patterns but very few things do
this. This means that we usually talk about average speeds and velocities.
If we look at the value at a particular point, this is referred to as an Instantaneous
speed or velocity.
Acceleration is a measure of how quickly something is changing velocity. If the value of the acceleration is negative then the object is decelerating. Acceleration is measured in meters per second every second ms -2 (or m/s/s or m/s 2 .)
|
Acceleration
= change in velocity/ time
|
e.g. If a car goes from stationary to 10 ms-1 in 2.5 seconds then what is its acceleration ?
Acceleration
= change in speed/ time
= (10 -
0)/2.5
= 4 ms
-2
Some Quick questions
a) If a car goes from 13 ms
-1
to 53 ms
-1
in 30s then what is its acceleration?
b) If the same car decelerates from 53 ms
-1
to 18 ms
-1
in 70s then what is its acceleration?
c) A car starts from rest and accelerates at 3 ms
-2
for 6 seconds, what is its speed?
d) A bird takes 5 seconds to reach a take-off speed of 20 ms
-1
, if it continues to accelerate at this rate until it is travelling at 100 ms-1
how long will this take?