Laws of Motion
Physics – BUMS (Unani)
“Understanding how bodies move is the cornerstone of both classical physics and the ancient Unani tradition of analyzing natural phenomena.”
1. Simple Explanation
- Motion is the change in position of an object over time.
- The laws of motion describe the relationship between an object’s motion and the forces acting on it.
- Newton’s three laws form the foundation of classical mechanics and are also used in Unani medical physics to explain bodily movements (e.g., pulse, respiration, circulation).
2. Key Points
| Law | Statement | Consequence | Symbolic Expression |
|---|---|---|---|
| First Law (Inertia) | An object at rest stays at rest, and an object in motion continues in a straight line at constant speed unless acted upon by a net external force. | Objects resist changes to their state of motion. | ( \sum \mathbf{F} = 0 \Rightarrow \text{constant }\mathbf{v} ) |
| Second Law (Dynamics) | The net force on an object is equal to the time rate of change of its momentum. For constant mass, ( \mathbf{F} = m\mathbf{a} ). | Force causes acceleration; heavier objects need more force for same acceleration. | ( \mathbf{F} = m\mathbf{a} ) |
| Third Law (Action–Reaction) | For every action there is an equal and opposite reaction. | Forces always occur in pairs; they act on different objects. | ( \mathbf{F}{12} = -\mathbf{F}{21} ) |
| Derived Concepts | • Momentum (p): ( \mathbf{p} = m\mathbf{v} ) • Impulse (J): ( \mathbf{J} = \int \mathbf{F},dt = \Delta \mathbf{p} ) |
Useful for analyzing collisions, rockets, and bodily motions. | – |
Unani Note – The concept of inertia can be loosely linked to the Unani idea of Qalb and Ruh (heart and spirit) maintaining their natural state unless influenced by Saff (forces).
3. Worked Example
Problem: A 1500 kg car starts from rest and accelerates uniformly to 20 m/s in 10 s.
Find: (a) Net force on the car (b) Distance travelled.
(a) Net Force
Using ( \mathbf{a} = \frac{\Delta v}{\Delta t} )
[ \mathbf{a} = \frac{20\ \text{m/s} - 0}{10\ \text{s}} = 2\ \text{m/s}^2 ]
[ \mathbf{F}_{\text{net}} = m\mathbf{a} = (1500\ \text{kg})(2\ \text{m/s}^2) = 3000\ \text{N} ]
(b) Distance Traveled
Using ( s = \frac{1}{2}a t^2 ) (since initial velocity (u = 0))
[ s = \frac{1}{2}(2\ \text{m/s}^2)(10^2\ \text{s}^2) = 100\ \text{m} ]
Answer
- Net force ≈ 3000 N (forward).
- Distance ≈ 100 m.
Unani Insight – The car’s inertia (mass) resists change; the applied force (engine thrust) overcomes this to set it moving, analogous to how a jaundice patient requires external treatment to restore normal blood flow.
4. Quick Revision Summary
- 1st Law – Inertia: no net force → no change in motion.
- 2nd Law – ( \mathbf{F} = m\mathbf{a} ): force causes acceleration.
- 3rd Law – Action ↔ Reaction: forces in equal–opposite pairs.
- Momentum ( \mathbf{p} = m\mathbf{v} ) and Impulse ( \mathbf{J} = \Delta \mathbf{p} ) link force over time to motion.
- Key Formulae:
- ( \mathbf{a} = \frac{\Delta v}{\Delta t} )
- ( s = ut + \frac12 at^2 )
- ( \mathbf{F}_{\text{net}} = \sum \mathbf{F} )
- Practical Tip – Always check if the net force is zero before applying the second law; if so, the object maintains a constant velocity.
5. Practice Questions (Optional)
- A 50 kg sled slides down a frictionless slope making 30° with the horizontal. What is its acceleration?
- Two astronauts of masses 70 kg and 80 kg push off each other