Motion, Force and Friction
Overview
Motion, Force and Friction form the foundation of classical mechanics and appear consistently in MAHA TET Paper II Science. This topic bridges everyday observations—a rolling ball, a book sliding off a desk, a car braking—with the scientific laws that explain them. For the exam, you must understand definitions, distinguish between types of motion, apply Newton's three laws qualitatively, and explain friction's dual role as both hindrance and necessity.
Mastery here also supports pedagogy questions on activity-based science teaching, as mechanics lends itself well to simple classroom demonstrations.
Key Concepts
- Motion is the change in position of an object with respect to time and a reference point. An object is at rest if its position remains unchanged relative to its surroundings.
- Distance vs Displacement: Distance is the total path length (scalar); displacement is the shortest straight-line separation between initial and final positions (vector).
- Speed vs Velocity: Speed = distance / time (scalar); velocity = displacement / time (vector). An object moving in a circle at constant speed has changing velocity because direction changes.
- Acceleration is the rate of change of velocity. Positive acceleration means speeding up; negative acceleration (retardation/deceleration) means slowing down.
- Force is a push or pull that can change an object's state of rest or uniform motion, its shape, or its direction of motion. Measured in newtons (N).
- Balanced vs Unbalanced Forces: Balanced forces result in zero net force and no change in motion. Unbalanced forces cause acceleration.
- Friction is the resistive force that opposes relative motion between two surfaces in contact. It depends on the nature of surfaces and the normal force pressing them together.
- Inertia is the tendency of an object to resist any change in its state of rest or uniform motion; it is directly related to mass.
Types of Motion
| Type | Description | Example |
|---|---|---|
| Rectilinear (Linear) | Motion along a straight line | A car on a straight road |
| Circular | Motion along a circular path | Earth orbiting the Sun |
| Rotational | Spinning about an axis | A spinning top |
| Oscillatory (Vibratory) | To-and-fro motion about a mean position | A swinging pendulum |
| Periodic | Motion that repeats after equal intervals | Hands of a clock |
| Random | Irregular, unpredictable motion | Movement of dust particles in air |
Formulas / Key Facts
- Speed = Distance / Time → measured in m/s or km/h.
- Velocity = Displacement / Time → measured in m/s (vector).
- Acceleration (a) = (Final velocity − Initial velocity) / Time = (v − u) / t → measured in m/s².
- Equations of uniformly accelerated motion (for reference):
- v = u + at
- s = ut + ½at²
- v² = u² + 2as
- Force (F) = Mass × Acceleration → F = m × a (Newton's second law).
- Weight (W) = Mass × Acceleration due to gravity → W = m × g (g ≈ 9.8 m/s² or ≈10 m/s² for calculations).
- Friction force depends on: (a) nature of surfaces, (b) normal reaction (pressing force). It does NOT depend on area of contact.
- Static friction > Sliding (kinetic) friction > Rolling friction in magnitude for same surfaces.
Newton's Laws of Motion
First Law (Law of Inertia)
An object remains at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force.
Example: Passengers jerk forward when a moving bus suddenly brakes—their bodies tend to continue moving forward due to inertia.
Second Law (Law of Acceleration)
The rate of change of momentum of an object is directly proportional to the applied unbalanced force and occurs in the direction of the force.
Simplified form: F = m × a
Example: A cricket ball hit with greater force accelerates faster; a heavier ball requires more force to achieve the same acceleration.
Third Law (Action-Reaction)
For every action, there is an equal and opposite reaction. The two forces act on different bodies.
Example: When you push a wall, the wall pushes you back with equal force; a swimmer pushes water backward, water pushes the swimmer forward.
Types of Friction
| Type | When it Acts | Magnitude |
|---|---|---|
| Static friction | Object is stationary; force applied but no motion yet | Highest |
| Sliding (kinetic) friction | Object slides over another surface | Less than static |
| Rolling friction | Object rolls over a surface | Least |
| Fluid friction (drag) | Object moves through liquid or gas | Depends on speed and medium |
Reducing friction: Lubricants, ball bearings, polishing surfaces, streamlined shapes.
Increasing friction: Treaded tyres, rough soles, sand on slippery roads.
Worked Examples
Example 1: Calculating Speed
A cyclist covers 450 m in 30 s. Find the speed.
Speed = Distance / Time = 450 m / 30 s = 15 m/s
Example 2: Applying Newton's Second Law
A force of 20 N acts on an object of mass 4 kg. Find the acceleration.
F = m × a → a = F / m = 20 N / 4 kg = 5 m/s²
Example 3: Identifying Friction Type
A heavy box is pushed but does not move. Which friction acts?
Since there is no motion, static friction acts. It exactly balances the applied force until the push exceeds the limiting static friction.
Common Mistakes
- Confusing speed with velocity → Speed is scalar (no direction); velocity is vector (magnitude + direction). A car going round a bend at constant speed has changing velocity.
- Thinking heavier objects always fall faster → In a vacuum, all objects fall at the same rate (g). Air resistance, not weight alone, causes differences in everyday situations.
- Believing friction is always harmful → Friction is essential for walking, writing, braking. Without it, tyres would spin uselessly on roads.
- Misapplying action-reaction pairs → Action and reaction act on different bodies. They do not cancel each other because they are not on the same object.
- Assuming larger surface area means more friction → Friction depends on normal force and surface nature, NOT on contact area (for rigid bodies).
Quick Reference
- Motion needs a reference point; no absolute rest exists.
- Inertia ∝ mass; more mass means harder to start or stop.
- F = m × a — fundamental equation linking force, mass, acceleration.
- Static friction > Sliding friction > Rolling friction.
- Newton's third law: Forces are equal, opposite, and act on different bodies.
- Friction can be friend (walking, brakes) or foe (wear, energy loss)—context matters.