IGCSE Physics: Motion, Forces and Energy

TL;DR

Physics is about how stuff moves, what makes it move, and the forms of "oomph" it has. You'll learn to describe motion with speed and acceleration, understand forces like gravity that cause changes in motion, and explore different types of energy that make things happen. This topic connects everyday experiences to fundamental physical principles.

1. The Mental Model

Imagine everything around you, from a rolling ball to a bouncing spring or even a hot cup of tea. This topic helps you understand why they do what they do, how fast they do it, and the hidden energy that lets it all happen. It's like having a secret decoder ring for the physical world.

2. The Core Material

Motion: How Stuff Moves

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Motion describes how an object changes its position. We use a few key terms:

• Distance: How far an object has travelled. It's a scalar quantity, meaning it only has magnitude (e.g., 10 metres).
• Displacement: How far an object is from its starting point and in what direction. It's a vector quantity (e.g., 10 metres East).
• Speed: How fast an object is moving. It's the distance travelled per unit time.
  • Formula: Speed = Distance / Time
  • Units: metres per second (m/s) or kilometres per hour (km/h)
• Velocity: How fast an object is moving and in what direction. It's the displacement per unit time.
  • Formula: Velocity = Displacement / Time
  • Units: metres per second (m/s) with a direction (e.g., 5 m/s North)
• Acceleration: The rate at which an object's velocity changes. This can be speeding up, slowing down, or changing direction.
  • Formula: Acceleration = (Change in Velocity) / Time
  • Units: metres per second squared (m/s²)

Graphs of Motion:
* Distance-time graphs: The gradient (slope) tells you the speed. A straight line means constant speed, a curved line means changing speed.
* Speed-time graphs: The gradient tells you the acceleration. The area under the graph tells you the distance travelled.

Forces: What Makes Stuff Move (or Not)

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A force is a push or a pull that can change an object's motion (or its shape). Forces are vector quantities – they have both magnitude and direction.

• Newton's First Law (Inertia): An object will stay at rest or keep moving at a constant velocity unless an unbalanced force acts on it.
• Newton's Second Law: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
  • Formula: Force (F) = Mass (m) × Acceleration (a)
  • Units: Newtons (N)
• Newton's Third Law: For every action, there is an equal and opposite reaction. (e.g., You push the wall, the wall pushes back on you.)

Types of Forces:
* Weight: The force of gravity acting on an object's mass.
* Formula: Weight (W) = Mass (m) × Gravitational Field Strength (g)
* On Earth, g is approximately 9.8 N/kg (often rounded to 10 N/kg for calculations).
* Friction: A force that opposes motion between two surfaces in contact.
* Air Resistance: A type of friction that opposes motion through the air.
* Normal Force: The support force exerted by a surface perpendicular to it.

Resultant Force: When multiple forces act on an object, the resultant force is the single force that has the same effect as all the individual forces combined. If the resultant force is zero, the object is in equilibrium (not accelerating).

Energy: The "Oomph" to Make Things Happen

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Energy is the ability to do work. It cannot be created or destroyed, only transferred from one form to another.

• Work Done: When a force causes an object to move a certain distance in the direction of the force.
  • Formula: Work Done (W) = Force (F) × Distance (d)
  • Units: Joules (J)
• Power: The rate at which work is done or energy is transferred.
  • Formula: Power (P) = Work Done / Time OR Power = Energy Transferred / Time
  • Units: Watts (W)

Forms of Energy:
* Kinetic Energy (KE): Energy due to motion.
* Formula: KE = ½ × Mass (m) × Velocity² (v²)
* Gravitational Potential Energy (GPE): Energy due to an object's position in a gravitational field (its height).
* Formula: GPE = Mass (m) × Gravitational Field Strength (g) × Height (h)
* Elastic Potential Energy: Energy stored in a stretched or compressed elastic object (like a spring).
* Thermal Energy: Energy associated with the random motion of atoms and molecules (heat).
* Chemical Energy: Energy stored in the bonds of chemical compounds.
* Electrical Energy: Energy associated with the movement of electric charges.
* Light Energy: Energy associated with electromagnetic waves.
* Sound Energy: Energy associated with vibrations.

Conservation of Energy: In a closed system, the total amount of energy remains constant, even if it changes form. For example, a falling object converts GPE into KE.

graph TD
    A["Initial Position"] --> B["Motion Begins"]
    B --> C["Speed, Velocity, Acceleration?"]
    C --> D{"Unbalanced Force?"}
    D -- Yes --> E["Change in Velocity (Acceleration)"]
    D -- No --> F["Constant Velocity or At Rest"]
    E --> G["Energy Conversion (e.g., GPE to KE)"]
    F --> G
    G --> H["Work Done? (Energy Transfer)"]
    H --> I["Final Position/State"]

3. Worked Example

Imagine you're pushing a box across a floor. The box has a mass of 10 kg. You apply a constant force of 50 N to push it 2 metres. There's a frictional force of 10 N opposing your motion.

1. Calculate the resultant force:

   • Force applied = 50 N (forward)
   • Frictional force = 10 N (backward)
   • Resultant Force = 50 N - 10 N = 40 N (forward)

2. Calculate the acceleration of the box:

   • Using F = ma:
   • 40 N = 10 kg × a
   • a = 40 N / 10 kg = 4 m/s²

3. Calculate the work done by you on the box:

   • Work Done = Force × Distance
   • Work Done = 50 N × 2 m = 100 J (This is the energy you put in, some of which is lost to friction as heat).

4. Calculate the work done against friction:

   • Work Done against friction = Frictional Force × Distance
   • Work Done against friction = 10 N × 2 m = 20 J (This energy is converted into thermal energy, making the floor and box slightly warmer).

5. Calculate the net work done on the box (leading to its kinetic energy):

   • Net Work Done = Resultant Force × Distance
   • Net Work Done = 40 N × 2 m = 80 J
   • This 80 J would be the kinetic energy the box gains if it started from rest.

4. Key Takeaways

• Speed is distance over time; velocity is displacement over time (including direction).
• Acceleration is the rate of change of velocity, caused by an unbalanced force.
• Newton's Laws explain how forces affect motion: inertia, F=ma, and action-reaction pairs.
• Weight is the force of gravity (mass × g); mass is a measure of inertia.
• Work is done when a force moves an object through a distance, and power is the rate of doing work.
• Energy comes in many forms (KE, GPE, thermal, etc.) and is always conserved, only changing forms.
• Understanding resultant force is crucial for predicting an object's motion.

Common Mistakes:
- Confusing speed with velocity, or distance with displacement. Remember vectors have direction!
- Forgetting that 'g' (gravitational field strength) has a value of approximately 10 N/kg on Earth when calculating weight or GPE.
- Thinking energy is used up rather than transferred or transformed.
- Not using the resultant force when calculating acceleration using F=ma.

5. Now Try It

Take a small object, like a toy car or a book. Design a simple experiment to investigate how friction affects its motion.

1. Plan: How will you measure the force needed to pull it at a constant speed? How will you change the surface to vary friction? (Hint: you could use a spring balance to pull it.)
2. Predict: What do you expect to happen to the force needed as the surface gets rougher?
3. Perform: Carry out your experiment, making sure to keep other factors (like the object's mass) constant.
4. Analyze: What did you observe? How did the force change?
5. Explain: Use the concepts of forces (especially friction) and Newton's laws to explain your observations. What does pulling at a constant speed tell you about the resultant force?

Success looks like: You can clearly describe your experiment, present your observations, and explain why the force changed based on the physics principles you've learned about forces and motion.