Force, Motion and Work
Overview
Force, Motion and Work form the foundational mechanics unit for TET-2 Science. This topic connects everyday experiences—pushing a cart, riding a bicycle, using a lever—to fundamental physics principles. Questions typically test conceptual understanding of Newton's laws, types of friction, work-energy relationships, and mechanical advantage of simple machines.
Expect 3–5 questions from this topic, often scenario-based: calculating work done, identifying the type of friction in a situation, or determining mechanical advantage. Mastery requires both formula recall and the ability to apply concepts to real-world examples that upper primary students encounter.
The key challenge is distinguishing closely related terms (force vs pressure, speed vs velocity, work vs energy) and remembering that physics definitions often differ from everyday usage—for instance, holding a heavy bag involves no "work" in physics terms if there's no displacement.
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).
- **Balanced vs Unbalanced Forces**: Balanced forces produce no change in motion (net force = 0). Unbalanced forces cause acceleration or deformation.
- **Friction** is the force that opposes relative motion between surfaces in contact. It depends on the nature of surfaces and the normal force, not on area of contact.
- **Types of Friction**: Static friction (prevents motion from starting) > Sliding friction > Rolling friction. Fluid friction acts in liquids and gases.
- **Newton's Laws of Motion**: First law (inertia)—objects resist change in motion. Second law—F = ma (force equals mass times acceleration). Third law—every action has an equal and opposite reaction.
- **Work** is done only when force causes displacement in its direction. W = F × d × cos θ. No displacement or perpendicular force means zero work.
- **Energy** is the capacity to do work. Forms include kinetic (motion), potential (position), heat, light, sound, and chemical energy. Energy transforms but is neither created nor destroyed (law of conservation).
- **Simple Machines** multiply force or change its direction. Six types: lever, pulley, wheel and axle, inclined plane, wedge, screw. They reduce effort but never reduce work.
Formulas / Key Facts
| Quantity | Formula | Unit | Notes | |----------|---------|------|-------| | Force | F = m × a | Newton (N) | 1 N = 1 kg·m/s² | | Weight | W = m × g | Newton | g ≈ 10 m/s² (approx) | | Work | W = F × d | Joule (J) | 1 J = 1 N·m | | Kinetic Energy | KE = ½mv² | Joule | Depends on velocity squared | | Potential Energy | PE = mgh | Joule | Height from reference level | | Power | P = W/t | Watt (W) | 1 W = 1 J/s | | Mechanical Advantage | MA = Load/Effort | No unit | Higher MA = less effort needed | | Velocity Ratio | VR = Distance moved by effort / Distance moved by load | No unit | — | | Efficiency | η = (MA/VR) × 100% | Percentage | Always < 100% in real machines |
**Key Facts for Quick Recall:**
- Friction can be reduced by lubrication, polishing, or using ball bearings
- Friction is essential for walking, writing, and braking
- Work done against gravity is stored as potential energy
- A lever has three classes based on position of fulcrum, load, and effort
- Inclined plane reduces effort but increases distance
Worked Examples
**Example 1: Calculating Work Done**
A boy pushes a box with a force of 50 N and moves it 4 m in the direction of the force. Calculate the work done.
*Solution:* Work = Force × Displacement W = 50 N × 4 m = 200 J
The work done is **200 Joules**.
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**Example 2: Finding Kinetic Energy**
A ball of mass 2 kg is moving with a velocity of 5 m/s. Find its kinetic energy.
*Solution:* KE = ½ × m × v² KE = ½ × 2 × (5)² KE = ½ × 2 × 25 KE = 25 J
The kinetic energy is **25 Joules**.
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**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. Calculate the mechanical advantage.
*Solution:* MA = Load / Effort MA = 600 N / 150 N = 4
The mechanical advantage is **4** (the lever multiplies the effort 4 times).
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**Example 4: Potential Energy**
A stone of mass 500 g is lifted to a height of 20 m. Calculate its potential energy. (Take g = 10 m/s²)
*Solution:* First convert mass: 500 g = 0.5 kg PE = m × g × h PE = 0.5 × 10 × 20 PE = 100 J
The potential energy is **100 Joules**.
Common Mistakes
- **Confusing mass and weight** → Mass is the amount of matter (kg), weight is gravitational force (N). Weight = mass × g. A 10 kg object has weight 100 N on Earth.
- **Assuming work is done when holding a load stationary** → No displacement means zero work in physics, even if you feel tired. The correct condition: work requires both force AND displacement in the force's direction.
- **Thinking friction is always harmful** → Friction enables walking, writing, braking vehicles, and holding objects. Without friction, we couldn't move. Recognise both advantages and disadvantages.
- **Believing simple machines reduce work** → Machines reduce effort, not work. The work output can never exceed work input. What you save in force, you lose in distance moved.
- **Mixing up speed and velocity** → Speed is scalar (magnitude only), velocity is vector (magnitude + direction). An object moving in a circle at constant speed has changing velocity because direction changes.
- **Forgetting to square velocity in KE formula** → KE depends on v², so doubling velocity quadruples kinetic energy. This is a favourite trap in numerical problems.
Quick Reference
- **Force changes motion or shape; unit is Newton (N)**
- **Work = Force × Displacement; done only when displacement occurs in force direction**
- **KE = ½mv²; PE = mgh; total mechanical energy is conserved in ideal systems**
- **Static friction > Sliding friction > Rolling friction**
- **Newton's 3rd Law: Action and reaction are equal, opposite, and act on different bodies**
- **Mechanical Advantage = Load ÷ Effort; efficiency is always less than 100% in real machines**