Physics becomes interesting when we start looking at the world around us and asking how things move. This article is about Kinematics from the Sindh Textbook Board Class 9th is all about learning the basic language of motion.
Before we talk about forces and why objects move, we first need to understand how to describe motion itself.
This chapter introduces terms like distance, displacement, speed, velocity, and acceleration that we use every day, but in physics they have very specific meanings.
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Once you get these definitions clear, solving numerical problems and understanding graphs becomes much easier.
REST AND MOTION
When we study kinematics, the first thing we need is a reference point.
MECHANICS:
The branch of physics which is related with the study of motion of objects. It has two sub branches: kinematics and dynamics.
KINEMATICS:
The branch of mechanics which deals with motion of objects without reference of force which causes motion. The word comes from Greek “Kinema” which means motion.
REST:
A body is said to be at rest if it does not change its position with respect to its surroundings. Example: a book lying on a table.
MOTION:
A body is said to be in motion if it changes its position with respect to its surroundings. Example: a moving bus, a person walking.
RELATIVE MOTION:
A motion observed in an object with respect to some other object. Example: passengers sitting in a moving car are at rest relative to each other, but moving 60 km/h relative to a person standing on the road.
EXAMPLE OF REST AND MOTION SIMULTANEOUSLY:
A person sitting in a moving train is at rest with respect to the train, but in motion with respect to the platform. This is how an object can be at rest and in motion at the same time.
TYPES OF MOTION
TRANSLATORY MOTION:
Type of motion in which every particle of a body moves parallel to each other along any path.
It has three types:
- LINEAR MOTION: Motion along a straight line. Example: a car moving on a straight road.
- CIRCULAR MOTION: Motion along a circular path. Example: a car moving on a roundabout.
- RANDOM MOTION: Motion with no definite path or direction. Example: motion of a ink particle in water.
ROTATORY MOTION:
Type of motion in which every particle of a body moves around a fixed point. Examples: motion of a fan, wheel, clock hands.
VIBRATORY MOTION:
The to and fro motion of a body along the same path about its mean position. Examples: motion of a pendulum, mass attached to a spring.
TERMS ASSOCIATED WITH MOTION
DISTANCE:
The length of a path between two points. It is a scalar quantity. Example: If you walk 3 m east then 4 m north, total distance = 7 m.
DISPLACEMENT:
The shortest distance between two points with magnitude and direction. It is a vector quantity. Example: For the same path 3 m east then 4 m north, displacement = 5 m north-east.
DIFFERENCE BETWEEN DISTANCE AND DISPLACEMENT:
Distance has only magnitude and depends on path, displacement has magnitude and direction and depends only on initial and final position. Distance is never negative, displacement can be zero or negative.
SPEED:
Distance covered by an object in unit of time. Formula: Speed = Distance / time. Unit: m/s. It is scalar.
UNIFORM SPEED:
When a body covers equal distance in equal intervals of time.
VELOCITY:
Displacement covered by an object in unit of time. Formula: Velocity = Displacement / time, or V = d / t. Unit: m/s. It is vector.
DIFFERENCE BETWEEN SPEED AND VELOCITY:
Speed has only magnitude, velocity has magnitude and direction. Speed of a car can be 50 km/h, velocity would be 50 km/h towards north.
UNIFORM VELOCITY:
When a body covers equal displacement in equal intervals of time. Means no change in speed or direction.
AVERAGE VELOCITY:
Total displacement divided by total time taken. Formula: Average velocity = total displacement / total time taken.
INSTANTANEOUS VELOCITY:
The velocity of a body for a very short interval of time, or at a specific moment.
ACCELERATION:
The rate of change of velocity of a body. Formula: acceleration = change in velocity / change in time = (Vf – Vi) / t. Unit: m/s².
UNIFORM ACCELERATION:
When the velocity of a body changes by an equal amount in equal intervals of time.
POSITIVE ACCELERATION:
When velocity of a body is increasing with time. Its direction is in the direction of velocity.
NEGATIVE ACCELERATION / RETARDATION / DECELERATION:
When velocity of a body is decreasing with time. Direction is opposite to velocity.
SCALAR AND VECTOR QUANTITIES
SCALAR QUANTITIES:
Physical quantities completely specified by magnitude and suitable unit only. Examples: speed, distance, mass, time, temperature.
VECTOR QUANTITIES:
Physical quantities specified by magnitude with suitable unit and particular direction. Examples: displacement, velocity, acceleration, force.
REPRESENTING VECTOR QUANTITIES BY DRAWING:
Vectors are shown by an arrow. The length of the arrow shows magnitude and the arrowhead shows direction. Example: 5 m displacement east is drawn as a 5 cm arrow pointing to the right if 1 cm = 1 m scale is used.
GRAPHICAL ANALYSIS
DISTANCE-TIME GRAPH:
Shows how distance changes with time.
- Slope of distance-time graph = speed.
- Straight horizontal line means body is at rest.
- Straight slanted line means body is moving with constant speed.
- Curved line means body is moving with variable speed.
SPEED-TIME GRAPH:
Shows how speed changes with time.
- Slope of speed-time graph = acceleration.
- Straight horizontal line means zero acceleration or constant speed.
- Straight slanted line means uniform acceleration.
- Curved line means non-uniform acceleration.
- Area under speed-time graph = distance travelled by the body.
EQUATIONS OF MOTION
For uniformly accelerated motion:
- vf = vi + at
- S = vi t + ½ at²
- 2aS = vf² – vi²
We can rearrange these equations according to what is required. Example: If we need time and we know vf, vi, a then we use t = (vf – vi) / a from the first equation.
FREELY FALLING BODIES
For freely falling bodies, replace ‘a’ with ‘g’ and ‘S’ with ‘h’. Take g = 10 m/s² as acceleration due to gravity.
- vf = vi + gt
- h = vi t + ½ gt²
- 2gh = vf² – vi²
For bodies falling down, g is positive. For bodies thrown up, g is negative.
Example: A ball is dropped from rest. After 2 s, vf = 0 + 10 × 2 = 20 m/s. Distance h = 0 + ½ × 10 × 4 = 20 m.
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Kinematics lays the foundation for the rest of physics. This blog train you to describe motion, use graphs, and solved numerical with the three equations of motion. The key is to remember which quantity is scalar and which is vector, and to read graphs carefully for slope and area. Practice rearranging the equations and use g = 10 m/s² for free fall problems. Once you can explain rest and motion together and handle the graphs, the rest of mechanics becomes much easier to handle.
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