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General Science: Motion (Physics)
📅 Date: 16 December 2025 (Tuesday)
Welcome to the world of Physics. This is where science moves from "what things are made of" (Chemistry) to "how things behave." For your exam, Physics questions are usually less about complex calculations and more about application and conceptual clarity. You need to visualize these concepts in real life.
1. Describing Position: The Reference Point
To say an object is moving, we must compare it to a fixed point called the Reference Point or Origin.
Relative Motion: Motion is relative. Example: To a person standing on the road, a bus is moving. To a passenger inside the bus, the fellow passenger is at rest, but the trees outside are moving backward.
2. Distance vs. Displacement (The Most Common Trap)
This is the most critical concept in this chapter.
- Distance: The total path length covered.
- It is a Scalar Quantity (magnitude only, no direction).
- It can never be zero if the object has moved.
- Displacement: The shortest straight-line distance between Initial and Final positions.
- It is a Vector Quantity (magnitude and direction).
- It can be zero even if the object has moved (e.g., running a lap and returning to the start).
If you throw a ball straight up to a height of 5 meters and catch it back:
Distance: 5m (up) + 5m (down) = 10m.
Displacement: 0 (Because your starting point and ending point are the same).
3. Speed vs. Velocity
Just like Distance and Displacement, these two are often confused.
- Speed: Distance per unit time. Scalar. (Formula: Distance/Time). Can never be zero for moving body.
- Velocity: Displacement per unit time (Speed with Direction). Vector. (Formula: Displacement/Time). Can be zero.
Average Speed: Total Distance / Total Time. (Used when speed changes during the trip).
Odometer: Measures Distance.
Speedometer: Measures Instantaneous Speed.
4. Acceleration (Rate of Change)
When an object changes its velocity (speeds up, slows down, or turns), it is accelerating.
- Formula: a = (v − u) / t
- u = Initial velocity, v = Final velocity, t = Time.
- Unit: m/s2
- Positive Acceleration: Speed increases.
- Negative Acceleration (Retardation): Speed decreases (e.g., applying brakes).
5. Graphical Representation
Examiners love to give you a graph and ask what it represents.
A. Distance-Time Graph (s−t graph)
- Slope: The slope (steepness) gives you Speed.
- Straight Line: Uniform speed.
- Parallel to X-axis: Body is at rest.
B. Velocity-Time Graph (v−t graph)
- Slope: The slope gives you Acceleration.
- Area Under the Curve: The area enclosed gives you the Displacement (Distance).
6. The Three Equations of Motion
Valid only when acceleration is uniform (constant).
- Velocity-Time: v = u + at
- Position-Time: s = ut + 1/2at2
- Position-Velocity: 2as = v2 − u2
- If a body starts from rest, take u = 0.
- If a body comes to a stop, take v = 0.
- If a body is falling freely, acceleration a = g (+9.8 m/s2).
- If a body is thrown upwards, acceleration a = −g (−9.8 m/s2).
7. Uniform Circular Motion (The "Trick" Concept)
When an object moves in a circle at a constant speed:
- Is the Speed constant? Yes.
- Is the Velocity constant? NO. (Because direction changes at every point).
- Is it accelerated motion? YES. (This is called Centripetal Acceleration).
Example: An athlete running on a circular track; Earth revolving around the Sun.
8. Mentor’s Final Drill (Exam-Ready Questions)
A: Zero. (He returned to the start).
Q: Can an object have constant speed but varying velocity?
A: Yes, in Uniform Circular Motion.
Q: What does the area under a Velocity-Time graph represent?
A: Displacement (or distance traveled).
Q: If a car travels 10 km North and then 10 km South, what is the total distance and displacement?
A: Distance = 20 km. Displacement = 0 km.
Q: Acceleration is a vector quantity. True or False?
A: True. It has direction.
Study Strategy: Don't get bogged down in solving long, complex mathematical problems. Focus on the logic behind the equations and the difference between scalar and vector quantities. Keep moving forward!