Force, Motion and Work form the backbone of classical mechanics and are essential topics for TN TET Paper II (Mathematics and Science). This unit tests your understanding of how objects move, what causes them to move or stop, and how energy is transferred through work. Questions typically involve definitions, laws of motion, numerical problems on work-energy, and identification of simple machines.
For TN TET, expect 3-5 questions from this area. You must be comfortable with Newton's laws, types of friction, calculation of work and energy, mechanical advantage of simple machines, and real-life applications. Conceptual clarity matters more than complex calculations at this level.
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Key Concepts
**Force** is a push or pull that can change an object's state of rest or motion, its shape, or its direction. SI unit: Newton (N). Force is a vector quantity (has magnitude and direction).
**Inertia** is the tendency of an object to resist change in its state of motion. Mass is the measure of inertia—greater mass means greater inertia.
**Friction** is the force that opposes relative motion between two surfaces in contact. It depends on the nature of surfaces and normal force, not on area of contact.
**Motion** is the change in position of an object with respect to time and a reference point. Types include rectilinear (straight line), circular, oscillatory and random motion.
**Work** is done when a force causes displacement in its direction. Work = Force × Displacement × cos θ. No displacement means no work done.
**Energy** is the capacity to do work. It exists in many forms—kinetic, potential, heat, light, sound, chemical, electrical.
**Simple machines** make work easier by multiplying force, changing direction of force, or increasing speed. They do not reduce the total work done.
**Mechanical Advantage (MA)** = Load / Effort. It tells how many times a machine multiplies the effort force.
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Formulas / Key Facts
| Quantity | Formula | Unit | |----------|---------|------| | Force | F = m × a | Newton (N) | | Weight | W = m × g (g ≈ 10 m/s²) | Newton (N) | | Work | W = F × d × cos θ | Joule (J) | | Kinetic Energy | KE = ½ × m × v² | Joule (J) | | Potential Energy | PE = m × g × h | Joule (J) | | Power | P = Work / Time = W / t | Watt (W) | | Mechanical Advantage | MA = Load / Effort | No unit | | Velocity Ratio | VR = Distance moved by effort / Distance moved by load | No unit | | Efficiency | η = (MA / VR) × 100% | Percentage |
**Key Facts:**
1 Joule = 1 Newton × 1 metre
1 Watt = 1 Joule per second
Static friction > Sliding friction > Rolling friction
Friction can be reduced by lubrication, polishing, using ball bearings, or streamlining
Law of Conservation of Energy: Energy can neither be created nor destroyed, only transformed
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Worked Examples
**Example 1: Calculating Work Done**
A boy pushes a box with a force of 50 N and moves it 8 m in the direction of the force. Find the work done.
*Solution:* Work = Force × Displacement W = 50 N × 8 m = 400 J
**Example 2: Kinetic Energy**
A car of mass 1000 kg is moving at 20 m/s. Calculate its kinetic energy.
*Solution:* KE = ½ × m × v² KE = ½ × 1000 × 20 × 20 KE = ½ × 1000 × 400 KE = 200000 J = 200 kJ
**Example 3: Mechanical Advantage of a Lever**
A lever is used to lift a load of 600 N by applying an effort of 150 N. Find the mechanical advantage.
*Solution:* MA = Load / Effort MA = 600 / 150 = 4
This means the lever multiplies the effort 4 times.
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Newton's Laws of Motion (Must Know)
**First Law (Law of Inertia):** An object at rest stays at rest, and an object in motion stays in motion with constant velocity, unless acted upon by an external force. *Example:* Passengers jerk forward when a bus stops suddenly.
**Second Law (Law of Force):** Force equals mass times acceleration: F = m × a The rate of change of momentum is proportional to the applied force. *Example:* A cricket ball hit harder travels faster.
**Third Law (Action-Reaction):** For every action, there is an equal and opposite reaction. *Example:* A gun recoils when a bullet is fired.
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Six Simple Machines
| Machine | Principle | Example | |---------|-----------|---------| | Lever | Fulcrum, load, effort | See-saw, scissors, crowbar | | Inclined Plane | Reduces effort over longer distance | Ramp, staircase | | Wedge | Two inclined planes back-to-back | Axe, knife, nail | | Screw | Inclined plane wrapped around cylinder | Screw, jack | | Wheel and Axle | Large wheel reduces effort | Steering wheel, door knob | | Pulley | Changes direction or multiplies force | Flagpole, crane |
**Three Classes of Levers:**
Class I: Fulcrum between load and effort (scissors)
Class II: Load between fulcrum and effort (wheelbarrow, nutcracker)
Class III: Effort between fulcrum and load (fishing rod, broom)
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Common Mistakes
**Confusing mass and weight** → Mass is the amount of matter (kg), weight is the gravitational force on mass (N). Weight = m × g.
**Thinking friction is always harmful** → Friction is essential for walking, writing, braking. Without friction, we cannot grip anything.
**Believing work is done when holding a heavy object** → No work is done if there is no displacement. Holding a bag stationary = zero work, even if you feel tired.
**Assuming simple machines reduce work** → Simple machines reduce effort, not total work. Work input ≈ Work output (minus friction losses).
**Mixing up speed and velocity** → Speed is scalar (magnitude only), velocity is vector (magnitude + direction). 40 km/h is speed; 40 km/h north is velocity.
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Quick Reference
Force changes motion; unit is Newton (N)
Work = Force × Displacement; unit is Joule (J)
KE = ½mv²; PE = mgh
Newton's Third Law: Action = Reaction (equal and opposite)
MA = Load / Effort; efficiency = (MA/VR) × 100%
Static friction > Sliding friction > Rolling friction
👥 Study this together
Invite your prep group — read the same notes, then discuss doubts in this topic's shared room.