Forces & Motion

Speed = distance ÷ time. Velocity = displacement ÷ time (includes direction). Acceleration = change in velocity ÷ time.

Distance-time graphs: gradient = speed. Horizontal line = stationary. Steeper line = faster speed. Velocity-time graphs: gradient = acceleration. Area under graph = distance travelled. Horizontal line = constant velocity.

Newton's Laws of Motion:

1. First Law (Inertia): An object stays at rest or constant velocity unless acted on by a resultant force.
2. Second Law: F = ma (Force = mass × acceleration). Larger force → more acceleration. Larger mass → less acceleration.
3. Third Law: Every action has an equal and opposite reaction.

Momentum = mass × velocity (kg m/s). Conservation of momentum: Total momentum before = total momentum after (in a closed system).

Friction is a force opposing motion. Weight = mass × g (gravitational field strength; g = 10 N/kg on Earth).

Stopping distance = thinking distance + braking distance. Increases with speed, tiredness, wet roads.

Deep dive: build the idea, then use it

Conceptual model

Motion describes change of position; forces explain changes in motion. Velocity includes direction, acceleration is change in velocity per time, and resultant force is the vector sum of all forces. Newton’s laws connect a zero resultant force with constant velocity and link force to F=ma. Momentum, p=mv, is conserved in an isolated interaction.

Worked example

A 1200 kg car accelerates from 4 to 10 m s⁻¹ in 3 s. a=(10-4)/3=2 m s⁻²; resultant force F=ma=1200×2=2400 N. This is the resultant, not automatically the engine force: resistive forces may also act, so an engine-force question needs a force diagram and direction convention.

Exam-method habit

Choose positive direction, draw a labelled free-body diagram, and resolve forces along the same line before using F=ma. On a velocity–time graph, gradient is acceleration and area is displacement. Convert km/h to m/s before substituting. In collisions, write total momentum before and after with signs.

Common errors to catch early

Calling velocity and speed interchangeable; using distance as the area under a velocity graph when direction matters; forgetting friction or air resistance; saying a moving object needs a force to keep moving; and assigning action–reaction forces to the same object.

Retrieval drill — close the notes, phir try karo

Define resultant force. A runner changes velocity from 2 to 8 m/s in 2 s: find acceleration. Sketch forces on a book at rest on a table. Explain why seatbelts increase stopping time and reduce force for the same momentum change.

A Pakistan-relevant use

When cyclists share busy city roads, stopping distance depends on speed, surface, reaction time, and braking—not courage alone. Physics supports practical choices such as leaving space and maintaining equipment, while traffic rules and local conditions remain essential.

Concise summary

Describe motion with direction and time, then explain it with resultant force. Diagrams, signs, and graph meanings prevent most avoidable mistakes.

A reliable self-check routine

Before accepting an answer, say what each quantity, symbol, particle, or graph feature means. Then check its unit, sign, direction, size, or conservation rule. In a calculation, write the relationship first, substitute with units, calculate, and decide whether the result is sensible. In an explanation, make a chain: cause → mechanism → observed result. This is not extra decoration; it is how a reader can follow your thinking and how you catch a copied digit or an attractive-but-wrong statement. If the question gives a new context, do not hunt for a memorised sentence. Identify the model underneath it and apply that model. Short, precise working beats a long paragraph that never answers the command word.

Practice plan

Try one straightforward question without notes, one mixed question where you choose the method, and one question where you explain why an answer is reasonable. Mark the exact first step that felt uncertain. Revisit that step the next day for two minutes instead of rereading everything. Small retrieval loops make the topic stick, yaar.

Connect and transfer

This topic becomes stronger when you deliberately meet it in an unfamiliar wrapper. A diagram may be rotated, a calculation may use an awkward unit, a practical may describe an everyday object, or a question may provide more information than you need. Pause and sort the information into three columns: given, wanted, and relationship. That small pause prevents the common rush of putting every number into the first formula remembered. If the answer is qualitative, decide whether the task is asking for a prediction, a description, or a mechanism; these need different sentences. If it is numerical, estimate its order of magnitude before the calculator. If it involves a graph or table, describe the relevant trend using the actual variables before explaining it.

Teach the idea out loud in sixty seconds as though a friend missed the lesson. Avoid specialist words you cannot unpack. Then add those words back with their exact meaning. This exposes the difference between recognition (“that looks familiar”) and recall (“I can construct the answer”). Keep an error log with a corrected example, not a list of scores. For the next attempt, cover the correction and reproduce the decision that led to it. The goal is calm, repeatable reasoning—not racing through a page. When your final answer differs from a friend’s, compare the model and assumptions before comparing calculators. Often the useful learning is in the first different step.