How to Solve Physics Exam Problems Step by Step
When a physics question looks unfamiliar, begin by describing what is happening. Identify the object, the quantities given and the quantity requested. A formula becomes useful once you know which physical relationship it represents and whether its assumptions fit the problem.
This guide develops a repeatable method through an original mechanics example. It is foundation practice rather than a complete official-exam syllabus. Use your current course requirements to choose the topics you need, and read every exam paper’s instructions separately.
Step 1: Write the target quantity
Before collecting formulas, state the unknown with a symbol and unit. “Find acceleration a, in m/s²” is clearer than “find the answer.” Distinguish quantities that can sound similar: distance and displacement, speed and velocity, or mass and weight.
Read the command as well. A request to explain, justify or draw requires more than a numerical answer. Break a multi-part problem into separate targets and keep the question numbering visible.
Step 2: List the data and convert units
Write values beside their quantities. Convert units before substituting into an equation that uses SI units. For example, 450 g = 0.450 kg and 90 km/h = 25 m/s.
Show the conversion if it is a possible source of error. For speed, multiplying by 1,000 metres per kilometre and dividing by 3,600 seconds per hour gives the conversion from km/h to m/s. The units explain why you divide by 3.6.
Step 3: Draw the simplest useful model
A force diagram, circuit sketch or labelled ray diagram can make relationships visible. Draw only what is relevant. In a force diagram, include forces acting on the chosen object and label their directions. A velocity arrow is not an extra force.
Choose a positive direction for one-dimensional motion. If right is positive, a leftward force enters the horizontal calculation with a negative sign. State assumptions such as negligible resistance only when the question gives them or when you explicitly introduce them in a model.
Step 4: Choose a law and explain why it applies
Use a relationship that connects the known quantities to the target. For a constant-mass object described in an inertial reference frame, Newton’s second law relates resultant force to acceleration. The force in that relationship is the sum of forces, not automatically the largest individual force.
For motion equations, check the constant-acceleration assumption. For an energy calculation, identify the system and account for the relevant transfers. Write the symbolic relationship before inserting numbers; this makes the reasoning easier to inspect.
A complete worked example
A 2.0 kg trolley starts from rest on a horizontal track. A constant horizontal pull of 8.0 N acts to the right, while a constant resistive force of 2.0 N acts to the left. Find the acceleration and the speed after 3.0 s. Assume the forces remain constant and the trolley continues moving to the right.
- Target: acceleration a, then final speed v.
- Data: mass m = 2.0 kg; initial velocity u = 0; time t = 3.0 s.
- Direction: right is positive. Vertically, the support force and weight balance because there is no vertical acceleration.
- Resultant horizontal force: F = 8.0 − 2.0 = 6.0 N.
- Acceleration: F = ma, so a = F/m = 6.0/2.0 = 3.0 m/s² to the right.
- Final velocity: v = u + at = 0 + 3.0 × 3.0 = 9.0 m/s to the right. The speed is 9.0 m/s.
The example’s numbers are invented for learning. They are not copied from an official paper.
Step 5: Check the result before moving on
First check units. A newton divided by a kilogram gives m/s², which matches acceleration. Next check direction: the larger force points right, so the acceleration should point right. Finally, check the trend: starting from rest with positive constant acceleration, the trolley’s speed should increase.
A plausible answer can still be wrong, so these checks complement the calculation. If you obtained 4.0 m/s², inspect whether you forgot the resistive force. If you obtained 18 m/s, inspect the mass division and time multiplication.
Practise the decisions, not only the arithmetic
Try a new version: a 1.5 kg trolley starts from rest with a 7.0 N pull and 2.5 N resistance in the opposite direction. Find its acceleration and speed after 4.0 s, using the same assumptions.
Answer: resultant force = 4.5 N; acceleration = 4.5/1.5 = 3.0 m/s²; speed = 3.0 × 4.0 = 12 m/s. Explain why using 7.0 N alone would give the wrong acceleration.
For your next question, use six lines: target, data, diagram, law, calculation, check. If you get stuck, identify which line needs help. That gives a teacher a much clearer starting point than an unexplained incorrect number.
Build the basics with Physics Foundations or its sample in the free resource library. Then choose relevant questions from the Lebanese official-exam archive and compare your reasoning with available corrections.