*Kinematics is the study of motion that deals only with how objects move without caring about what made them move.
*Dynamics is the study that deals with 'what' caused an object to move without caring about how that object moved.
Types of motion
There are mainly four types of motion
- Random or zigzag motion
- Translational motion
- Rotational motion
- Oscillatory motion
Distance and direction (displacement)
distance indicates how far an object has moved. Looking at the diagram above, let us assume that an Ant initially at point "O" crawled 15cm away from it can you be able to say for sure its final position? (P,Q,B,R,C,S)
Yes! you can guess but you can't be so sure because those other points P,Q,B,R,C,S are all 15cm away from point O.
In science we don't guess...
Therefore knowing the distance alone is not sufficient to tell us the final position of the Insect.
In the study of motion it is really necessary to specify the direction in which a body is moving. If the Ant crawled 15cm eastward, then its final position is at P, if it crawled 15cm westward, the final position is at R, if it crawled northward, the final position is at Q , if it is in the south direction then its final position is at S also if the direction is south-west or north-west, then its final positions is at C or B respectively.
"Displacement" is one word in physics used to explain both distance and direction at thesame time.
Speed and velocity
Speed is an everyday word used by everybody, it is common to every student. Speed is simply the rate at which a body covered a particular distance. For instance; if it took that ant 10 seconds to move 30cm from O to P (eastward) then the speed is given by
therefore the formular for calculating speed is
the speed in this case is actually the average speed because maybe in the course of the travel, the ant had to stop on the way to pick food then continue. That time at which the ant stopped was not subtracted from the 4 seconds, all were actually counted to make it up to the 4 seconds knowing fully well that at the point where it stopped, that the speed then was zero. So this type of speed is known as the total/average speed of travel.
Sometimes you are asked about the instantaneous speed this is simply the speed at a particular instant in time like assuming the ant stopped at the sixth second and continued at the seventh you maybe asked what was the speed of the ant on the sixth seconds mark: the answer is simply 0 cm per second. And that is the instantaneous speed.
Uniform or constant speed
A body is said to be in uniform or constant speed when it covers equal distances in equal time intervals. When you are in a moving car and you notice the speedometer fixed at a particular number e.g 50, then you must be travelling with a constant or uniform speed which is 50km/h
Velocity is simply speed-with-details the difference between this two is that speed has no specific direction while velocity has a specific direction.
Note: Average speed is similar to average velocity, instantaneous speed is similar to instantaneous velocity but uniform speed is completely different from uniform velocity
Uniform velocity
A body is said to be moving with a uniform velocity if it covers equal displacement in equal time intervals "In thesame direction."
Note: A body moving in a circular path at constant speed doesn't move with a constant velocity because its direction is constantly changing.
Acceleration
Acceleration of a body is simply defined as the rate at which velocity changes for example: If we say that the acceleration of a body is 10m/s , this simply means that the velocity (speed) of this body increases by 10 every second e.g if the initial velocity is
0m/s
then in the first second, the velocity would be equal to
0 + 10 = 10m/s
in the next second, the velocity would be equal to
(0 + 10) + 10 = 20m/s
the formular for acceleration is
the unit of acceleration is
when the velocity is decreasing, the name of acceleration changes to retardation.
thus suppose a car moves from a velocity (speed) of 20m/s to a velocity of 30m/s in 5 seconds, its average acceleration is
Uniform acceleration
Uniform acceleration is a case whereby the velocity of a body increases by equal amount every second. For instance if the velocity of a moving body increases by 2m/s every second for 5 seconds, then at the end of the 5th second, the velocity of the car can be predicted to be 10m/s
(recall that the speed of this car increases by 2m/s every second)
therefore at the end of the 5 seconds, the final speed would be
(0 + 2) + 2 + 2 + 2 + 2 = 10m/s
' 0 ' is the initial velocity and it means the body is at rest (not moving)
So you must now understand that for these 5 seconds, the car travelled with a constant or uniform
acceleration which is = 2m/s
Graphical representation of acceleration (velocity-time graph)
The slope of a velocity time graph is the acceleration.
Let us assume an object travelling along a straight path, has the following velocities for the first 6 seconds of its motion
If we plot a graph of velocity (on the vertical axis) against time (on the horizontal axis), we can obtain a velocity-time graph which describes this motion
Motion description (using the graph)
Along 0A, the velocity of the object increases with equal rate leading to the straight line (uniform/constant acceleration).
Along AB the body traveled with a constant velocity. Here, the acceleration is zero (because the velocity was not changing).
Along BC, the velocity of the body begins to drop (reduce) but with thesame rate thus retardation.
The retardation = gradient/slope along BC
the length of line QC is
6 - 2 = 2
therefore the slope along BC
Causes of motion (Dynamics)
Motion is caused by force. According to Isaac Newton, a body will remain in its state of motion (even for a thousand years) unless a force acts on it. With this, you can now understand why the book you leave on top of your reading table remains there until you remove it, you can also understand why earth moves with a constant speed round the sun.
The speed at which our planet revolves round the sun can only increase if a giant force pushes it to move faster. If this case should happen, then it will no longer take us 365 days to see Christmas (probably something shorter).
Types of forces
There are just two kinds of forces in existence;
- contact forces e.g push and pull forces
- Non contact forces e.g magnetic forces
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