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Physics Study Guide
Physics explains how objects move, how forces act, how energy changes, and how the natural world behaves. This guide introduces problem-solving methods, speed, force, SI units, common errors, and practice questions with worked answers.
1. How to solve physics problems
- Read carefully: Identify what is known and what needs to be found.
- Choose a formula: Select a relationship that connects the known quantities to the unknown one.
- Convert units: Convert measurements when the formula requires different units.
- Substitute and calculate: Show the calculation step by step.
- Check your result: Include the appropriate unit and consider whether the answer is reasonable.
2. SI units and measurements
The International System of Units (SI) provides a standard way to express physical measurements.
- Length — metre (m)
- Mass — kilogram (kg)
- Time — second (s)
- Temperature — kelvin (K)
- Electric current — ampere (A)
- Force — newton (N)
- Energy — joule (J)
- Power — watt (W)
Always check the units before substituting values into a formula. For example, speed is commonly expressed in metres per second (m/s) or kilometres per hour (km/h).
3. Worked example: calculating speed
Question: A cyclist travels 150 metres in 30 seconds. Calculate the average speed.
The formula is:
Average speed = distance ÷ time
- Distance = 150 m.
- Time = 30 s.
- Average speed =
150 ÷ 30 = 5 m/s.
Answer: 5 m/s.
Average speed describes the total distance travelled divided by the total time taken. It is not necessarily the speed at every moment during the journey.
Another example
A car travels 120 kilometres in 2 hours.
Average speed = 120 ÷ 2 = 60 km/h.
Answer: 60 km/h.
4. Worked example: calculating force
Newton's second law relates net force, mass, and acceleration:
Force = mass × acceleration
Question: A 4 kg object accelerates at 3 m/s². Find the net force.
- Mass = 4 kg.
- Acceleration = 3 m/s².
- Force =
4 × 3 = 12 N.
Answer: 12 newtons.
This formula uses the net force acting on the object. If several forces act at once, consider their directions when determining the net force.
5. Understanding energy
Energy is the capacity to do work. It can take different forms, including kinetic, potential, thermal, chemical, and electrical energy.
Kinetic energy: Energy associated with motion. For an object of mass m moving at speed v:
Kinetic energy = ½ × m × v²
Example: A 2 kg object moves at 3 m/s.
KE = ½ × 2 × 3² = 9 J
Answer: 9 joules.
Energy can be transferred or transformed from one form to another. In an isolated system, total energy is conserved, although some energy may become less useful for a particular task.
6. Common physics mistakes
- Using a formula without checking whether it applies to the situation.
- Forgetting to convert minutes to seconds or kilometres to metres when needed.
- Leaving units out of the final answer.
- Confusing mass, measured in kilograms, with weight, which is a force measured in newtons.
- Ignoring direction when working with vector quantities such as force and velocity.
- Rounding intermediate values too early.
7. Practice questions with answers
Try each question yourself before opening the answer.
Question 1: A runner travels 100 m in 20 s. What is the average speed?
Average speed = distance ÷ time = 100 ÷ 20 = 5 m/s.
Answer: 5 m/s.
Question 2: What force accelerates a 5 kg object at 2 m/s²?
Force = mass × acceleration = 5 × 2 = 10 N.
Answer: 10 N.
Question 3: What is the kinetic energy of a 4 kg object moving at 2 m/s?
KE = ½ × mass × speed² = ½ × 4 × 2² = 8 J.
Answer: 8 J.
Question 4: Name the SI unit of force.
The SI unit of force is the newton (N).
Question 5: Convert 2 hours to seconds.
One hour contains 3,600 seconds, so 2 × 3,600 = 7,200 seconds.
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